U.S. patent application number 12/634895 was filed with the patent office on 2010-04-08 for surface emitting laser, and transceiver, optical transceiver, and optical communication system employing the surface emitting laser.
This patent application is currently assigned to The FURUKAWA ELECTRIC CO., LTD.. Invention is credited to Setiagung Casimirus, Takeshi Hama, Norihiro Iwai, Yasukazu Shiina, Hitoshi SHIMIZU.
Application Number | 20100086311 12/634895 |
Document ID | / |
Family ID | 34637391 |
Filed Date | 2010-04-08 |
United States Patent
Application |
20100086311 |
Kind Code |
A1 |
SHIMIZU; Hitoshi ; et
al. |
April 8, 2010 |
SURFACE EMITTING LASER, AND TRANSCEIVER, OPTICAL TRANSCEIVER, AND
OPTICAL COMMUNICATION SYSTEM EMPLOYING THE SURFACE EMITTING
LASER
Abstract
A surface emitting laser includes a lower semiconductor
multilayer mirror formed of a plurality of pairs of a
high-refractive-index area and a low-refractive-index area; an
active layer vertically sandwiched by cladding layers; a current
confinement layer of Al.sub.zGa.sub.1-zAs having an oxide area in a
peripheral portion of the current confinement layer, where
0.95.ltoreq.z.ltoreq.1; and an upper semiconductor multilayer
mirror formed of a plurality of pairs of a high-refractive-index
area and a low-refractive-index area. The low-refractive-index area
of at least one of the lower semiconductor multilayer mirror and
the upper semiconductor multilayer mirror includes an
Al.sub.z1Ga.sub.1-z1As layer with a thickness thinner than that of
the current confinement layer, where z.ltoreq.z1.
Inventors: |
SHIMIZU; Hitoshi;
(Chiyoda-ku, JP) ; Casimirus; Setiagung;
(Chiyoda-ku, JP) ; Shiina; Yasukazu; (Chiyoda-ku,
JP) ; Hama; Takeshi; (Chiyoda-ku, JP) ; Iwai;
Norihiro; (Chiyoda-ku, JP) |
Correspondence
Address: |
OBLON, SPIVAK, MCCLELLAND MAIER & NEUSTADT, L.L.P.
1940 DUKE STREET
ALEXANDRIA
VA
22314
US
|
Assignee: |
The FURUKAWA ELECTRIC CO.,
LTD.
Chiyoda-ku
JP
|
Family ID: |
34637391 |
Appl. No.: |
12/634895 |
Filed: |
December 10, 2009 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
10958125 |
Oct 5, 2004 |
7656924 |
|
|
12634895 |
|
|
|
|
PCT/JP03/04413 |
Apr 7, 2003 |
|
|
|
10958125 |
|
|
|
|
Current U.S.
Class: |
398/139 ;
398/135 |
Current CPC
Class: |
H01S 5/34313 20130101;
B82Y 20/00 20130101; H01S 5/18347 20130101; H01S 5/02251 20210101;
H01S 5/18391 20130101; H01S 5/18313 20130101; H01S 5/18305
20130101 |
Class at
Publication: |
398/139 ;
398/135 |
International
Class: |
H04B 10/00 20060101
H04B010/00 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 5, 2002 |
JP |
2002-104392 |
Apr 5, 2002 |
JP |
2002-104393 |
Jun 11, 2002 |
JP |
2002-170574 |
Jul 22, 2002 |
JP |
2002-212830 |
Claims
1. A transceiver comprising: an optical transmitting unit that
includes a surface emitting laser that emits a laser beam in a
wavelength range between 1.2 micrometers and 1.6 micrometers in a
vertical direction with respect to a semiconductor substrate; and a
control circuit that controls a current injected into the surface
emitting laser based on an electric signal input; and an optical
receiving unit that includes a photoelectric conversion element
that receives an optical signal input from outside, and converts
the optical signal into an electric signal, wherein the surface
emitting laser includes an active layer stacked on the
semiconductor substrate; a reflection-side
semiconductor-multilayer-mirror having a reflectivity of equal to
or more than 99.9 percent with respect to the laser beam; and an
emission-side semiconductor-multilayer-mirror having a reflectivity
of equal to or more than 99.4 percent and equal to or less than
99.8 percent with respect to the laser beam.
2. An optical transceiver comprising: a surface emitting laser that
emits a laser beam in a wavelength range between 1.2 micrometers
and 1.6 micrometers in a vertical direction with respect to a
semiconductor substrate; a signal multiplexing circuit that
multiplexes a plurality of electric signals; a control circuit that
controls the surface emitting laser based on an electric signal
output from the signal multiplexing circuit; a photoelectric
conversion element that receives an optical signal input from
outside, and converts the optical signal into an electric signal;
and a signal demultiplexing circuit that demultiplexes the electric
signal output from the photoelectric conversion element into a
plurality of electric signals, wherein the surface emitting laser
includes an active layer stacked on the semiconductor substrate; a
reflection-side semiconductor-multilayer-mirror having a
reflectivity of equal to or more than 99.9 percent with respect to
the laser beam; and an emission-side
semiconductor-multilayer-mirror having a reflectivity of equal to
or more than 99.4 percent and equal to or less than 99.8 percent
with respect to the laser beam.
3. An optical communication system comprising: a surface emitting
laser that emits a laser beam in a wavelength range between 1.2
micrometers and 1.6 micrometers in a vertical direction with
respect to a semiconductor substrate; a control circuit that
controls the surface emitting laser; an optical fiber that
transmits an optical signal emitted from the surface emitting
laser; and a photoelectric conversion element that receives the
optical signal from the optical fiber, and converts the optical
signal into an electric signal, wherein the surface emitting laser
includes an active layer stacked on the semiconductor substrate; a
reflection-side semiconductor-multilayer-mirror having a
reflectivity of equal to or more than 99.9 percent with respect to
the laser beam; and an emission-side
semiconductor-multilayer-mirror having a reflectivity of equal to
or more than 99.4 percent and equal to or less than 99.8 percent
with respect to the laser beam.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a Divisional of U.S. Ser. No.
10/958,125, filed Oct. 5, 2004, the entire content of which is
incorporated herein by reference. U.S. Ser. No. 10/958,125 is a
Continuation of PCT/JP03/04413 filed Apr. 7, 2003, which claims the
benefit of priority from Japanese Patent Application Nos.
2002-104392 filed Apr. 5, 2002, 2002-104393 filed Apr. 5, 2002,
2002-170574 filed Jun. 11, 2002 and 2002-212830 filed Jul. 22,
2002.
TECHNICAL FIELD
[0002] The present invention relates to a surface emitting laser of
a vertical cavity type, and a transceiver, an optical transceiver,
and an optical communication system using the surface emitting
laser.
BACKGROUND ART
[0003] In recent years, a vertical cavity surface emitting laser
(VCSEL, hereinafter simply referred to as "surface emitting
laser"), in which light resonates in a vertical direction with
respect to a substrate surface as indicated by the name, has been
attracting attentions as a light source for communication like
optical interconnection.
[0004] Compared with a conventional edge emitting laser, the
surface emitting laser has advantages that: a two-dimensional array
of the lasers can be easily formed; it is unnecessary to cleave the
element to form a mirror unlike the edge emitting laser; laser
oscillation is possible with an extremely low threshold value
because a volume of an active layer is considerably small; and
power consumption is low.
[0005] Since the surface emitting laser has an extremely short
cavity length of about one wavelength due to the inherent
structure, the surface emitting laser has a characteristic that a
basic mode oscillation is naturally obtained for an oscillation
spectrum. Therefore, the surface emitting diode maintains a single
longitudinal mode easier than the edge emitting laser, such as a
distributed feedback (DFB) laser. In addition, the surface emitting
laser attracts attentions as a laser essentially more suitable for
optical communication than the DFB laser or the like because, for
example, it is possible to obtain a narrow far field pattern (FFP)
and a relatively low intensity noise.
[0006] With such advantages, the surface emitting laser attracts
attentions as a signal light source in an optical communication
network and optical interconnection that transmits information by
optically connecting computers and as a device for other various
applications.
[0007] FIG. 7 is a perspective sectional view of a conventional
surface emitting laser. FIG. 8 is an explanatory diagram for
explaining structures of a lower semiconductor multilayer mirror
and an upper semiconductor multilayer mirror. Note that, portions
common to FIG. 7 and FIG. 8 are denoted by identical reference
numerals. To manufacture a surface emitting laser 100 shown in FIG.
7, first, a lower semiconductor multilayer mirror (lower
distributed bragg reflector (DBR) mirror) 112 is formed on an
n-type GaAs substrate 11 by a metal organic chemical vapor
deposition (MOCVD) method. As shown in FIG. 8, in the lower
semiconductor multilayer mirror 112, a stacked structure of an
n-type high-refractive-index area 141 and an n-type
low-refractive-index area 142 having respective thicknesses of
.lamda./4n (.lamda. is an oscillation wavelength and n is a
refractive index) forms one pair, and for example, thirty-five
pairs are stacked. The n-type high-refractive-index area 141 is
formed of, for example, n-type Al.sub.0.2Ga.sub.0.8As, and the
n-type low-refractive-index area 142 is formed of, for example,
n-type Al.sub.0.9Ga.sub.0.1As.
[0008] Then, a quantum well (QW) active layer 32 vertically
sandwiched by cladding layers 31 and 33 is formed on the lower
semiconductor multilayer mirror 112. Further, an
Al.sub.zGa.sub.1-zAs (0.95.ltoreq.z.ltoreq.1) layer 15 for forming
a current confinement layer in a later process is formed. Usually,
AlAs is used as the current confinement layer. Moreover, an upper
semiconductor multilayer mirror 116 (upper DBR mirror) is formed on
the Al.sub.zGa.sub.1-zAs (0.95.ltoreq.z.ltoreq.1) layer 15. Here,
as shown in FIG. 8, in the upper semiconductor multilayer mirror
116, assuming that a stacked structure of a p-type
high-refractive-index area 145 and a p-type low-refractive-index
area 146 having respective thicknesses of .lamda./4n (.lamda. is an
oscillation wavelength and n is a refractive index) forms one pair,
for example, twenty-five pairs are stacked. The p-type
high-refractive-index area 145 is formed of, for example, p-type
Al.sub.0.2Ga.sub.0.8As, and the p-type low-refractive-index area
146 is formed of, for example, p-type Al.sub.0.9Ga.sub.0.1As. In
addition, a p-type GaAs contact layer 17 is formed on the upper
semiconductor multilayer mirror 116.
[0009] Next, an outer edge part of a stacked structure, which
consists of the upper semiconductor multilayer mirror 116, the AlAs
layer 15, the cladding layer 33, the QW active layer 32, the
cladding layer 31, and a part of the lower semiconductor multilayer
mirror 112 is removed by a photolithography process and an etching
process (dry etching or wet etching). Consequently, for example, a
columnar mesa-post with a diameter of 30 micrometers is formed.
[0010] Next, oxidation treatment is performed at temperature of
about 400.degree. C. in a moisture vapor atmosphere to selectively
oxidize the Al.sub.zGa.sub.1-zAs (0.95.ltoreq.z.ltoreq.1) layer 15
from a sidewall of the mesa-post and form an Al oxide layer 14. For
example, when a diameter of the mesa-post is 30 micrometers and the
Al oxide layer 14 is formed in a ring shape with a band width of 10
micrometers, an area of the Al.sub.zGa.sub.1-zAs
(0.95.ltoreq.z.ltoreq.1) layer 15 in the center, that is, an area
of an aperture to which a current is injected is about 80
.mu.m.sup.2 (with a diameter of 10 micrometers).
[0011] Then, a silicon nitride film 19 functioning as a protective
layer is formed on an upper surface and a side surface of the
mesa-post and an exposed upper surface of the lower semiconductor
multilayer mirror 112. Subsequently, periphery of the mesa-post, on
which the silicon nitride film 19 is formed, is filled with
polyimide 22. The silicon nitride film 19 formed on the upper
surface of the mesa-post is removed in a circular shape with a
diameter of 30 micrometers to further form a p-type electrode 18 of
a ring shape with an inner diameter of 20 micrometers and an outer
diameter of 30 micrometers on the p-type GaAs contact layer 17
exposed by the removal. After grinding the substrate to have a
thickness of, for example, 200 micrometers, an n-type electrode 21
is formed on the back of the n-type GaAs substrate 11. An electrode
pad 20, on which a wire is bonded, is formed on the polyimide 22 to
come into contact with the p-type electrode 18.
[0012] The characteristic in the structure explained above is that
the Al.sub.zGa.sub.1-zAs (0.95.ltoreq.z.ltoreq.1) layer 15 with a
resistance lower than that of the surrounding Al oxide layer 14 is
arranged on a central part of the QW active layer 32. This
Al.sub.zGa.sub.1-zAs (0.95.ltoreq.z.ltoreq.1) layer 15 makes it
possible to flow a current intensively only in a narrow part of the
active layer 13. Such a structure is called an oxidation
confinement type surface emitting laser. Laser characteristics like
a laser oscillation threshold value are improved significantly.
[0013] In the surface emitting laser, the current confinement
structure is important. In addition, from the viewpoint of
selection of an oscillation wavelength, improvement of a thermal
conductivity, and the like, structures of the lower semiconductor
multilayer mirror 112 and the upper semiconductor multilayer mirror
116 vertically sandwiching the active layer 13 are also very
important. It is known that, in the lower semiconductor multilayer
mirror 112 and the upper semiconductor multilayer mirror 116, a
refractive index difference increases as a difference of Al
composition between a high-refractive-index area and a
low-refractive-index area increases, and a satisfactory
reflectivity is obtained. In addition, it is also known that the
thermal conductivity increases as the Al composition difference
increases (Afromowitz M A et al, Journal of Applied Physics 44, pp
1292, 1973). If the reflectivity is large, the number of pairs of
semiconductor multilayer mirrors can be reduced. In addition, if
the thermal conductivity is large, a surface emitting laser, which
has satisfactory thermal saturation characteristics of optical
output power and operates stably at high power even in a
high-temperature operation environment, can be manufactured.
[0014] However, to obtain a large refractive index difference and a
high thermal conductivity, if a composition y of an
Al.sub.yGa.sub.1-yAs layer (x<y<1), which is a
low-refractive-index area of any one of a lower semiconductor
multilayer mirror and an upper semiconductor multilayer mirror or
both, is set close to 1, a state in which the low-refractive-index
area is easily oxidized is created. In particular, when the
composition y is set too large in the upper semiconductor
multilayer mirror 116, if oxidation treatment is performed in a
moisture vapor atmosphere to obtain the Al oxide layer 14, the
Al.sub.yGa.sub.1-yAs layer (x<y<1), which is the
low-refractive-index area of the upper semiconductor multilayer
mirror 116, may be oxidized together with the Al.sub.zGa.sub.1-zAs
(0.95.ltoreq.z.ltoreq.1) layer 15. When a volume of an oxide film
increases in the lower semiconductor multilayer mirror 112 or the
upper semiconductor multilayer mirror 116, characteristics
deteriorate, for example, an oscillation threshold value increases
and dislocation occurs often.
[0015] As a background art of the invention, "Optoelectronics
semiconductor device with mesa" disclosed in U.S. Pat. No.
5,408,105 is characterized in that an entire lower semiconductor
multilayer mirror is used as an AlAs mirror layer, and a lower
semiconductor multilayer film is not etched.
[0016] Incidentally, when a surface emitting laser is used as a
signal light source, a surface emitting laser, which has an
emission wavelength of 0.8 micrometer to 1.65 micrometers including
a low-loss waveband of an optical fiber serving as a transmission
medium, is required. In surface emitting lasers in this wavelength
band, for a long time, it has been impossible to realize a surface
emitting laser, which oscillates a laser beam having a long
wavelength, for example, a wavelength of 1.2 micrometers or more,
due to difficulty in crystal growth. However, recently, a surface
emitting laser, which oscillates a laser beam having a wavelength
of 1.2 micrometers to 1.3 micrometers, has been realized by the
inventors (Japanese Patent Application Laid-Open No.
2001-124300).
[0017] FIG. 19 shows a structure of the surface emitting laser
described in Japanese Patent Application Laid-Open No. 2001-124300.
This surface emitting laser has a structure in which a buffer layer
1102, a lower reflective layer 1103, a lower cladding layer 1104,
an active layer including a QW layer 1105, and an upper cladding
layer 1106 are sequentially stacked on a substrate 1101. Further,
the surface emitting laser has a stacked structure of a current
confinement layer 1108 processed in a mesa shape, an upper
reflective layer 1109, and a contact layer 1110 on the upper
cladding layer 1106. The current confinement layer 1108 is formed
of a current injection area 1107a consisting of an AlAs layer in a
central part and a selectively oxidized area 1107b formed by
selectively oxidizing an end of the AlAs layer. In addition, an n
side electrode 1114 is arranged on a lower surface of the substrate
1101. Then, in the active layer including the QW layer 1105, by
adding a small amount of Sb in GaInNAs forming the QW layer, a
crystallographic quality of the active layer including the QW layer
1105 is improved. In this way, recently, laser oscillation of a
surface emitting laser in a 1.3-micrometer-band has been performed
utilizing the improvement in a structure of a QW layer and a
selective oxidation technique of an AlAs layer.
[0018] To use a surface emitting laser as a signal light source in
an optical communication network, it is necessary to realize a
surface emitting laser that emits a laser beam having a wavelength
with a low loss when the laser beam is transmitted through an
optical fiber for transmission and having a fixed intensity.
Therefore, a surface emitting laser having an emission wavelength
of 1.2 micrometers or more has been developed, and an example of
realizing laser oscillation using a GaInNAs material for an active
layer has been reported according to the progress of a crystal
growth technique in recent years.
[0019] For example, in the Post Deadline Paper (PD1.2) of the
LEOS-2001 Annual Meeting, the group of Agilent Technologies
Laboratories reported about a surface emitting laser of an
oxidation confinement type. According to this report, there is a
surface emitting laser that has a lower semiconductor multilayer
mirror in which forty layers of n type DBR mirror are stacked
sequentially, an active layer including a triple QW layer formed of
GaInNAs, and an upper semiconductor multilayer mirror in which
twenty-eight layers of a p type DBR mirror and includes an opening
portion with a diameter of 11 micrometers by arranging a current
confinement layer in a part of the p-type upper semiconductor
multilayer mirror. With such a structure, continuous oscillation at
a room temperature is realized, and a surface emitting laser with a
threshold current of about 6 milliamperes and maximum optical
output power of about 0.7 milliwatt is realized.
[0020] FIG. 35 is a perspective sectional view of the conventional
surface emitting laser. FIG. 36 is an explanatory diagram for
explaining structures of a lower semiconductor multilayer mirror
and an upper semiconductor multilayer mirror. Note that, portions
common to FIG. 35 and FIG. 36 are denoted by identical reference
numerals. To manufacture a surface emitting laser 3100 shown in
FIG. 35, first, a lower semiconductor multilayer mirror (lower DBR
mirror) 3112 is formed on an n-type GaAs substrate 3111 by an MOCVD
method. Here, as shown in FIG. 36, in the lower semiconductor
multilayer mirror 3112, assuming that a stacked structure of an
n-type high-refractive-index area 3141 and an n-type
low-refractive-index area 3142 having respective thicknesses of
.lamda./4n (.lamda. is an oscillation wavelength and n is a
refractive index) forms one pair, for example, thirty-five pairs
are stacked. The n-type high-refractive-index area 3141 is formed
of, for example, n-type GaAs, and the n-type low-refractive-index
area 3142 is formed of, for example, n-type
Al.sub.0.9Ga.sub.0.1As.
[0021] However, "Optoelectronics semiconductor device with mesa"
disclosed in U.S. Pat. No. 5,408,105 also has a problem in that
etching accuracy has to be extremely strict.
[0022] In addition, there are problems that should be solved in
using a surface emitting laser for an application like a signal
light source. First, it is necessary to unify lateral modes of an
oscillating laser beam. When a mode higher in a lateral direction
is present in the lateral modes, this causes marked deterioration
in a signal waveform in proportion to a transmission distance at
the time of optical transmission, in particular, at the time of
high-speed modulation. Therefore, it is necessary to realize single
lateral mode oscillation to realize long distance transmission.
[0023] In a surface emitting laser, it is naturally difficult to
stabilize lateral modes due to a structure thereof. Therefore, in a
surface emitting laser including selectively oxidized areas, single
lateral mode oscillation is realized by adjusting a diameter of a
current injection area sandwiched by the selectively oxidized
areas. However, conventionally, it is difficult from the viewpoint
of controllability to realize the single lateral mode oscillation
by adjusting only the diameter of the current injection area in a
surface emitting laser in a 1300-nanometer-band (in a range of
about 1260 nanometers to 1360 nanometers).
[0024] In addition, even if the single lateral mode oscillation can
be realized, when a value of a threshold current increases, a
problem like an increase in power consumption is caused. Therefore,
it is necessary to realize the single lateral mode oscillation
while controlling the increase in a value of a threshold current.
For this purpose, for example, it is necessary to set a diameter of
a current injection layer to, for example, .phi.5 micrometers,
which is disadvantageous from the viewpoint of a working voltage
and optical output power. Moreover, reliability of the surface
emitting laser has to be secured. This is because the surface
emitting laser is required to have sufficient reliability to use
the surface emitting laser element for a signal light source or the
like.
[0025] Moreover, when the surface emitting laser is used for a
signal light source or the like, it is necessary that direct
modulation is possible at a level of 10 Gbit/s. This is a numerical
value necessary for actually using the surface emitting laser as a
signal light source according to an increase in a channel capacity
in recent years.
[0026] When the surface emitting laser reported by the group of
Agilent Technologies Laboratories is actually used as a signal
light source, a new problem occurs. Since a signal beam is
transmitted in a long distance in an optical communication system,
in general, a laser beam outputted from a signal light source is
required to have a light intensity of about 1 milliwatt at the
minimum. Since a maximum light intensity of the surface emitting
laser is only about 0.7 milliwatt, it is inappropriate to use the
surface emitting laser as a signal light source at the present
point.
[0027] To directly modulate a laser beam at 2.4 GBit/s or more, for
example, 10 GBit/s, in general, it is necessary to drive the
surface emitting laser with an injection current five times as
large as a threshold current. In the case of the surface emitting
laser, since the threshold current is 6 milliamperes, the injection
current at the time of driving is 30 milliampere or more. Thus, it
is unrealistic to use the surface emitting laser in terms of power
consumption and taking into account the fact that thermal
saturation occurs actually. To use the surface emitting laser as a
signal light source, it is desirable that the threshold current is
about 1 milliampere and the injection current at the time of
driving is about 5 milliamperes to 6 milliamperes. To realize the
light intensity of 1 milliwatt when the injection current is 5
milliamperes, it is necessary to set slope efficiency to 0.25
mW/mA, and when the injection current is 6 milliampere, it is
necessary to set slope efficiency to 0.2 mW/mA. Thus, it is
inappropriate to use the surface emitting laser as a signal light
source from the viewpoint of a slope efficiency as well.
[0028] Moreover, in the surface emitting laser in the
1300-nanometer-band (1260 nanometers to 1360 nanometers)
oscillation under the present situation, since crystal growth is
difficult for any of the above-mentioned active layers, a low
oscillation threshold value and a high slope efficiency cannot be
realized. In particular, in the surface emitting laser, oscillation
by direct modulation is stable in a high frequency band. The
surface laser element is advantageous in this respect compared with
the edge-emitting laser like a distributed DFB laser. However, a
new problem occurs if it is attempted to realize oscillation with a
wavelength longer than VCSEL in a 0.85 micrometer to
0.98-micrometer-band like 1.2 micrometers to 1.3 micrometers in the
surface emitting laser. More specifically, laser oscillation is
made unstable due to inter-valence-band absorption or free carrier
absorption in a semiconductor multilayer mirror. In the present
situation, satisfactory characteristics are not realized even in
serial transmission in 10 kilometers to 20 kilometers with direct
modulation at about 10 Gbps.
[0029] The invention has been devised in view of the drawbacks of
the conventional technique, and it is an object of the invention to
provide a surface emitting laser with an improved reflectivity and
temperature characteristics by causing an AlAs layer to be present
inside a semiconductor multilayer mirror, which is not oxidized
easily, according to film thickness control of the AlAs layer
rather than controlling oxidation speed according to a difference
of composition of Al as in the conventional technique.
[0030] The invention has been devised in view of the drawbacks of
the conventional technique, and it is another object of the
invention to provide a surface emitting laser that has a lower
threshold current and is highly reliable and with which single
lateral mode oscillation is possible and direct modulation is
possible, and a transceiver, an optical transceiver, and an optical
communication system using the surface emitting element.
[0031] The invention has been devised in view of the drawbacks of
the conventional technique, and it is still another object of the
invention to provide a surface emitting laser with which a
threshold current is controlled to be about 1 milliampere and slope
efficiency is 0.2 mW/mA or more, and an optical transceiver, an
optical communication device, and an optical communication system
using the surface emitting laser.
[0032] The invention has been devised in view of the drawbacks of
the conventional technique, and it is still another object of the
invention to provide a surface emitting laser of a structure having
a long wavelength band of 1.2 micrometers or more as an oscillating
wavelength, which can realize a low oscillation threshold value,
high slope efficiency, and high frequency direct modulation by
reducing an absorption loss due to a p-type semiconductor
reflector, and a transceiver, an optical transceiver, and an
optical communication system using the surface emitting laser.
DISCLOSURE OF THE INVENTION
[0033] It is an object of the present invention to solve at least
the above problems in the conventional technology.
[0034] A surface emitting laser according to one aspect of the
present invention includes a lower semiconductor multilayer mirror
formed of a plurality of pairs of a high-refractive-index area and
a low-refractive-index area on a semiconductor substrate; an active
layer arranged above the lower semiconductor multilayer mirror and
vertically sandwiched by cladding layers; a current confinement
layer of Al.sub.zGa.sub.1-zAs having an oxide area in a peripheral
portion of the current confinement layer, where
0.95.ltoreq.z.ltoreq.1; and an upper semiconductor multilayer
mirror formed of a plurality of pairs of a high-refractive-index
area and a low-refractive-index area. The low-refractive-index area
of at least one of the lower semiconductor multilayer mirror and
the upper semiconductor multilayer mirror includes an
Al.sub.z1Ga.sub.1-z1As layer having a thickness thinner than that
of the current confinement layer, where z.ltoreq.z1.
[0035] A transceiver according to another aspect of the present
invention includes an optical transmitting unit that includes a
surface emitting laser, and a control circuit that controls a
current injected into the surface emitting laser based on an
electric signal input; and an optical receiving unit that includes
a photoelectric conversion element that receives an optical signal
input from outside, and converts the optical signal into an
electric signal. The surface emitting laser includes a lower
semiconductor multilayer mirror formed of a plurality of pairs of a
high-refractive-index area and a low-refractive-index area on a
semiconductor substrate; an active layer arranged above the lower
semiconductor multilayer mirror and vertically sandwiched by
cladding layers; a current confinement layer of
Al.sub.zGa.sub.1-zAs having an oxide area in a peripheral portion
of the current confinement layer, where 0.95.ltoreq.z.ltoreq.1; and
an upper semiconductor multilayer mirror formed of a plurality of
pairs of a high-refractive-index area and a low-refractive-index
area. The low-refractive-index area of at least one of the lower
semiconductor multilayer mirror and the upper semiconductor
multilayer mirror includes an Al.sub.z1Ga.sub.1-z1As layer having a
thickness thinner than that of the current confinement layer, where
z.ltoreq.z1.
[0036] An optical transceiver according to still another aspect of
the present invention includes a surface emitting laser; a signal
multiplexing circuit that multiplexes a plurality of electric
signals; a control circuit that controls the surface emitting laser
based on an electric signal output from the signal multiplexing
circuit; a photoelectric conversion element that receives an
optical signal input from outside, and converts the optical signal
into an electric signal; and a signal demultiplexing circuit that
demultiplexes the electric signal output from the photoelectric
conversion element into a plurality of electric signals. The
surface emitting laser includes a lower semiconductor multilayer
mirror formed of a plurality of pairs of a high-refractive-index
area and a low-refractive-index area on a semiconductor substrate;
an active layer arranged above the lower semiconductor multilayer
mirror and vertically sandwiched by cladding layers; a current
confinement layer of Al.sub.zGa.sub.1-zAs having an oxide area in a
peripheral portion of the current confinement layer, where
0.95.ltoreq.z.ltoreq.1; and an upper semiconductor multilayer
mirror formed of a plurality of pairs of a high-refractive-index
area and a low-refractive-index area. The low-refractive-index area
of at least one of the lower semiconductor multilayer mirror and
the upper semiconductor multilayer mirror includes an
Al.sub.z1Ga.sub.1-z1As layer having a thickness thinner than that
of the current confinement layer, where z.ltoreq.z1.
[0037] An optical communication system according to still another
aspect of the present invention includes a surface emitting laser;
a control circuit that controls the surface emitting laser; an
optical fiber that transmits an optical signal emitted from the
surface emitting laser; and a photoelectric conversion element that
receives the optical signal from the optical fiber, and converts
the optical signal into an electric signal. The surface emitting
laser includes a lower semiconductor multilayer mirror formed of a
plurality of pairs of a high-refractive-index area and a
low-refractive-index area on a semiconductor substrate; an active
layer arranged above the lower semiconductor multilayer mirror and
vertically sandwiched by cladding layers; a current confinement
layer of Al.sub.zGa.sub.1-zAs having an oxide area in a peripheral
portion of the current confinement layer, where
0.95.ltoreq.z.ltoreq.1; and an upper semiconductor multilayer
mirror formed of a plurality of pairs of a high-refractive-index
area and a low-refractive-index area. The low-refractive-index area
of at least one of the lower semiconductor multilayer mirror and
the upper semiconductor multilayer mirror includes an
Al.sub.z1Ga.sub.1-z1As layer having a thickness thinner than that
of the current confinement layer, where z.ltoreq.z1.
[0038] A surface emitting laser according to still another aspect
of the present invention includes a lower reflective layer, a lower
cladding layer, an active layer, an upper cladding layer, and an
upper reflective layer sequentially stacked on a substrate; a
selectively oxidized area that is arranged in an area distant from
a center of the active layer in a stacking direction by equal to or
more than 370 nanometers and equal to or less than 780 nanometers
inside of either of the lower reflective layer and the upper
reflective layer; and a current injection area that sandwiched by
the selectively oxidized area. A difference between a first
effective refractive index of a first area in the stacking
direction including the current injection area and a second
effective refractive index of a second area in the stacking
direction including the selectively oxidized area is equal to or
less than 0.038.
[0039] A transceiver according to still another aspect of the
present invention includes an optical transmitting unit that
includes a surface emitting laser, and a control circuit that
controls a current injected into the surface emitting laser based
on an electric signal input; and an optical receiving unit that
includes a photoelectric conversion element that receives an
optical signal input from outside, and converts the optical signal
into an electric signal. The surface emitting laser includes a
lower reflective layer, a lower cladding layer, an active layer, an
upper cladding layer, and an upper reflective layer sequentially
stacked on a substrate; a selectively oxidized area that is
arranged in an area distant from a center of the active layer in a
stacking direction by equal to or more than 370 nanometers and
equal to or less than 780 nanometers inside of either of the lower
reflective layer and the upper reflective layer; and a current
injection area that sandwiched by the selectively oxidized area. A
difference between a first effective refractive index of a first
area in the stacking direction including the current injection area
and a second effective refractive index of a second area in the
stacking direction including the selectively oxidized area is equal
to or less than 0.038.
[0040] An optical transceiver according to still another aspect of
the present invention includes a surface emitting laser; a signal
multiplexing circuit that multiplexes a plurality of electric
signals; a control circuit that controls the surface emitting laser
based on an electric signal output from the signal multiplexing
circuit; a photoelectric conversion element that receives an
optical signal input from outside, and converts the optical signal
into an electric signal; and a signal demultiplexing circuit that
demultiplexes the electric signal output from the photoelectric
conversion element into a plurality of electric signals. The
surface emitting laser includes a lower reflective layer, a lower
cladding layer, an active layer, an upper cladding layer, and an
upper reflective layer sequentially stacked on a substrate; a
selectively oxidized area that is arranged in an area distant from
a center of the active layer in a stacking direction by equal to or
more than 370 nanometers and equal to or less than 780 nanometers
inside of either of the lower reflective layer and the upper
reflective layer; and a current injection area that sandwiched by
the selectively oxidized area. A difference between a first
effective refractive index of a first area in the stacking
direction including the current injection area and a second
effective refractive index of a second area in the stacking
direction including the selectively oxidized area is equal to or
less than 0.038.
[0041] An optical communication system according to still another
aspect of the present invention includes a surface emitting laser;
a control circuit that controls the surface emitting laser; an
optical fiber that transmits an optical signal emitted from the
surface emitting laser; and a photoelectric conversion element that
receives the optical signal from the optical fiber, and converts
the optical signal into an electric signal. The surface emitting
laser includes a lower reflective layer, a lower cladding layer, an
active layer, an upper cladding layer, and an upper reflective
layer sequentially stacked on a substrate; a selectively oxidized
area that is arranged in an area distant from a center of the
active layer in a stacking direction by equal to or more than 370
nanometers and equal to or less than 780 nanometers inside of
either of the lower reflective layer and the upper reflective
layer; and a current injection area that sandwiched by the
selectively oxidized area. A difference between a first effective
refractive index of a first area in the stacking direction
including the current injection area and a second effective
refractive index of a second area in the stacking direction
including the selectively oxidized area is equal to or less than
0.038.
[0042] A surface emitting laser according to still another aspect
of the present invention includes an active layer stacked on a
semiconductor substrate; a reflection-side
semiconductor-multilayer-mirror having a reflectivity of equal to
or more than 99.9 percent with respect to the laser beam; and an
emission-side semiconductor-multilayer-mirror having a reflectivity
of equal to or more than 99.4 percent and equal to or less than
99.8 percent with respect to the laser beam.
[0043] A transceiver according to still another aspect of the
present invention includes an optical transmitting unit that
includes a surface emitting laser that emits a laser beam in a
wavelength range between 1.2 micrometers and 1.6 micrometers in a
vertical direction with respect to a semiconductor substrate, and a
control circuit that controls a current injected into the surface
emitting laser based on an electric signal input; and an optical
receiving unit that includes a photoelectric conversion element
that receives an optical signal input from outside, and converts
the optical signal into an electric signal. The surface emitting
laser includes an active layer stacked on the semiconductor
substrate; a reflection-side semiconductor-multilayer-mirror having
a reflectivity of equal to or more than 99.9 percent with respect
to the laser beam; and an emission-side
semiconductor-multilayer-mirror having a reflectivity of equal to
or more than 99.4 percent and equal to or less than 99.8 percent
with respect to the laser beam.
[0044] An optical transceiver according to still another aspect of
the present invention includes a surface emitting laser that emits
a laser beam in a wavelength range between 1.2 micrometers and 1.6
micrometers in a vertical direction with respect to a semiconductor
substrate; a signal multiplexing circuit that multiplexes a
plurality of electric signals; a control circuit that controls the
surface emitting laser based on an electric signal output from the
signal multiplexing circuit; a photoelectric conversion element
that receives an optical signal input from outside, and converts
the optical signal into an electric signal; and a signal
demultiplexing circuit that demultiplexes the electric signal
output from the photoelectric conversion element into a plurality
of electric signals. The surface emitting laser includes an active
layer stacked on the semiconductor substrate; a reflection-side
semiconductor-multilayer-mirror having a reflectivity of equal to
or more than 99.9 percent with respect to the laser beam; and an
emission-side semiconductor-multilayer-mirror having a reflectivity
of equal to or more than 99.4 percent and equal to or less than
99.8 percent with respect to the laser beam.
[0045] An optical communication system according to still another
aspect of the present invention includes a surface emitting laser
that emits a laser beam in a wavelength range between 1.2
micrometers and 1.6 micrometers in a vertical direction with
respect to a semiconductor substrate; a control circuit that
controls the surface emitting laser; an optical fiber that
transmits an optical signal emitted from the surface emitting
laser; and a photoelectric conversion element that receives the
optical signal from the optical fiber, and converts the optical
signal into an electric signal. The surface emitting laser includes
an active layer stacked on the semiconductor substrate; a
reflection-side semiconductor-multilayer-mirror having a
reflectivity of equal to or more than 99.9 percent with respect to
the laser beam; and an emission-side
semiconductor-multilayer-mirror having a reflectivity of equal to
or more than 99.4 percent and equal to or less than 99.8 percent
with respect to the laser beam.
[0046] A surface emitting laser according to still another aspect
of the present invention includes an n-type semiconductor
multilayer mirror formed of a plurality of pairs of a
high-refractive-index area and a low-refractive-index area; an
active layer that is vertically sandwiched by cladding layers, and
has an oscillation wavelength of equal to or more than 980
nanometers; and a p-type semiconductor multilayer mirror formed of
a plural pairs of a high-refractive-index area and a
low-refractive-index area on a substrate. The high-refractive-index
layer of the p-type semiconductor multilayer mirror within a
predetermined number of pairs from the active layer in the p-type
semiconductor multilayer mirror includes a first
high-refractive-index area that is adjacent to an interface with
the low-refractive-index layer, and p-type-doped with a first
impurity concentration; and a second high-refractive-index area
that is provided outside the first high-refractive-index area, and
is p-type-doped with a second impurity concentration lower than the
first impurity concentration. The low-refractive-index layer of the
p-type semiconductor multilayer mirror within a predetermined
number of pairs from the active layer in the p-type semiconductor
multilayer mirror includes a first low-refractive-index area that
is adjacent to an interface with the high-refractive-index layer,
and p-type-doped with a third impurity concentration; and a second
low-refractive-index area that is provided outside the first
low-refractive-index area, and is p-type-doped with a fourth
impurity concentration lower than the third impurity
concentration.
[0047] A transceiver according to still another aspect of the
present invention includes an optical transmitting unit that
includes a surface emitting laser, and a control circuit that
controls a current injected into the surface emitting laser based
on an electric signal input; and an optical receiving unit that
includes a photoelectric conversion element that receives an
optical signal input from outside, and converts the optical signal
into an electric signal. The surface emitting laser includes an
n-type semiconductor multilayer mirror formed of a plurality of
pairs of a high-refractive-index area and a low-refractive-index
area; an active layer that is vertically sandwiched by cladding
layers, and has an oscillation wavelength of equal to or more than
980 nanometers; and a p-type semiconductor multilayer mirror formed
of a plural pairs of a high-refractive-index area and a
low-refractive-index area on a substrate. The high-refractive-index
layer of the p-type semiconductor multilayer mirror within a
predetermined number of pairs from the active layer in the p-type
semiconductor multilayer mirror includes a first
high-refractive-index area that is adjacent to an interface with
the low-refractive-index layer, and p-type-doped with a first
impurity concentration; and a second high-refractive-index area
that is provided outside the first high-refractive-index area, and
is p-type-doped with a second impurity concentration lower than the
first impurity concentration. The low-refractive-index layer of the
p-type semiconductor multilayer mirror within a predetermined
number of pairs from the active layer in the p-type semiconductor
multilayer mirror includes a first low-refractive-index area that
is adjacent to an interface with the high-refractive-index layer,
and p-type-doped with a third impurity concentration; and a second
low-refractive-index area that is provided outside the first
low-refractive-index area, and is p-type-doped with a fourth
impurity concentration lower than the third impurity
concentration.
[0048] An optical transceiver according to still another aspect of
the present invention includes a surface emitting laser; a signal
multiplexing circuit that multiplexes a plurality of electric
signals; a control circuit that controls the surface emitting laser
based on an electric signal output from the signal multiplexing
circuit; a photoelectric conversion element that receives an
optical signal input from outside, and converts the optical signal
into an electric signal; and a signal demultiplexing circuit that
demultiplexes the electric signal output from the photoelectric
conversion element into a plurality of electric signals. The
surface emitting laser includes an n-type semiconductor multilayer
mirror formed of a plurality of pairs of a high-refractive-index
area and a low-refractive-index area; an active layer that is
vertically sandwiched by cladding layers, and has an oscillation
wavelength of equal to or more than 980 nanometers; and a p-type
semiconductor multilayer mirror formed of a plural pairs of a
high-refractive-index area and a low-refractive-index area on a
substrate. The high-refractive-index layer of the p-type
semiconductor multilayer mirror within a predetermined number of
pairs from the active layer in the p-type semiconductor multilayer
mirror includes a first high-refractive-index area that is adjacent
to an interface with the low-refractive-index layer, and
p-type-doped with a first impurity concentration; and a second
high-refractive-index area that is provided outside the first
high-refractive-index area, and is p-type-doped with a second
impurity concentration lower than the first impurity concentration.
The low-refractive-index layer of the p-type semiconductor
multilayer mirror within a predetermined number of pairs from the
active layer in the p-type semiconductor multilayer mirror includes
a first low-refractive-index area that is adjacent to an interface
with the high-refractive-index layer, and p-type-doped with a third
impurity concentration; and a second low-refractive-index area that
is provided outside the first low-refractive-index area, and is
p-type-doped with a fourth impurity concentration lower than the
third impurity concentration.
[0049] An optical communication system according to still another
aspect of the present invention includes a surface emitting laser;
a control circuit that controls the surface emitting laser; an
optical fiber that transmits an optical signal emitted from the
surface emitting laser; and a photoelectric conversion element that
receives the optical signal from the optical fiber, and converts
the optical signal into an electric signal. The surface emitting
laser includes an n-type semiconductor multilayer mirror formed of
a plurality of pairs of a high-refractive-index area and a
low-refractive-index area; an active layer that is vertically
sandwiched by cladding layers, and has an oscillation wavelength of
equal to or more than 980 nanometers; and a p-type semiconductor
multilayer mirror formed of a plural pairs of a
high-refractive-index area and a low-refractive-index area on a
substrate. The high-refractive-index layer of the p-type
semiconductor multilayer mirror within a predetermined number of
pairs from the active layer in the p-type semiconductor multilayer
mirror includes a first high-refractive-index area that is adjacent
to an interface with the low-refractive-index layer, and
p-type-doped with a first impurity concentration; and a second
high-refractive-index area that is provided outside the first
high-refractive-index area, and is p-type-doped with a second
impurity concentration lower than the first impurity concentration.
The low-refractive-index layer of the p-type semiconductor
multilayer mirror within a predetermined number of pairs from the
active layer in the p-type semiconductor multilayer mirror includes
a first low-refractive-index area that is adjacent to an interface
with the high-refractive-index layer, and p-type-doped with a third
impurity concentration; and a second low-refractive-index area that
is provided outside the first low-refractive-index area, and is
p-type-doped with a fourth impurity concentration lower than the
third impurity concentration.
[0050] The other objects, features, and advantages of the present
invention are specifically set forth in or will become apparent
from the following detailed description of the invention when read
in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0051] FIG. 1 is a perspective sectional view of a surface emitting
laser according to a first embodiment of the invention;
[0052] FIG. 2 is an explanatory diagram for explaining structures
of a lower semiconductor multilayer mirror and an upper
semiconductor multilayer mirror of a surface emitting laser
according to the first embodiment;
[0053] FIG. 3 is a diagram of current-optical output power
characteristics of the surface emitting laser according to the
first embodiment;
[0054] FIGS. 4A and 4B are diagrams of structures of one pair of
semiconductor multilayer mirrors in other arrangement examples of
an AlAs layer in the surface emitting laser according to the first
embodiment;
[0055] FIG. 5 is a perspective sectional view of a surface emitting
laser according to a second embodiment of the invention;
[0056] FIG. 6 is an explanatory diagram for explaining structures
of a lower semiconductor multilayer mirror and an upper
semiconductor multilayer mirror of a surface emitting laser
according to the second embodiment;
[0057] FIG. 7 is a perspective sectional view of a conventional
surface emitting laser;
[0058] FIG. 8 is an explanatory diagram for explaining a lower
semiconductor multilayer mirror and an upper semiconductor
multilayer mirror of the conventional surface emitting laser;
[0059] FIG. 9 is a diagram of current-optical output power
characteristics of the conventional surface emitting laser;
[0060] FIG. 10 is a sectional view showing a structure of a surface
emitting laser according to a third embodiment of the
invention;
[0061] FIGS. 11A, 11B, and 11C are sectional views of a structure
of an 850 nm band surface emitting laser used in measurement;
[0062] FIG. 12 is a schematic diagram for explaining an effective
refractive index difference;
[0063] FIG. 13 is a sectional view of a structure of a surface
emitting laser according to a modification of the third
embodiment;
[0064] FIG. 14 is a sectional view of a structure of a surface
emitting laser according to a fourth embodiment of the
invention;
[0065] FIG. 15 is a graph showing a relation between a film
thickness of a selectively oxidized area and an effective
refractive index difference for each mirror layer in which the
selectively oxidized area is arranged;
[0066] FIG. 16 is a sectional view of a structure of a surface
emitting laser according to a modification of the fourth
embodiment;
[0067] FIG. 17 is a block diagram of a structure of an optical
transceiver according to a fifth embodiment of the invention;
[0068] FIG. 18 is a block diagram of a structure of an optical
communication system according to a sixth embodiment of the
invention;
[0069] FIG. 19 is a sectional view of a structure of a conventional
surface emitting laser;
[0070] FIG. 20 is a sectional bird's eye view of a structure of a
surface emitting element according to a seventh embodiment of the
invention;
[0071] FIG. 21 is a graph showing a relation between a reflectivity
and slope efficiency of an emission-side
semiconductor-multilayer-mirror;
[0072] FIG. 22 is a graph showing a relation between a reflectivity
and a threshold current value of the emission-side
semiconductor-multilayer-mirror;
[0073] FIG. 23A is a table showing a relation between the number of
stacked layers of n-type DBR mirrors and a reflectivity;
[0074] FIG. 23B is a table showing a relation between the number of
stacked layers of p-type DBR mirrors and a reflectivity;
[0075] FIG. 24 is a sectional view of a structure of a surface
emitting laser according to an eighth embodiment;
[0076] FIG. 25 is a block diagram of a structure of an optical
transceiver according to a ninth embodiment of the invention;
[0077] FIG. 26 is a block diagram of a structure of an optical
communication system according to a tenth embodiment of the
invention;
[0078] FIG. 27 is a perspective sectional view of a surface
emitting laser according to an eleventh embodiment of the
invention;
[0079] FIG. 28 is an explanatory diagram for explaining a structure
of an n-type lower semiconductor multilayer mirror and a p-type
upper semiconductor multilayer mirror of the surface emitting laser
according to the eleventh embodiment;
[0080] FIG. 29 is a graph in which a relation between a threshold
current density J.sub.th in a vertical direction and a mirror loss
with an absorption loss in a semiconductor multilayer mirror as a
parameter using laser parameters extracted by an edge emitting type
laser;
[0081] FIG. 30 is a table showing laser parameters used for
calculating the graph in FIG. 29;
[0082] FIG. 31 is a publicly-known graph in which a relation
between a doping concentration and an absorption coefficient for
p-type GaAs is arranged with respect to incident rays of 1.3
micrometers and 1.55 micrometers;
[0083] FIG. 32 is a table showing an increase in resistance with
respect to an oxidation confinement type surface emitting laser in
a 850-nanometer-band at the time when an impurity concentration of
areas other than interface areas of low-refractive-index layers and
high-refractive-index layers is changed for five pairs from a GaAs
cladding layer in an upper part of a p-type upper semiconductor
multilayer mirror;
[0084] FIG. 33 is a block diagram of a schematic structure of an
optical transceiver according to a twelfth embodiment of the
invention;
[0085] FIG. 34 is a schematic diagram of a schematic structure of
an optical communication system according to a thirteenth
embodiment;
[0086] FIG. 35 is a perspective sectional view of a conventional
surface emitting laser; and
[0087] FIG. 36 is an explanatory diagram for explaining structures
of a lower semiconductor multilayer mirror and an upper
semiconductor multilayer mirror in the conventional surfaced
emitting laser element.
BEST MODE FOR CARRYING OUT THE INVENTION
[0088] Exemplary embodiments of a surface emitting laser, and a
transceiver, an optical transceiver, and an optical communication
system employing the surface emitting laser according to the
present invention will be explained in detail with reference to the
accompanying drawings. Note that the invention is not limited by
the embodiments. In the description of the drawings, identical or
similar portions are denoted by identical or similar reference
numerals and signs. It should be noted that the drawings are
schematic, and a relation among thicknesses and widths of
respective layers and a ratio of the respective layers are
different from actual ones. It is needless to mention that a
relation of dimensions and a ratio of the dimensions are different
in some portions among the drawings.
[0089] First, a surface emitting laser according to a first
embodiment will be explained. The surface emitting laser according
to the first embodiment is characterized in that a reflectivity and
temperature characteristics are improved compared with the
conventional technique by including thin AlAs layers, which are not
oxidized easily, in an upper semiconductor multilayer mirror and a
lower semiconductor multilayer mirror.
[0090] A relation between a thickness of the AlAs layer and an
oxidation rate will be explained. As shown in FIG. 8B on page 916
of the IEEE Journal of Selected topics in Quantum Electronics Vol.
3, 3, June 1997, it is known that the easiness of oxidation of AlAs
rapidly increases according to an increase in a film thickness up
to a certain film thickness. As a result of detailed experiments
concerning the relation between a film thickness of AlAs and an
oxidation rate, the inventors found that, although the oxidation
rate varies depending on conditions of epitaxial growth and
oxidation conditions, AlAs is stable and is rarely oxidized when
the film thickness is 10 nanometers or less. In other words, the
inventors came to obtain knowledge that a surface emitting laser,
which has a high reflectivity and satisfactory temperature
characteristics and can be manufactured by the same simple process
as in the past, can be provided by including an AlAs layer with a
film thickness of 10 nanometer or less in a semiconductor
multilayer mirror.
[0091] FIG. 1 is a perspective sectional view of the surface
emitting laser according to the first embodiment. FIG. 2 is an
explanatory diagram for explaining structures of a lower
semiconductor multilayer mirror and an upper semiconductor
multilayer mirror of the surface emitting laser according to the
first embodiment. Note that, portions common to FIG. 1 and FIG. 7
are denoted by identical reference numerals, and portions common to
FIG. 1 and FIG. 2 are denoted by identical reference numerals.
[0092] A surface emitting laser 10 shown in FIG. 1 is different
from a conventional surface emitting laser shown in FIG. 7 in
respective layer structures of a lower semiconductor multilayer
mirror 12 and an upper semiconductor multilayer mirror 16. Thus,
the large difference is explained in FIG. 2. To manufacture the
surface emitting laser 10 shown in FIG. 1, first, the lower
semiconductor multilayer mirror (lower DBR mirror) 12 is formed on
an n-type GaAs substrate 11 by the MOCVD method. Here, in the lower
semiconductor multilayer mirror 12, as shown in FIG. 2, assuming
that a stacked structure of an n-type high-refractive-index area 41
and an n-type low-refractive-index area 42 having respective
thicknesses of .lamda./4n (.lamda. is an oscillation wavelength and
n is a refractive index) forms one pair, thirty-five pairs are
stacked.
[0093] Note that, although the n-type high-refractive-index area 41
is formed of n-type Al.sub.0.2Ga.sub.0.8As as in the past, the
n-type low-refractive-index area 42 is formed of three layers,
namely, a first n-type AlAs layer 51, an n-type
Al.sub.0.9Ga.sub.0.1As layer 52, and a second n-type AlAs layer 53.
In particular, thicknesses of the first n-type AlAs layer 51 and
the second n-type AlAs layer 53 are about 5 nanometers,
respectively, and a thickness of the n-type Al.sub.0.9Ga.sub.0.1As
layer 52 is calculated by subtracting a sum of thicknesses of the
first n-type AlAs layer 51 and the second n-type AlAs layer 53 from
.lamda./4n.
[0094] Then, a QW active layer 32 vertically sandwiched by cladding
layers 31 and 33 is formed on the lower semiconductor multilayer
mirror 12. In addition, the Al.sub.zGa.sub.1-zAs
(0.95.ltoreq.z.ltoreq.1) layer 15 for forming a current confinement
layer in a later process is formed. Further, an upper semiconductor
multilayer mirror 16 (upper DBR mirror) is formed on the
Al.sub.zGa.sub.1-zAs (0.95.ltoreq.z.ltoreq.1) layer 15. Here, as
shown in FIG. 2, in the upper semiconductor multilayer mirror 16,
assuming that a stacked structure of a p-type high-refractive-index
area 45 and a p-type low-refractive-index area 46 having respective
thicknesses of .lamda./4n (.lamda. is an oscillation wavelength and
n is a refractive index) forms one pair, twenty-five pairs are
stacked.
[0095] Note that, although the p-type high-refractive-index area 45
is formed of p-type Al.sub.0.2Ga.sub.0.8As as in the past, the
p-type low-refractive-index area 46 is formed of three layers,
namely, a first p-type AlAs layer 56, an p-type
Al.sub.0.9Ga.sub.0.1As layer 57, and a second p-type AlAs layer 58.
In particular, thicknesses of the first p-type AlAs layer 56 and
the second p-type AlAs layer 58 are about 5 nanometers,
respectively, and a thickness of the p-type Al.sub.0.9Ga.sub.0.1As
layer 57 is calculated by subtracting a sum of thicknesses of the
first p-type AlAs layer 56 and the second p-type AlAs layer 58 from
.lamda./4n. In addition, a p-type GaAs contact layer 17 is formed
on the upper semiconductor multilayer mirror 16.
[0096] Next, a photolithography process and an etching process (dry
etching or wet etching) are performed. An outer edge of a stacked
structure, which consists of the upper semiconductor multilayer
mirror 16, the Al.sub.zGa.sub.1-zAs (0.95.ltoreq.z.ltoreq.1) layer
15, the cladding layer 33, the QW active layer 32, the cladding
layer 31, and a part of the lower semiconductor multilayer mirror
12, is removed. Consequently, for example, a columnar mesa-post
with a diameter of 30 micrometers is formed.
[0097] Next, oxidation treatment is performed at temperature of
about 400.degree. C. for twenty minutes in a moisture vapor
atmosphere to selectively oxidize the Al.sub.zGa.sub.1-zAs
(0.95.ltoreq.z.ltoreq.1) layer 15 from a sidewall of the mesa-post
to form the Al oxide layer 14. For example, when the Al oxide layer
14 is formed in a ring shape with a band width of 10 micrometers,
an area of the Al.sub.zGa.sub.1-zAs (0.95.ltoreq.z.ltoreq.1) layer
15 in the center, that is, an area of an aperture to which a
current is injected is about 80 .mu.m.sup.2 (with a diameter of 10
micrometers). Here, in particular, an oxidation amount of the first
n-type AlAs layer 51 and the second n-type AlAs layer 53 in the
lower semiconductor multilayer mirror 12 and the first p-type AlAs
layer 56, the p-type Al0.9Ga0.1As layer 57, and the second p-type
AlAs layer 58 was only 0.2 micrometers from peripheries
thereof.
[0098] Then, the silicon nitride film 19 functioning as a
protective layer is formed on an upper surface and a side surface
of the mesa-post and an exposed upper surface of the lower
semiconductor multilayer mirror 12. Subsequently, periphery of the
mesa-post, on which the silicon nitride film 19 is formed, is
filled with the polyimide 22. The silicon nitride film 19 formed on
the upper surface of the mesa-post is removed in a circular shape
with a diameter of 30 micrometers to further form the p-type
electrode 18 of a ring shape with an inner diameter of 20
micrometers and an outer diameter of 30 micrometers on the p-type
GaAs contact layer 17 exposed by the removal. After grinding the
substrate to have a thickness of, for example, 200 micrometers, the
n-type electrode 21 is formed on the back of the n-type GaAs
substrate 11. The electrode pad 20, on which a wire is bonded, is
formed on the polyimide 22 to come into contact with the p-type
electrode 18.
[0099] FIG. 3 is a diagram of current-optical output power
characteristics of the surface emitting laser according to the
first embodiment. As shown in FIG. 3, it is seen that the surface
emitting laser according to the first embodiment has satisfactory
thermal saturation characteristics of optical output power and
operates stably at high power even in a high temperature
environment. Note that, results of other characteristic evaluation
performed by the inventors indicate that an oscillation threshold
value is also satisfactory and there is no dislocation error.
[0100] In the above explanation, the two AlAs layers are arranged
to sandwich the Al.sub.0.9Ga.sub.0.1As layer in the n-type
low-refractive-index area 42 and the p-type low-refractive-index
area 46, respectively. However, only one AlAs layer may be arranged
in a low-refractive-index area if the AlAs layer has a thickness of
10 nanometers or less. FIGS. 4A and 4B are diagrams showing a
structure of one pair of semiconductor multilayer mirrors in that
case. In other words, the p-type low-refractive-index area 46 can
be formed of the a p-type AlAs layer 59 with a thickness of 10
nanometers and p-type Al.sub.0.9Ga.sub.0.1As layer 57 as shown in
FIG. 4A, and the n-type low-refractive-index area 42 can be formed
of a p-type AlAs layer 54 with a thickness of 10 nanometers and the
n-type Al.sub.0.9Ga.sub.0.1As layer 52 as shown in FIG. 4B. The
invention is not limited to this, and three or more AlAs layers may
be arranged in an identical low-refractive-index area as long as a
condition that an AlAs layer has a thickness of 10 nanometers or
less is satisfied. In addition, although it is preferable to
include AlAs layers in all pairs forming the lower semiconductor
multilayer mirror 12 and the upper semiconductor multilayer mirror
16, the AlAs layers may be included in a part of the pairs.
[0101] As described above, according to the surface emitting laser
according to the first embodiment, AlAs layers with a thickness of
10 nanometers or less, which is not easily oxidized, are included
in both the lower semiconductor multilayer mirror 12 and the upper
semiconductor multilayer mirror 16 sandwiching the active layer 13.
Thus, characteristics of a low refractive index and high thermal
conductivity inherent in the AlAs layer can be adopted in the
semiconductor multilayer mirrors. Consequently, a reflectivity and
temperature characteristics are improved to make laser oscillation
at high power possible.
[0102] Note that, it is also possible to design the surface
emitting laser such that one of the lower semiconductor multilayer
mirror 12 and the upper semiconductor multilayer mirror 16 is
formed of a pair including an AlAs layer, although the effect is
reduced.
[0103] Next, a surface emitting laser according to a second
embodiment of the invention will be explained. The surface emitting
laser according to the second embodiment is characterized in that
thin AlAs layers, which are not easily oxidized, are included in a
lower semiconductor multilayer mirror and an upper semiconductor
multilayer mirror and inclined composition layers are arranged as a
layers adjacent to the AlAs layers.
[0104] FIG. 5 is a perspective sectional view of the surface
emitting laser according to the second embodiment. FIG. 6 is an
explanatory diagram for explaining structures of the lower
semiconductor multilayer mirror and the upper semiconductor
multilayer mirror according to the second embodiment. Note that,
portions common to FIG. 1 and FIG. 5 are denoted by the identical
reference numerals, and portions common to FIG. 5 and FIG. 6 are
denoted by the identical reference numerals.
[0105] The surface emitting laser shown in FIG. 5 is different from
the surface emitting laser according to the first embodiment shown
in FIG. 1 in that the lower semiconductor multilayer mirror is
divided into two areas of a first lower semiconductor multilayer
mirror 61 and a second semiconductor multilayer mirror 62 and in a
layer structure of an upper semiconductor multilayer mirror 63.
Thus, the large difference is explained in FIG. 6. To manufacture a
surfaced emitting laser element 60 shown in FIG. 5, first, the
first lower semiconductor multilayer mirror (lower DBR mirror) 61
is formed on the n-type GaAs substrate 11 by the MOCVD method.
[0106] As shown in FIG. 6, in the first lower semiconductor
multilayer mirror 61, assuming that a semiconductor layer 70
consisting of lamination of an n-type low-refractive-index area and
an n-type high-refractive-index area forms one pair, twenty pairs
are stacked. A first inclined composition layer 71 is formed of
n-type Al.sub.iGa.sub.1-iAs (i=0.2.fwdarw.1.0), an Al composition
of which increases gently from 20 percent to 100 percent. A
thickness of the first inclined composition layer 71 is usually 10
to 30 nanometers and is 20 nanometers in the second embodiment. In
addition, an n-type low-refractive-index area 72 is formed of
n-type AlAs.
[0107] A second inclined composition layer 73 is formed of n-type
Al.sub.jGa.sub.1-jAs (j=1.0.fwdarw.0.2), an Al composition of which
decreases gently from 100 percent to 20 percent. Note that a
thickness of the second inclined composition layer 73 is the same
as that of the first inclined composition layer 71. In addition, an
n-type high-refractive-index area 74 is formed of n-type
Al.sub.0.2Ga.sub.0.8As. A thickness of the n-type
low-refractive-index area (n-type AlAs) 72 is calculated by
subtracting half a thickness of the first inclined composition
layer 71 and further subtracting half a thickness of the second
inclined composition layer 73 from .lamda./4n. Thus, for example,
when the thickness of the first inclined composition layer 71 is 20
nanometers and the thickness of the second inclined composition
layer 73 is 20 nanometers, the thickness of the n-type
low-refractive-index area (n-type AlAs) 72 is (.lamda./4n-10-10)
nanometers. A thickness of the high-refractive-index layer 74 is
calculated in the same manner and is (.lamda./4n-10-10) nanometers
in this case. Note that, since the n-type low-refractive-index area
(n-type AlAs) 72 in this embodiment is never exposed by etching in
a later process, the thickness of the n-type low-refractive-index
area (n-type AlAs) 72 may be 10 nanometers or more. Here, the
inclined composition layer such as the first inclined composition
layer 71 or the second inclined composition layer 73 is publicly
known as a structure with which an effect of reducing an electric
resistance is obtained.
[0108] On the other hand, as shown in FIG. 6, the second lower
semiconductor multilayer mirror 62 is a layer in which, assuming
that a semiconductor layer 80 consisting of lamination of an n-type
low-refractive-index area and an n-type high-refractive-index area
forms one pair, fifteen pairs are stacked. A first inclined
composition layer 81 is formed of n-type Al.sub.iGa.sub.1-iAs
(i=0.2.fwdarw.1.0), an Al composition of which increases gently
from 20 percent to 100 percent. In addition, the n-type
low-refractive-index area is formed of three layers of a first
n-type AlAs layer 82, an n-type Al.sub.0.9Ga.sub.0.1As layer 83,
and a second n-type AlAs layer 84. Note that it is necessary to set
thicknesses of the first n-type AlAs layer 82 and the second AlAs
layer 84 to 10 nanometers or less as explained in the first
embodiment. Here, the thicknesses are set to 5 nanometers,
respectively.
[0109] A second inclined composition layer 85 is formed of n-type
Al.sub.jGa.sub.1-jAs (j=1.0.fwdarw.0.2), an Al composition of which
decreases gently from 100 percent to 20 percent. Note that a
thickness of the second inclined composition layer 85 is the same
as that of the first inclined composition layer 81. An n-type
low-refractive-index area 86 is formed of an n-type
Al.sub.0.2Ga.sub.0.8As layer. A thickness of the n-type
Al.sub.0.9Ga.sub.0.1As layer 83 is calculated by subtracting half a
thickness of the first inclined composition layer 81, further
subtracting a thickness of the first AlAs layer 82, and further
subtracting a thickness of the second AlAs layer 84 from .lamda.4n.
In this case, the thickness of the n-type Al.sub.0.9Ga.sub.0.1As
layer 83 is (.lamda./4n-10-5-5-10) nanometers. In addition, a
thickness of the n-type low-refractive-index area (AlAs) 86 is
calculated by subtracting half a thickness of the first inclined
composition layer 81 and further subtracting half a thickness of
the second inclined composition layer 85 from .lamda./4n.
[0110] Next, the QW active layer 32 vertically sandwiched by the
cladding layers 31 and 33 is formed on the second lower
semiconductor multilayer mirror 62. In addition, the
Al.sub.zGa.sub.1-zAs (0.95.ltoreq.z.ltoreq.1) layer 15 for forming
a current confinement layer in a later process is formed. Further,
the upper semiconductor multilayer mirror 63 (upper DBR mirror) is
formed on the Al.sub.zGa.sub.1-zAs (0.95.ltoreq.z.ltoreq.1) layer
15.
[0111] As shown in FIG. 6, the upper semiconductor multilayer
mirror 63 is a layer in which, assuming that a semiconductor layer
90 consisting of lamination of a p-type low-refractive-index area
and a p-type high-refractive-index area forms one pair, fifteen
pairs are stacked. A first inclined composition layer 91 is formed
of p-type AliGa1-iAs (i=0.2.fwdarw.1.0), an Al composition of which
increases gently from 20 percent to 100 percent. The p-type
low-refractive-index area is formed of three layers of a first
p-type AlAs layer 92, a p-type Al0.9Ga0.1As layer 93, and second
p-type AlAs layer 94. Note that it is necessary to set thicknesses
of the first p-type AlAs layer 92 and the second p-type AlAs layer
94 to 10 nanometers or less as explained in the first embodiment.
Here, the thicknesses are set to 5 nanometers, respectively.
[0112] A second inclined composition layer 95 is formed of p-type
Al.sub.jGa.sub.1-jAs (j=1.0.fwdarw.0.2), an Al composition of which
decreases gently from 100 percent to 20 percent. Note that a
thickness of the second inclined composition layer 95 is the same
as that of the first inclined composition layer 91. A
low-refractive-index area 96 is formed of an p-type
Al.sub.0.2Ga.sub.0.8As layer. A thickness of the p-type
Al.sub.0.9Ga.sub.0.1As layer 93 is calculated by subtracting half a
thickness of the first inclined composition layer 91, further
subtracting a thickness of the first AlAs layer 92, further
subtracting a thickness of the second AlAs layer 94, and further
subtracting a thickness of the inclined composition layer 95 from
.lamda.4n. In this case, the thickness of the p-type
Al.sub.0.9Ga.sub.0.1As layer 93 is (.lamda./4n-10-5-5-10)
nanometers. In addition, a thickness of the p-type AlAs 96 is
calculated by subtracting half a thickness of the first inclined
composition layer 91 and further subtracting half a thickness of
the second inclined composition layer 95 from .lamda./4n. As shown
in FIG. 6, the upper semiconductor multilayer mirror 63 is a layer
in which, assuming that the semiconductor layer 90 consisting of
lamination of a p-type low-refractive-index area and a p-type
high-refractive-index area having respective thicknesses of
.lamda./4n (.lamda. is an oscillation wavelength and n is a
refractive index) forms one pair, twenty-five pairs are stacked. In
the semiconductor layer 90, the p-type low-refractive-index layer
is formed of four layers of the first inclined composition layer
91, the first p-type AlAs layer 92, the p-type
Al.sub.0.9Ga.sub.0.1As layer 93, and the second p-type AlAs layer
94. Here, in particular, the first inclined composition layer 91 is
formed of p-type Al.sub.iGa.sub.1-iAs (i=0.2.fwdarw.1.0), an Al
composition of which increases gently from 20 percent to 100
percent. Note that it is necessary to set thicknesses of the first
p-type AlAs layer 92 and the second p-type AlAs layer 94 to 10
nanometers or less as explained in the first embodiment. Here, the
thicknesses are set to 5 nanometers, respectively. Thus, for
example, when the thickness of the first inclined composition layer
91 is set to 20 nanometers, the thickness of the p-type
Al.sub.0.9Ga.sub.0.1As layer 93 is calculated by subtracting 30
nanometers from .lamda./4n.
[0113] Then, the p-type GaAs contact layer 17 is formed on the
upper semiconductor multilayer mirror 63. Next, a photolithography
process and an etching process (dry etching or wet etching) are
performed. With the processes, an outer edge of a stacked
structure, which consist of the upper semiconductor multilayer
mirror 63, the Al.sub.zGa.sub.1-zAs (0.95.ltoreq.z.ltoreq.1) layer
15, the cladding layer 33, the QW active layer 32, the cladding
layer 31, and a part of the lower semiconductor multilayer mirror
62, is removed. Consequently, for example, a columnar mesa-post
with a diameter of 30 micrometer is formed.
[0114] Next, oxidation treatment is performed at temperature of
about 400.degree. C. for twenty minutes in a moisture vapor
atmosphere to selectively oxidize the Al.sub.zGa.sub.1-zAs
(0.95.ltoreq.z.ltoreq.1) layer 15 from a sidewall of the mesa-post
to form the Al oxide layer 14. For example, when the Al oxide layer
14 is formed in a ring shape with a band width of 10 micrometers,
an area of the Al.sub.zGa.sub.1-zAs (0.95.ltoreq.z.ltoreq.1) layer
15 in the center, that is, an area of an aperture to which a
current is injected is about 80 .mu.m.sup.2 (with a diameter of 10
micrometers).
[0115] Then, the silicon nitride film 19 functioning as a
protective layer is formed on an upper surface and a side surface
of the mesa-post and an exposed upper surface of the lower
semiconductor multilayer mirror 112. Subsequently, periphery of the
mesa-post, on which the silicon nitride film 19 is formed, is
filled with the polyimide 22. The silicon nitride film 19 formed on
the upper surface of the mesa-post is removed in a circular shape
with a diameter of 30 micrometers to further form the p-type
electrode 18 of a ring shape with an inner diameter of 20
micrometers and an outer diameter of 30 micrometers on the p-type
GaAs contact layer 17 exposed by the removal. After grinding the
substrate to have a thickness of, for example, 200 micrometers, the
n-type electrode 21 is formed on the back of the n-type GaAs
substrate 11. The electrode pad 20, on which a wire is bonded, is
formed on the polyimide 22 to come into contact with the p-type
electrode 18.
[0116] As explained above, according to the surface emitting laser
according to the second embodiment, the AlAs layers with a
thickness of 10 nanometers or less, which are not easily oxidized,
are included in the second lower semiconductor multilayer mirror 62
and the upper semiconductor multilayer mirror 63, respectively.
Thus, the same effects as those in the first embodiment can be
realized. In addition, the introduction of an inclined composition
layer further reduces an electric resistance of a semiconductor
multilayer mirror to make it possible to further increase
power.
[0117] Note that, although the respective layers are manufactured
by the MOCVD method in the first and the second embodiment, the
layers may be manufactured by a molecular beam epitaxy (MBE) method
or the like. In addition, although the respective layers are formed
on the n-type GaAs substrate 11 to obtain the surface emitting
laser in the first and the second embodiment, a p-type GaAs
substrate may be used instead of the n-type GaAs substrate 11. In
this case, a p-type lower semiconductor multilayer mirror and an
n-type upper semiconductor multilayer mirror are used, and an
electrode material corresponding to the p-type lower semiconductor
multilayer mirror and the n-type upper semiconductor multilayer
mirror is used. Moreover, the surface emitting laser does not limit
an oscillation wavelength. The surface emitting laser can be
applied to a structure for oscillating a wavelength in a bandwidth
between 700 nanometers to 1600 nanometers, more specifically, a
wavelength of 780 nanometers, 850 nanometers, 980 nanometers, 1300
nanometers, or 1550 nanometers.
[0118] First, a surface emitting laser according to a third
embodiment will be explained. In the surface emitting laser
according to the third embodiment, a structure of a selectively
oxidized area is optimized for a surface emitting laser in a
1300-nanometer-band. FIG. 10 is a sectional view of a structure of
a surface emitting laser according to the third embodiment. The
structure of the surface emitting laser according to the third
embodiment will be explained with reference to FIG. 10 according to
circumstances.
[0119] The surface emitting laser according to the third embodiment
has a structure in which a lower reflective layer 1002 is stacked
on a substrate 1001. An upper area of the lower reflective layer
1002 is formed in a mesa shape, and a lower cladding layer 1003, an
active layer 1004, an upper cladding layer 1005, and an upper
reflective layer 1006 are sequentially stacked on the area formed
in the mesa shape. Note that the mesa shape is formed such that a
horizontal sectional shape thereof is circular. Moreover, a contact
layer 1007 is stacked on the upper reflective layer 1006, and a p
side electrode 1008 formed in an annular shape including a current
injection area is arranged in the center on the contact layer 1007,
and an n side electrode 1009 is arranged on a lower surface of the
substrate 1001. In the upper reflective layer 1006, a current
confinement layer 1020, which consists of a current injection area
1019a arranged near the mesa center and having a circular shape in
a horizontal section and a selectively oxidized area 1019b provided
adjacent to the current injection area 1019a, is arranged. A
specific structure of the selectively oxidized area 1019b will be
explained in detail later.
[0120] The substrate 1001 consists of an n-type GaAs substrate.
Usually, the substrate 1001 has a (100) surface as a principal
plane, and a thin film structure of the lower reflective layer 1002
and layers above the upper reflective layers 1002 are stacked on
the (100) surface. The lower reflective layer 1002 is a layer for
reflecting and feeding back light of an emission wavelength in
light generated in the active layer 1004. More specifically, the
lower reflective layer 1002 has a structure in which a large number
of mirror layers 1012 formed by a stacked structure of a
high-refractive-index layer 1010 and a low-refractive-index layer
1011 are stacked.
[0121] The high-refractive-index layer 1010 is formed of an n-type
GaAs layer, and the low-refractive-index layer 1011 is formed of an
n-type Al.sub.0.9Ga.sub.0.1As layer. Film thicknesses of the
high-refractive-index layer 1010 and the low-refractive-index layer
1011 are adjusted such that an optical length thereof is 1/4 of an
emission wavelength .lamda. to reflect only light of the emission
wavelength. More specifically, since an emission wavelength of the
surface emitting laser according to the embodiment is 1300
nanometers, the film thickness of the high-refractive-index layer
1010 is set to about 94 nanometers and the film thickness of the
low-refractive-index layer 1011 is set to about 110 nanometers.
With such a structure, the mirror layer 1012 has a function of
reflecting light of the emission wavelength at a fixed ratio in a
combination of the high-refractive-index layer 1010 and the
low-refractive-index layer 1011. To increase a reflectivity in the
lower reflective layer 1002 as a whole, the lower reflective layer
1002 is formed by stacking 34.5 mirror layers 1012. Note that a
fraction below the decimal point is due to a layer consisting only
of the high-refractive-index layer 1010.
[0122] The active layer 1004 has a structure including a QW layer.
More specifically, as shown in FIG. 10, the active layer 1004 is
formed of a barrier layer 1013a, a QW layer 1014a, a barrier layer
1013b, a QW layer 1014b, a barrier layer 1013c, a QW layer 1014c,
and a barrier layer 1013d that are sequentially stacked on the
lower cladding layer 1003. In other words, the active layer 1004
has a structure in which the three QW layers 1014a to 1014c are
sandwiched among the four barrier layers 1013a to 1013d.
[0123] The QW layers 1014a to 1014c are layers for confining a
carrier at high efficiency with a quantum confinement effect and
are formed of GaInNAsSb layers. The QW layers 1014a to 1014c have
high quality crystallinity when a very small amount of Sb is added.
In addition, the QW layers 1014a to 1014c are required to be formed
thin to show the quantum confinement effect. In the third
embodiment, film thicknesses of the respective QW layers are set to
about 7 nanometers.
[0124] The barrier layers 1013a to 1013d are layers for separating
the QW layers 1014a to 1014c from each other. Film thicknesses of
the barrier layer 1013a and the barrier layer 1013d are about 30
nanometers, and film thicknesses of the barrier layer 1013 and the
barrier layer 1013c are about 20 nanometers.
[0125] The lower cladding layer 1003, the active layer 1004, and
the upper cladding layer 1005 are formed such that a sum of optical
length of the film thicknesses of the respective layers is twice as
long as the emission wavelength .lamda. and function as an optical
cavity. Therefore, in the following explanation, the lower cladding
layer 1003, the active layer 1004, and the upper cladding layer
1005 are generally referred to as a 2.lamda. cavity 1015. In the
third embodiment, the lower cladding layer 1003 consists of an
n-type GaAs layer and the upper cladding layer 1005 consists of a
p-type GaAs layer. Film thicknesses of the respective layers are
about 297 nanometers.
[0126] Like the lower reflective layer 1002, the upper reflective
layer 1006 is a layer for reflecting and feeding back light of an
emission wavelength in light generated in the active layer 1004.
More specifically, the upper reflective layer 1006 has a structure
in which a large number of mirror layers 1018, which are formed of
a pair of a low-refractive-index layer 1016 and a
high-refractive-index layer 1017 stacked sequentially, are stacked.
The low-refractive-index layer 1016 has a p-type
Al.sub.0.9Ga.sub.0.1As layer, and the high-refractive-index layer
1017 has a p-type GaAs layer. The upper reflective layer 1006 has a
high reflectivity to feeding back a laser beam generated in the
active layer 1004. Since the surface emitting laser according to
the third embodiment emits a laser beam in a vertically upward
direction with respect to the substrate 1001, the upper reflective
layer 1006 is required to have a reflectivity lower than that of
the lower reflective layer 1002. Therefore, the upper reflective
layer 1006 has a structure consisting of twenty-five mirror layers
1018 that are smaller in number than the mirror layers in the lower
reflective layer 1002. In the following explanation, concerning the
twenty-five mirror layers 1012 or mirror layers 1018 forming the
lower reflective layer 1002 or the upper reflective layer 1006, a
mirror layer arranged on a side closest to the active layer 1004 is
referred to as a mirror layer in a first round, a mirror layer
arranged in contact with the mirror layer in the first round is
referred to as a mirror layer in a second round, a mirror layer
arranged in contact with the mirror layer in the second round is
referred to as a mirror layer in a third round, and so on.
[0127] Among the low-refractive-index layers 1016 forming the upper
reflective layer 1006, the low-refractive-index layer 1016 forming
the mirror layer in the first round adjacent to the lowermost
layer, that is, the upper cladding layer 1005 has a structure in
which a p-type Al.sub.0.9Ga.sub.0.1As layer and a p-type AlAs layer
are stacked sequentially. Further, the current confinement layer
1020 is formed of the current injection area 1019a formed in an
area near the center of the AlAs layer and the selectively oxidized
area 1019b formed by selective oxidation in a part near the end of
the AlAs layer.
[0128] The selectively oxidized area 1019b is an area for
constricting a current injected from the p-side electrode 1008 to
increase a density of a current flowing into the active layer 1004
and reduce an oscillation threshold value. In addition, since a
refractive index of the selectively oxidized area 1019b has a value
different from those of the current injection area 1019a and the
Al.sub.0.9Ga.sub.0.1As layer present around the selectively
oxidized area 1019b, the refractive index affects light confinement
in the horizontal direction. In other words, since the selectively
oxidized area 1019b is present, the surface emitting laser
according to the third embodiment includes a refractive
index-guiding type waveguide.
[0129] In the third embodiment, a film thickness of the selectively
oxidized area 1019b is set to 13 nanometers. In addition, a
diameter of the current injection area 1019a defined by a width of
the selectively oxidized area 1019b is set to 5.3 micrometers. A
reason for adopting such a structure will be hereinafter
explained.
[0130] The surface emitting laser according to the third embodiment
improves oscillation characteristics by optimizing a structure of
the selectively oxidized area 1019b. More specifically, it is
necessary to examine a position where the selectively oxidized area
1019b is arranged in a mirror layer, a mirror layer in which the
selectively oxidized area 1019b is arranged, and a width and a film
thickness of the selectively oxidized area 1019b. Optimization of
these requirements will be hereinafter explained.
[0131] First, it will be explained in which position in a mirror
layer the selectively oxidized area 1019b should be arranged. The
current injection area 1019a adjacent to the selectively oxidized
area 1019b has a refractive index close to that of an
Al.sub.0.9Ga.sub.0.1As layer. Therefore, it is necessary to arrange
the AlAs layers, which form the selectively oxidized area 1019b and
the current injection area 1019a, in the low-refractive-index layer
1011 or the low-refractive-index layer 1016. Moreover, to control a
loss of light generated in the active layer 1004, it is preferable
to arrange the selectively oxidized area 1019b in a position close
to a part of a node of a standing wave (position where a field
intensity is minimized), that is, an area spaced apart from the
active layer 1004 in the low-refractive-index layer 1011 or the
low-refractive-index layer 1016, that is, a position near a lower
surface of the low-refractive-index layer 1011 or an upper surface
of the low-refractive-index layer 1016.
[0132] Next, it will be explained in which mirror layer the
selectively oxidized area 1019b should be arranged. As already
described, the selectively oxidized area 1019b includes a current
confinement function. From the viewpoint of constricting a current
to increase a density of a current flowing into the active layer
1004, it is preferable that the selectively oxidized area 1019b is
arranged in a position near the active layer 1004. This is because,
when the selectively oxidized area 1019b is arranged in a position
spaced apart from the active layer 1004, the current constricted by
the selectively oxidized area 1019b diffuses again by the time when
the current is flown into the active layer 1004, and the current
density falls. Therefore, from the viewpoint of controlling an
oscillation threshold value to be low, it is preferable to arrange
the selectively oxidized area 1019b near the active layer 1004.
[0133] On the other hand, when the selectively oxidized area 1019b
is arranged near the active layer 1004, other problems occur. The
selectively oxidized area 1019b is formed by once stacking an AlAs
layer and other layers to process the layers in a mesa shape and
then heating the layers in a moisture vapor atmosphere to introduce
oxygen atoms from the outside and oxidize the layers selectively.
Since an initial crystalline order is damaged by introducing the
oxygen atoms from the outside, dislocation occurs in the
selectively oxidized area 1019b. Therefore, when the selectively
oxidized area 1019b is arranged too close to the active layer 1004,
the dislocation in the selectively oxidized area 1019b affects a
crystal structure of the active layer 1004, and reliability as a
surface emitting laser is deteriorated. Thus, from the viewpoint of
securing reliability of the surface emitting laser, it is
preferable to arrange the selectively oxidized area 1019b spaced
apart from the active layer 1004.
[0134] As described above, the control of an oscillation threshold
value and the securing of reliability of the surface emitting laser
is in a relation of tradeoff, there is an optimum value for a
position in a stacking direction of the selectively oxidized area
1019b. Therefore, concerning the surface emitting laser, the
inventors changes the position of the selectively oxidized area
1019b in the stacking direction to check characteristics of the
surface emitting laser and derived the optimum value.
[0135] More specifically, concerning a surface emitting laser with
an emission wavelength of about 850 nanometers (hereinafter
referred to as "850 nm band surface emitting laser"), the inventors
changed a position for arranging a selectively oxidized area in a
stacking direction thereof and measured values of threshold
currents and reliability of the surface emitting laser. The
inventors used the 850 nm band surface emitting laser as an object
of measurement because the 850 nm band surface emitting laser has
been widely studied and characteristics thereof has been grasped
well. Therefore, it is possible to eliminate an influence on a
measurement result due to portions other than the selectively
oxidized area and grasp an influence of the selectively oxidized
area on characteristics of the surface emitting laser. Note that it
is assumed that a structure of the 850-nm-band surface emitting
laser used in the measurement is identical with the structure of
the surface emitting laser according to the third embodiment other
than portions corresponding to a wavelength like an optical length
of an optical cavity.
[0136] The inventors measured a threshold current and reliability
concerning the 850 nm band surface emitting laser having a .lamda.
cavity in which current confinement layers 1020a, 1020b, and 1020c
were formed of selectively oxidized areas 1019b-1, 1019b-2, and
1019b-3, current injection areas 1019a-1, 1019a-2, and 1019a-3 in a
mirror layer in a first round, a mirror layer in a third round, and
a mirror layer in a fifth round, respectively, as shown in FIGS.
11A to 11C. As a result, it was found that there was a problem in
reliability when the selectively oxidized area 1019b-1 was arranged
in the mirror layer in the first round, and it was inappropriate to
put the surface emitting laser to practical use. However, when the
selectively oxidized area 1019b-2 was arranged in the mirror layer
in the third round, the reliability was high and the threshold
current had a low value. When the selectively oxidized area 1019b-3
was arranged in the mirror layer in the fifth round, the
reliability could be secured but the threshold current showed a
tendency of increase compared with the time when the selectively
oxidized area 1019b-2 was arranged in the mirror layer in the third
round. Since an increase in the threshold current was within an
allowable range when the selectively oxidized area 1019b-3 was
arranged in the mirror layer in the fifth round, concerning the
850-nm-band surface emitting laser, the inventors concluded that it
was preferable to arrange a selectively oxidized area in the mirror
layer in the third round or the fifth round.
[0137] In the 850-nm-band surface emitting laser used in the
measurement, a film thickness of an upper cladding layer and film
thicknesses of a low-refractive-index layer and a
high-refractive-index layer are different from those in the surface
emitting laser according to the third embodiment due to a
difference of an emission wavelength. Therefore, in optimizing the
position in the stacking direction of the selectively oxidized area
1019b in the third embodiment, a distance from the center of the
active layer was taken into account concerning the measurement
result. In the 850-nm-band surface emitting laser used for the
measurement, a distance from the center of the active layer to a
lower end of the mirror layer in the third round is 390 nanometers,
and a distance from the center of the active layer to a lower end
of the mirror layer in the fifth round is 660 nanometers. Taking
these values into account, in the surface emitting laser according
to the third embodiment, a mirror layer, in which the selectively
oxidized area 1019b is arranged, is decided as a mirror layer
having a lower end present in a range of 370 nanometers to 680
nanometers from the center of the active layer 1004.
[0138] In the third embodiment, the mirror layer in the first round
is selected as a mirror layer suitable for this numerical value
range. This is because the distance from the center of the active
layer 1004 is 375 nm in the mirror layer in the first round. In
addition, concerning a position in the mirror layer, when the
selectively oxidized area 1019b is provided in the upper reflective
layer as already explained, it is preferable that the selectively
oxidized area 1019b is arranged near an upper surface of a
low-refractive-index layer. Therefore, from these viewpoints, the
position in the stacking direction of the selectively oxidized area
1019b is decided as a place shown in FIG. 10.
[0139] Next, from the viewpoint of realizing the single lateral
mode oscillation, optimization of the width in the horizontal
direction and the film thickness in the stacking direction of the
selectively oxidized area 1019b will be explained one after
another. Note that, since the width in the horizontal direction of
the selectively oxidized area 1019b is determined by optimizing a
diameter of the current injection area 1019a, optimization of the
diameter of the current injection area 1019a will be hereinafter
mainly explained.
[0140] First, optimization of the diameter of the current injection
area 1019a depending upon the width of the selectively oxidized
area 1019b will be explained. In general, it is possible to realize
the single lateral mode oscillation by reducing the diameter of the
current injection area 1019a. On the other hand, a problem occurs
in that a threshold current increases due to a diffraction loss as
the diameter of the current injection area 1019a decreases. Thus,
it is necessary to increase the diameter of the current injection
area 1019a to control the increase in a threshold current.
Therefore, a relation of tradeoff is also established between the
control of a threshold current and conditions under which the
single lateral mode oscillation is possible, and there is an
optimum value in the diameter of the current injection area
1019a.
[0141] Thus, concerning the 850 nm surface emitting laser, the
inventors measured an oscillation lateral mode at the time when a
diameter of a current injection area was changed and checked an
optimum value of the diameter of the current injection area. More
specifically, as shown in FIG. 11B, the inventors inserted a
selectively oxidized area in the mirror layer in the third round,
fixed a film thickness of the selectively oxidized area at 20
nanometers, and varied the diameter of the current injection area
1019a to measure a threshold current and a form of a lateral
mode.
[0142] As a result of the measurement, it was found that a maximum
value of the diameter of the current injection area, with which the
single lateral mode oscillation was possible, was 3.5 micrometers.
In addition, when the diameter was 3.5 micrometers, a value of the
threshold current could be controlled to be within an allowable
range. Note that an effective refractive index difference to be
described later was 0.0165 in this surface emitting laser.
[0143] Based on this result, the inventors set a condition that the
diameter of the current injection area 1019a of the surface
emitting laser according to the third embodiment was 3.5
micrometers or more. This is because, the control of the increase
in the threshold current has little correlation with an emission
wavelength of the surface emitting laser, and when the threshold
current is controlled to be within the allowable range in the
850-nm-band surface emitting laser, it is considered that the
threshold current can also be controlled to be within the allowable
range in the case of 1300 nanometers. However, the inventors
thought that it was preferable to increase the diameter of the
current injection area 1019a according to a ratio of the emission
wavelength and considered an optimum value of the diameter of the
current injection area 1019a was 5.3 micrometers calculated by
multiplying 3.5 micrometers by (1300/850).
[0144] Next, under such conditions concerning the diameter of the
current injection area 1019a, an optimum value of the film
thickness of the selectively oxidized area 1019b necessary for
realizing the single lateral mode oscillation will be examined.
This is because not only the diameter of the current injection area
1019a but also the effective refractive index difference in the
refractive index-guiding type waveguide affects the lateral mode
oscillation. In the following explanation, first, the effective
refractive index difference will be explained briefly and an
optimum value of the effective refractive index difference will be
derived, and then conditions for the film thickness of the
selectively oxidized area 1019b necessary for realizing the optimum
value will be derived.
[0145] As described above, since the selectively oxidized area
1019b is present, the surface emitting laser according to the third
embodiment includes the refractive index-guiding type waveguide.
For example, as shown in FIG. 12, the refractive index-guiding type
waveguide means a structure in which, due to an effective
refractive index difference that is a difference of equivalent
refractive indexes in a first area 1021, which is a stacking
direction area including the current injection area 1019a, and
second areas 1022 and 1023 including the selectively oxidized area
1019b, the first area 1021 functions as a waveguide. A structure of
the refractive index-guiding type waveguide is explained by a
refractive index distribution of the surface emitting laser. More
specifically, the structure of the refractive index guiding wave
guide and the lateral mode can be analyzed by equivalently
replacing light confinement in the horizontal direction with a
planar waveguide having the equivalent refractive indexes of the
first area 1021 and the second areas 1022 and 1023 and evaluating
the light confinement.
[0146] For example, in the surface emitting laser according to the
third embodiment, a refractive index of the selectively oxidized
area 1019b has a small value compared with a refractive index of
AlAs forming the current injection area 1019a. Therefore, the
equivalent refractive index of the second areas 1022 and 1023 has a
value smaller than the equivalent refractive index of the first
area 1021, and light generated from the active layer 1004 is guided
through the first area 1021 and emitted to the outside. Since the
structure of the refractive index-guiding type waveguide
corresponds to the effective refractive index difference, which is
a differential value between the equivalent refractive index of the
first area 1021 and the equivalent refractive index of the second
areas 1022 and 1023, a form of light guiding changes according to
the effective refractive index difference.
[0147] It is known that a relation between a diameter .PHI..sub.c
of the current injection area 1019a, with which the single lateral
mode oscillation can be performed, and an effective refractive
index difference .DELTA.n is approximated as follows.
.PHI..sub.c.varies./(.DELTA.n).sup.1/2 (1)
Therefore, the width in the horizontal direction and the film
thickness of the selectively oxidized area 1019b in the surface
emitting laser according to the third embodiment can be determined
using the measurement result obtained about the 850 nm band surface
emitting laser and expression (1).
[0148] First, a conditional expression for the effective refractive
index difference .DELTA.n at the time when the diameter of the
current injection area 1019a is set to 3.5 micrometers at the
minimum to control the increase in the threshold current is
calculated as follows from the measurement result and expression
(1).
.PHI..sub.c(.lamda.=1.3 .mu.m)/.PHI..sub.c(.lamda.=0.85
.mu.m)={1.3/(.DELTA.n).sup.1/2}/{0.85/(0.0165).sup.1/2} (2)
Here, since .PHI..sub.c(.lamda.=0.85 .mu.m)=3.5 .mu.m, if a value
of the left part of expression (2) is 1 or more, the diameter of
the current injection area 1019a is also 3.5 .mu.m or more in the
surface emitting laser according to the third embodiment. When
expression (2) is calculated for .DELTA.n, to satisfy the above
condition, the effective refractive index difference .DELTA.n only
has to be equal to or less than 0.038. Since a value of the
effective refractive index difference .DELTA.n can be controlled by
the film thickness of the selectively oxidized area 1019b, a
maximum value of the film thickness of the selectively oxidized
area 1019b can be calculated from this condition. In addition, it
is preferable to set the effective refractive index difference
.DELTA.n to 0.0165 as in the measurement result for the 850 nm band
surface emitting laser because the diameter of the current
injection area 1019a increases to be 1.5 times as large according
to a wave length ratio.
[0149] Here, it is possible to derive the film thickness of the
selectively oxidized area 1019b, which is necessary for realizing
the effective refractive index difference, by carrying out a
calculation known to those skilled in the art taking into account
the position in the stacking direction of the selectively oxidized
area 1019b. As a conclusion, since the selectively oxidized area
1019b is arranged in the first mirror layer in the third
embodiment, to reduce the effective refractive index difference
.DELTA.n to 0.038 or less, the film thickness of the selectively
oxidized area 1019b only has to be set to 32 nanometers or less. In
addition, the film thickness of the selectively oxidized area 1019b
at the time when the effective refractive index difference is set
to 0.0165 is 13 nanometers. Therefore, to realize the single
lateral mode oscillation with the surface emitting laser according
to the third embodiment, it is necessary to reduce the film
thickness of the selectively oxidized area 1019b to 32 nm or less
and, more preferably to 13 nanometers or less.
[0150] Next, a minimum value of the film thickness of the selective
oxidized area 1019b will be examined. As described already, the
selectively oxidized area 1019b is formed by stacking an AlAs layer
once and then introducing oxygen atoms to selectively oxidize the
AlAs layer in a moisture vapor atmosphere. Here, in forming the
selectively oxidized area 1019b, if a film thickness of the AlAs
layer is too thin, it becomes difficult to introduce oxygen atoms
and to form the selectively oxidized area 1019b. It is considered
that the film thickness of the selectively oxidized area 1019b at
least required for introducing oxygen atoms is 6 nanometers. If the
selectively oxidized area 1019b preferably has a film thickness of
10 nanometers or more, it is possible to perform the selective
oxidation easily.
[0151] From the above discussion, a range of a film thickness d of
the selectively oxidized area 1019b, which is required for
controlling the threshold current to be low and realizing the
single lateral oscillation mode is as follows.
6 nm.ltoreq.d.ltoreq.32 nm (3)
In addition, a preferable range of the film thickness d, with which
the selective oxidation can be performed promptly and the diameter
of the current injection area 1019a can be set large, is as
follows
10 nm.ltoreq.d.ltoreq.13 nm (4)
Since it is preferable that the selectively oxidized area 1019b has
a thickness satisfying expression (4), in the third embodiment, the
film thickness of the selectively oxidized area 1019b is set to 13
.mu.m, and the diameter of the current injection area 1019a is set
to 5.3 .mu.m.
[0152] The inventors actually manufactured a surface emitting laser
satisfying the above-mentioned conditions to check characteristics
of the surface emitting laser. More specifically, an n-type GaAs
buffer layer was stacked 0.1 .mu.m on an n-type GaAs (100) surface
substrate and 34.5 mirror layers consisting of n-type
Al.sub.0.9Ga.sub.0.1As and GaAs were stacked to form a lower
reflective layer. In addition, an active layer included a triple QW
layer, and an optical length of an optical cavity including the
active layer was set to 2.lamda.. Further, twenty-five mirror
layers consisting of p-type Al.sub.0.9Ga.sub.0.1As and GaAs were
stacked to form an upper reflective layer. These semiconductor
layers were grown by any one of a gas source MBE method, the MBE
method, and the MOCVD method and doped with carbon (C) as a p-type
impurity and silicon (Si) as an n-type impurity. An AlAs layer was
arranged in an upper end of the Al.sub.0.9Ga.sub.0.1As layer
forming a lowermost mirror layer (a mirror layer in the first
round) of the upper reflective layer, and a film thickness thereof
was set to 12 nanometers. An outer diameter of a horizontal section
of a part formed in a mesa shape was set to 40 .mu.m. After forming
the mesa, the AlAs layer was held in a moisture vapor atmosphere of
420.degree. C. for twenty minutes to form a selectively oxidized
area. A diameter of a current injection area formed by the AlAs
layer, which was not oxidized, was set to 5.2 .mu.m.
[0153] When oscillation characteristics was checked about the
surface emitting laser manufactured in this way, a value of a
threshold current was 0.5 microampere and slope efficiency was 0.25
W/A, and continuous oscillation was possible at 100.degree. C. or
more. In addition, if an injection current value was 10
microamperes or less, single lateral mode oscillation was possible.
When an optical signal directly modulated at 10 Gbit/s was made
incident in an optical fiber for transmission, a transmittable
distance was 15 kilometers or more, and a satisfactory eye pattern
of the optical signal was obtained after the optical signal was
transmitted 15 kilometers.
[0154] Next, a structure of a surface emitting laser according to a
modification of the third embodiment will be explained. FIG. 13 is
a sectional view showing the structure of the surface emitting
laser according to the modification. Although the mirror layer, in
which the selectively oxidized area 1019b is arranged is the mirror
layer in the first round in the third embodiment, it is also
possible to arrange the selectively oxidized area 1019b in the
mirror layer in the second round. This is because, since a distance
from the center to the lower end of the active layer 1004 is 580
nanometers in the mirror layer in the second round, the distance is
included in the numerical value range of 370 nanometers to 680
nanometers defined for a position in the stacking direction, and
the securing of reliability and the control of the threshold
current are possible.
[0155] Since the mirror layer, in which the selectively oxidized
area 1019b is arranged, is the mirror layer in the second round, to
maintain a diameter of the current injection area 1019a and
maintain an effective refractive index difference, a film thickness
of the selectively oxidized area 1019b changes. This is because
equivalent refractive indexes also change according to the position
in the stacking direction of the selectively oxidized area 1019b.
As a result of calculation, the diameter of the current injection
area 1019a is 3.5 .mu.m or more, a range of the film thickness d,
with which selective oxidation is possible, is calculated as
follows,
6 nm.ltoreq.d.ltoreq.46 nm (5)
the diameter of the current injection area 1019a is 5.3 .mu.m or
more, and a range of the film thickness d, with which selective
oxidation is possible sufficiently, is calculated as follows
10 nm.ltoreq.d.ltoreq.20 nm (6)
[0156] Next, a surface emitting laser according to a fourth
embodiment of the invention will be explained. As shown in FIG. 14,
the surface emitting laser according to the fourth embodiment has
an emission wavelength of 1300 nanometers and has a .lamda. cavity
structure in which an optical length of an optical cavity formed of
an active layer and an upper cladding layer is equal to a
wavelength of emitted light. More specifically, the surface
emitting laser according to the fourth embodiment has a structure
in which the lower reflective layer 1002 is stacked on the
substrate 1001. An upper area of the lower reflective layer 1002 is
formed in a mesa shape, and a lower cladding layer 1026, the active
layer 1004, an upper cladding layer 1027, and the upper reflective
layer 1006 are sequentially stacked on the area formed in the mesa
shape. Note that the mesa shape is formed such that a horizontal
sectional shape thereof is circular. Moreover, the contact layer
1007 is stacked on the upper reflective layer 1006, and the p side
electrode 1008 formed in an annular shape including a current
injection area is arranged in the center on the contact layer 1007,
and the n side electrode 1009 is arranged on a lower surface of the
substrate 1001. In the upper cladding layer 1027, a current
confinement layer 1030, which consists of a current injection area
1029a arranged near the mesa center and having a circular shape in
a horizontal section and a selectively oxidized area 1029b provided
adjacent to the current injection area 1029a, is arranged. Note
that, in the fourth embodiment, portions denoted by reference
numerals identical with or similar to those in the third embodiment
have the identical or similar structures and show the identical or
similar functions unless specifically noted otherwise.
[0157] In the fourth embodiment, an optical length of a .lamda.
cavity 1028 formed of the lower cladding layer 1026, the active
layer 1004, and the upper cladding layer 1027 is equal to a
wavelength of emitted light. Therefore, a position in a stacking
direction and an optimum value of a film thickness of the
selectively oxidized area 1029b are different from those in the
third embodiment. Optimization of the selectively oxidized area
1029b of the surface emitting laser having the emission wavelength
of 1300 nanometers and having the .lamda. cavity 1028 will be
hereinafter explained.
[0158] First, optimization of the position in the stacking
direction of the selectively oxidized area 1029b will be explained.
In the third embodiment, the inventors found that it is preferable
to arrange the selectively oxidized area 1029b in a mirror layer,
an end facet on the active layer side of which is present in a
range of 370 nanometers to 680 nanometers from the center of the
active layer 1004. In the fourth embodiment, it is necessary to
determine an appropriate mirror layer taking into account a
difference of an optical length of the .lamda. cavity 1028, and a
mirror layer in a second round is selected as the appropriate
mirror layer. This is because a distance from the center of the
active layer 1004 to the mirror layer in the second round is 392
nanometers, which meets the above-mentioned condition. In addition,
as in the third embodiment, in the mirror layer of the second
round, it is preferable to arrange the selectively oxidized area
1029b is arranged on a far side from an active layer of a
low-refractive-index layer. Consequently, the position in the
stacking direction of the selectively oxidized area 1029b is
determined to be a position shown in FIG. 14.
[0159] Next, optimization of the diameter of the current injection
area 1029a defined by a width in a horizontal direction of the
selectively oxidized area 1029b will be explained. As explained in
the third embodiment, the diameter of the current injection area
1029a is decided from the viewpoint of controlling an increase in a
threshold current and realizing single lateral mode oscillation and
is determined irrespective of an optical length of an optical
cavity. Therefore, the same discussion as the third embodiment is
established. From the measurement result for the 850 nm band
surface emitting laser, the diameter of the current injection area
1029a is required to be 2.5 .mu.m or more, and when a wavelength
ratio is taken into account, it is preferable that the diameter is
5.3 .mu.m.
[0160] Lastly, optimization of the film thickness of the selective
oxidized area 1029b will be explained. From the viewpoint of
realizing the single lateral mode oscillation, the relation of
expression (1) is established between the effective refractive
index difference of the first area 1021 and the second areas 1022
and 1023 shown in FIG. 12 and the diameter of the current injection
area 1029a. As conditions under which the single lateral mode
oscillation is possible with the diameter of the current injection
area 1029a of 3.5 .mu.m or more, the effective refractive index
difference is required to be 0.038 or less. To realize the single
lateral mode oscillation when the diameter of the current injection
area 1029a is 5.3 .mu.m, the effective refractive index difference
is 0.0165.
[0161] In determining the film thickness of the selectively
oxidized area 1029b necessary for realizing such an effective
refractive index difference, a graph shown in FIG. 15 is used in
the fourth embodiment. FIG. 15 is a graph showing, concerning a
surface emitting laser having a .lamda. cavity, a relation between
a film thickness of a selectively oxidized area and an effective
refractive index difference for each mirror layer in which the
selectively oxidized area is arranged (K. D. Choquette et al.,
Proceedings of SPIE Vertical-Cavity Surface-Emitting Lasers, vol.
3003, PP. 194-200, 1997). In FIG. 15, a curve I.sub.1 indicates a
case in which the selectively oxidized layer is arranged in a first
round, and curves I.sub.2, I.sub.3, I.sub.4, and I.sub.5 indicate
cases in which the selectively oxidized layer is arranged in mirror
layers in a second round, a third round, a fourth round, and a
fifth round, respectively. A horizontal axis of the graph indicates
a film thickness of the selectively oxidized area, and a vertical
axis of the graph indicates an effective refractive index
difference.
[0162] In the surface emitting laser according to the fourth
embodiment, since the selectively oxidized area 1029b is arranged
in the mirror layer in the second round, the curve I.sub.2 will be
referred to in the graph in FIG. 15. Referring to the curve
I.sub.2, it is seen that the film thickness is required to be 32
nanometers or less such that the effective refractive index
difference is 0.038 or less. In addition, it is seen that the film
thickness is 13 nanometers such that the effective refractive index
difference is 0.0165.
[0163] The same discussion as the third embodiment is established
concerning a minimum value of the film thickness. As a result, a
range of a film thickness d of the selectively oxidized area 1029b
necessary for realizing the single lateral oscillation mode while
controlling a threshold current to be low is as follows
6 nm.ltoreq.d.ltoreq.32 nm (7)
A preferable range of the film thickness d, with which selective
oxidation can be performed promptly and the diameter of the current
injection area 1029a can be increased, is as follows
10 nm.ltoreq.d.ltoreq.13 nm (8)
The structure of the selectively oxidized area 1029b is optimized
as described above in the surface emitting laser according to the
fourth embodiment.
[0164] Next, a surface emitting laser according to a modification
of the fourth embodiment will be explained. FIG. 16 is a sectional
view of a structure of the surface emitting laser according to the
modification. The modification is different from the fourth
embodiment in that the selectively oxidized area 1029b is arranged
in a mirror layer in a third round.
[0165] As described already, in the fourth embodiment, it is also
possible to arrange the selectively oxidized area 1029b in the
mirror layer, a lower end of which is present in the range of 370
nanometers to 680 nanometers from the center of the active layer
1004. It is seen that, when an emission wavelength is 1300
nanometers and an optical length of an optical cavity is equal to a
wavelength of emitted light, a lower end of the mirror layer in the
third round is spaced apart from the center of the active layer
1004 by 596 nanometers and is included in the above-mentioned
range. Therefore, it is possible to adopt a structure in which the
selectively oxidized area 1029b-1 is arranged in the mirror layer
in the third round.
[0166] The conditions for the diameter of the current injection
area 1029a-1 and the effective refractive index difference can be
considered the same as those in the surface emitting laser
according to the fourth embodiment. As a conclusion, it is
necessary to set the diameter of the current injection area 1029a-1
to 3.5 .mu.m or more, preferably 5.3 .mu.m, and set the effective
refractive index difference to 0.038 or less, preferably
0.0165.
[0167] A film thickness satisfying the effective refractive index
difference will be examined with reference to the graph shown in
FIG. 15. Since the selectively oxidized area 1029b-1 is arranged in
the mirror layer in the third round in the surface emitting laser
according to the modification, it is necessary to refer to the
curve I.sub.3 in the figure. According to the curve I.sub.3, the
film thickness is required to be 46 nanometers or less such that
the effective refractive index difference is 0.038 or less and is
required to be 20 nanometers such that the effective refractive
index difference is 0.0165. Here, the optimization of the structure
of the selectively oxidized area 1029b-1 according to the
modification ends.
[0168] Note that the surface emitting laser according to the
invention has been explained using the third embodiment and the
fourth embodiment and the modifications thereof. However, it is
also possible to adopt structures other than those explained above.
For example, in the third embodiment and the fourth embodiment and
the modifications thereof, the emission wavelength of the surface
emitting laser is 1300 nanometers. The invention is not limited to
this, and optimization of a structure of a selectively oxidized
area is also possible for a surface emitting laser having a long
wavelength equal to or longer than 850 nanometers. This will be
hereinafter explained.
[0169] First, concerning a position of the selectively oxidized
area in a mirror layer, regardless of an emission wavelength, it is
preferable to arrange the selectively oxidized area near an
interface on a far side viewed from an active layer of a
low-refractive-index layer. It is determined in which mirror layer
the selectively oxidized area is arranged from the viewpoint of
controlling an increase in a threshold current and securing
reliability. These viewpoints have little correlation with the
emission wavelength. Further, a diameter of the current injection
area is also derived from the measurement result concerning the 850
nm surface emitting laser. As described above, the measurement
result is used irrespective of the emission wavelength. Concerning
a film thickness of the selectively oxidized area, a film thickness
for realizing a necessary effective refractive index difference
only has to be derived by a known method. Therefore, concerning a
surface emitting laser with a long wavelength of 850 nanometers or
more, optimization of the structure of the selectively oxidized
area can be performed using the method described in the third
embodiment or the fourth embodiment. Note that, due to the same
reason, optimization of the structure of the selectively oxidized
area can be performed for a surface emitting laser including an
optical cavity of a different optical wavelength. Here, concerning
the position in the stacking direction of the selectively oxidized
area is set in a range of 370 nanometers to 680 nanometers from the
center of the active layer. However, concerning a position of the
selectively oxidized area itself, it is preferable to set an upper
limit of a distance from the center of the active layer to 780
nanometers taking into account a film thickness of the
low-refractive-index layer forming the mirror layer.
[0170] Other than using carbon as a p-type impurity, zinc (Zn) or
beryllium (Be) can be used. The same holds true for an n-type
impurity, and a dopant other than silicon may be used.
[0171] As a semiconductor material forming the selectively oxidized
area and the current injection area, AlAs is used. Other than AlAs,
it is possible to perform selective oxidation using
Al.sub.xGa.sub.1-xAs (0.97.ltoreq.x.ltoreq.1) and form a current
injection area.
[0172] Concerning the mirror layers forming the upper reflective
layer and the lower reflective layer, when the low-refractive-index
layer is formed of Al.sub.xGa.sub.1-xAs (0.5.ltoreq.x.ltoreq.1) and
the high-refractive-index layer is formed of Al.sub.xGa.sub.1-xAs
(0.ltoreq.x.ltoreq.0.2), the low-refractive-index layer and the
high-refractive-index layer can reflect light of the emission
wavelength and function as mirror layers. In addition, an inclined
composition layer for mitigating a refractive index difference of
the low-refractive-index layer and the high-refractive-index layer
may be arranged near a boundary surface of the low-refractive-index
layer and the high-refractive-index layer.
[0173] Concerning the substrate, when an InP substrate, a GaInAs
substrate, or the like is used other than the GaAs substrate, it is
also possible to realize the surface emitting laser according to
the invention.
[0174] Concerning the active layer, instead of the structure
consisting of the triple QW layer and the barrier layers separating
the QW layers, the active layer may be formed of a single QW layer
or may have QW layers of other numbers. In addition, it is possible
to form the QW layer with a GaInAs or GaAsSb semiconductor
material. A quantum dot formed of (Ga)InAs or the like may be
adopted instead of the QW layer. Further, a surface emitting laser
of a double hetero structure may be simply adopted.
[0175] Concerning the semiconductor material forming the surface
emitting laser, it is possible to reverse a conduction type. For
example, the substrate, the lower cladding layer, and the lower
reflective layer may be formed of a p-type semiconductor, and the
upper reflective layer and the upper cladding layer may be formed
of an n-type semiconductor.
[0176] Moreover, it is preferable to arrange the current
confinement layer consisting of the selectively oxidized area and
the current injection layer in a p-type reflective layer. However,
the discussion is established even if the current confinement layer
is arranged in an n-type reflective layer, and the current
confinement layer can show the equivalent effects.
[0177] Next, an optical transceiver according to a fifth embodiment
of the invention will be explained. FIG. 17 is a block diagram of a
structure of the optical transceiver according to the fifth
embodiment. The optical transceiver according to the fifth
embodiment includes a transceiver 1031 that has an optical
transmitting unit 1034 and an optical receiving unit 1035 for
transmitting and receiving an optical signal, a signal multiplexing
circuit 1032 that inputs an electric signal to the transceiver
1031, and a signal separating circuit 1033 that separates an
electric signal obtained from an optical signal received by the
transceiver 1031.
[0178] The optical transmitting unit 1034 is a unit for converting
an electric signal inputted from the signal multiplexing circuit
1032 into an optical signal and transmitting the optical signal.
More specifically, the optical transmitting unit 1034 includes a
surface emitting laser 1036 that emits an optical signal, a control
circuit 1037 that controls the surface emitting laser 1036 based on
the inputted electric signal, and an output optical system 1038 for
outputting the optical signal emitted from the surface emitting
laser 1036 to the outside.
[0179] As the surface emitting laser 1036 included in the optical
transmitting unit 1034, the surface emitting laser according to the
third embodiment or the fourth embodiment is used. Therefore, the
surface emitting laser 1036 has a low threshold current and high
reliability and can perform single lateral mode oscillation.
[0180] The optical receiving unit 1035 is a unit for converting an
optical signal received from the outside into an electric signal
and outputting the electric signal to the signal separating circuit
1033. More specifically, the optical receiving unit 1035 includes a
photoelectric conversion element 1039 for receiving an optical
signal and converting the optical signal into an electric signal,
an input optical system 1040 for guiding the optical signal to the
photoelectric conversion element 1039, and an amplifier circuit
1041 that amplifies the electric signal outputted from the
photoelectric conversion element 1039. The photoelectric conversion
element 1039 outputs an electric signal based on intensity of the
received optical signal. As the photoelectric conversion element
1039, it is possible to use a photoresistor and the like other than
a photodiode.
[0181] The signal multiplexing circuit 1032 is a circuit for
multiplexing plural electric signals inputted from the outside into
one electric signal. The one electric signal obtained by
multiplexing the electric signals is outputted to the optical
transmitting unit 1034 constituting the transceiver 1031.
[0182] The signal separating circuit 1033 is a circuit for
separating an electric signal obtained from the optical receiving
unit 1035 constituting the transceiver 1031 into plural electric
signals. This is because, since an optical signal received by the
optical receiving unit 1035 originally includes plural signals, to
extract information, it is necessary to separate an electric signal
obtained by subjecting the optical signal to photoelectric
conversion into plural electric signals.
[0183] Operations of the optical transceiver according to the fifth
embodiment will be explained. The optical transceiver according to
the fifth embodiment is an optical transceiver for transmitting and
receiving plural electric signals. A transmission operation will be
explained in the first place.
[0184] First, plural electric signals inputted from the outside are
converted into a single electric signal by the signal multiplexing
circuit 1032. Then, this single electric signal is inputted to the
control circuit 1037 from the signal multiplexing circuit 1032, and
the control circuit 1037 controls a current to be injected into the
surface emitting laser 1036 based on this electric signal. More
specifically, an optical signal having a waveform corresponding to
an electric signal waveform is emitted from the surface emitting
laser 1036 by the control circuit 1037. Note that, since the
surface emitting laser element 1036 consists of the surface
emitting laser according to the third embodiment or the fourth
embodiment, direct optical modulation is possible at 10 Gbit/s at
the maximum. Therefore, it is possible to add a large amount of
information to an optical signal to transmit the optical signal.
The optical signal outputted from the surface emitting laser 1036
is outputted to the outside via the output optical system 1038.
Here, the transmission operation ends.
[0185] Next, a reception operation will be explained. An optical
signal transmitted from the outside is made incident in the optical
transceiver via the input optical system 1040 and received by the
photoelectric conversion element 1039. The photoelectric conversion
element 1039 has a function of outputting an electric signal having
a waveform corresponding to a change in intensity of the received
optical signal. The converted electric signal is inputted to the
amplifier circuit 1041. Since intensity of the optical signal
inputted from the outside is generally feeble, intensity of the
electric signal outputted from the photoelectric conversion element
1039 is also feeble and is amplified by the amplifier circuit 1041.
Thereafter, the amplified electric signal is inputted to the signal
separating circuit 1033 and separated into plural electric signals.
Here, the reception operation ends.
[0186] The optical transceiver according to the fifth embodiment
has the surface emitting laser according to the third embodiment or
the fourth embodiment. Therefore, in the fifth embodiment, the
surface emitting laser 1036 has a low value of a threshold current
and has high reliability. In addition, direct modulation is
possible at 10 Gbit/s, which makes it possible to output an optical
signal having a large amount of information. Moreover, when an
outputted optical signal is transmitted by an optical fiber, a
transmittable distance is 15 kilometers or more, which makes it
possible to perform long distance transmission.
[0187] Next, an optical communication system according to a sixth
embodiment of the invention will be explained. FIG. 18 is a
schematic diagram of a structure of the optical communication
system according to the sixth embodiment. The optical communication
system according to the sixth embodiment uses the surface emitting
laser according to the third embodiment or the fourth embodiment as
a signal light source. More specifically, the optical communication
system according to the sixth embodiment includes a signal
multiplexing circuit 1042, a control circuit 1043 connected to the
signal multiplexing circuit 1042, a surface emitting laser 1044
connected to the control circuit 1043, an optical fiber for
transmission 1046, and an optical system 1045 for optically
combining the surface emitting laser 1044 and an end of the optical
fiber for transmission 1046. In addition, the optical communication
system further includes an photoelectric conversion element 1048
optically combined with the other end of the optical fiber for
transmission 1046 via the optical system 1047, an amplifier circuit
1049 connected to the photoelectric conversion element 1048, and a
signal separating circuit 1050 connected to the amplifier circuit
1049.
[0188] The single electric signal obtained by the signal
multiplexing circuit 1042 is inputted to the control circuit 1043.
The control circuit 1043 controls a current, which is injected into
the surface emitting laser 1024, based on this electric signal.
Consequently, an optical signal outputted from the surface emitting
laser 1044 has a waveform corresponding to an electric signal
obtained by the signal multiplexing circuit 1042. The optical
signal outputted from the surface emitting laser 1044 is made
incident in an end of the optical fiber for transmission 1046 via
the optical system 1045 and transmitted through the optical fiber
for transmission 1046.
[0189] Then, the optical signal transmitted through the optical
fiber for transmission 1046 is emitted from the other end of the
optical fiber for transmission 1046 and is made incident in the
photoelectric conversion element 1048 via the optical system 1047.
The photoelectric conversion element 1048 outputs an electric
signal based on the received optical signal. The electric signal is
amplified by the amplifier circuit 1049 and then inputted to the
signal separating circuit 1050.
[0190] The signal separating circuit 1050 separates the inputted
electric signal into individual electric signals before being
multiplexed by the signal multiplexing circuit 1042 and restores
information. In this way, the optical communication system
according to the sixth embodiment transmits the information.
[0191] In the optical communication system according to the sixth
embodiment, the surface emitting laser according to the third
embodiment or the fourth embodiment is used as a signal light
source on a transmission side. Therefore, it is possible to use a
signal light source having a low threshold value and high
reliability. In addition, since the single lateral mode oscillation
is possible, a signal waveform is not broken in the course of
transmission, and an optical signal can be transmitted surely. More
specifically, it is possible to transmit an optical signal directly
modulated at 10 Gbit/s even if a fiber length of the optical fiber
for transmission 1046 is set to 15 km or more.
[0192] Since the surface emitting laser according to the third
embodiment or the fourth embodiment can change an emission
wavelength in the range of 850 nanometers to 1650 nanometers, it is
possible to select a wavelength at which a loss is reduced in the
optical fiber for transmission 1046. In addition, the surface
emitting laser also has an advantage that an existing optical
communication system can be used in these wavelength bands. For
example, it is also possible that the emission wavelength is set to
980 nanometers and an erbium doped fiber amplifier (EDFA) is
arranged in the optical fiber for transmission 1046. In this case,
since intensity of an optical signal can be amplified by the EDFA,
a transmission distance can be further extended. Similarly, a
thulium doped fiber amplifier (TDFA), a Raman amplifier, or the
like may be used.
[0193] Next, exemplary embodiments of the surface emitting laser
according to the invention, and the optical transceiver, the
optical communication device, and the optical communication system
using the surface emitting laser will be explained with reference
to the drawings. In the drawings, identical or similar portions are
denoted by identical or similar reference numerals and signs. In
addition, it should be noted that the drawings are schematic, and a
relation among thicknesses and widths of respective layers and a
ratio of the respective layers are different from actual ones. It
is needless to mention that a relation of dimensions and a ratio of
the dimensions are different in some portions among the
drawings.
[0194] First, a surface emitting laser according to a seventh
embodiment of the invention will be explained. FIG. 20 is a
sectional bird's eye view of a structure of the surface emitting
laser according to the seventh embodiment. In the surface emitting
laser according to the seventh embodiment, reduction of a threshold
current value and improvement of slope efficiency are performed by
optimizing structures of a reflection-side
semiconductor-multilayer-mirror and an emission-side
semiconductor-multilayer-mirror. A specific structure of the
surface emitting laser will be explained with reference to FIG.
20.
[0195] In the surface emitting laser according to the seventh
embodiment, a reflection-side semiconductor-multilayer-mirror 2002,
a lower cladding layer 2003, an active layer 2004, an upper
cladding layer 2007, and an emission-side
semiconductor-multilayer-mirror 2008 are sequentially stacked on an
n-type substrate 2001. In the emission-side
semiconductor-multilayer-mirror 2008, the surface emitting laser
also includes an opening portion 2005 and a selectively oxidized
layer 2006 arranged around the opening portion 2005. An upper area
of the lower cladding layer 2003 and semiconductor layers stacked
above the lower cladding layer 2003 are formed in a mesa-post
shape. The entire mesa-post shaped area and the entire upper
surface of the lower cladding layer 2003 are covered by a
protective layer 2010 except a part of an upper surface of the
emission-side semiconductor-multilayer-mirror 2008. A polyimide
layer 2011 is arranged at a periphery of the mesa-post shaped area,
and a p side electrode 2012, which includes an opening portion and
is in contact with an exposed part of the upper surface of the
semiconductor multilayer mirror 2008, is arranged in the center of
the mesa-post shaped area. In addition, an n side electrode 2013 is
arranged on a lower surface of the n-type substrate 2001. Note that
the n-type substrate 2001 is formed of GaAs having a conduction
type of an n-type.
[0196] The lower cladding layer 2003 and the upper cladding layer
2007 are stacked to vertically sandwich the active layer 2004 and
form an optical cavity together with the active layer 2004. In the
seventh embodiment, it is assumed that an optical length of the
optical cavity is equal to the emission wavelength. However, the
optical cavity may have other optical lengths like a value twice as
large as the emission wavelength. The lower cladding layer 2003 is
formed of n-type GaAs, and the upper cladding layer 2007 is formed
of p-type GaAs. Note that it is preferable that the lower cladding
layer 2003 and the upper cladding layer 2007 have film thicknesses
for realizing a substantially equal optical length such that an
antinode of a standing wave in the cavity coincides with the part
of the active layer 2004.
[0197] The active layer 2004 has a structure including multiple QW
layers. More specifically, the active layer 2004 is formed of a
barrier layer 2014a, a QW layer 2015a, a barrier layer 2014b, a QW
layer 2015b, a barrier layer 2014c, a QW layer 2015c, and a barrier
layer 2014d that are stacked sequentially. In other words, the
active layer 2004 has a structure in which three QW layers are
sandwiched by four barrier layers.
[0198] The QW layers 2015a to 2015c have a structure for
efficiently confining a carrier with a quantum confinement effect
and are formed of Ga.sub.x3In.sub.1-x3N.sub.y3As.sub.1-y3
(0.3.ltoreq.x.sub.3<1, 0<y.sub.3<1). The barrier layers
2014a to 2014d are layers for spatially separating the plural QW
layers from each other and are formed of GaNAs or GaAs.
[0199] The opening portion 2005 is formed of p-type AlAs. In
addition, the selectively oxidized layer 2006 is formed by
selectively oxidizing AlAs. The selectively oxidized layer 2006 has
insulating properties and has a function of constricting a current
injected from the p-side electrode 12 to increase a current density
in the active layer 2004. In addition, the selectively oxidized
layer 2006 has a refractive index different from that of the
opening part 2005 and also has a function of controlling an
oscillation lateral mode.
[0200] The p side electrode 2012 has a structure in which an
opening portion is provided in the center. This opening portion
functions as an emission window for outputting light generated in
the active layer 2004 to the outside. In addition, since the p-side
electrode 2012 has a structure extending not only onto the
emission-side semiconductor-multilayer-mirror 2008 but also onto
the polyimide layer 2011, the polyimide layer 2011 is formed of a
substance with a low dielectric constant from the viewpoint of
reducing parasitic capacitance.
[0201] The reflective side semiconductor multilayer mirror 2002 has
a conduction type of an n-type and has a structure in which plural
n-type DBR mirrors, which include a high-refractive-index area and
a low-refractive-index area as a pair, are stacked. In addition,
the emission-side semiconductor-multilayer-mirror 2008 has a
conduction type of a p-type and has a structure in which plural
p-type DBR mirrors, which include a high-refractive-index area and
a low-refractive-index area as a pair, are stacked. The
high-refractive-index area is formed of p-type or n-type GaAs, and
the low-refractive-index area is formed of p-type or n-type
Al.sub.0.9Ga.sub.0.1As. Here, the n-type DBR mirrors forming the
reflective side semiconductor multilayer mirror 2002 are stacked in
thirty layers, and the p-type DBR mirrors forming the emission-side
semiconductor-multilayer-mirror 2008 are stacked in twenty to
thirty layers. Note that it is preferable to interpose inclined
composition areas in interfaces of the respective areas to reduce a
resistance.
[0202] Next, a reason why the numbers of stacking the n-type DBR
mirrors and the p-type DBR mirrors are determined as described
above will be explained. As explained already, to use the surface
emitting laser as a signal light source or the like, it is
necessary to control a threshold current to be low and improve
slope efficiency. First, reflectivities of the reflection-side
semiconductor-multilayer-mirror 2002 and the emission-side
semiconductor-multilayer-mirror 2008 necessary for satisfying these
conditions are derived, and the numbers of stacking n-type DBR
mirrors and p-type DBR mirrors necessary for realizing the derived
reflectivities are determined.
[0203] In the first place, conditions necessary for improving slope
efficiency will be examined. Efficiency .eta..sub.f of a laser beam
outputted from the opening portion provided in the p side electrode
2012 can be represented as follows using external differential
quantum efficiency .eta..sub.d, a reflectivity R.sub.f of the
emission-side semiconductor-multilayer-mirror 2008, and a
reflectivity R.sub.r of the reflection-side
semiconductor-multilayer-mirror 2002.
.eta..sub.f=.eta..sub.d(1+[R.sub.f/R.sub.r{1=(R.sub.r/1-R.sub.f)}].sup.1-
/2).sup.-1 (11)
Efficiency of a laser beam to be outputted has a large value as the
efficiency .eta..sub.f is larger. A relation between the efficiency
and slope efficiency S.sub.f is represented as follows using a
wavelength .lamda. of the laser beam.
S.sub.f=1.24.eta..sub.f/.lamda. (12)
Therefore, conditions under which the slope efficiency S.sub.f is
equal to or higher than 0.2 mW/mA can be found using expression
(11) and expression (12). Here, since the external differential
quantum efficiency .eta..sub.d and the wavelength .lamda. are known
values, desired slope efficiency can be realized by optimizing the
reflectivity R.sub.f of the emission-side
semiconductor-multilayer-mirror 2008 and the reflectivity R.sub.r
of the reflection-side semiconductor-multilayer-mirror 2002. FIG.
21 shows a result of specific calculation. In a graph of FIG. 21, a
horizontal axis indicates a reflectivity of the emission-side
semiconductor-multilayer-mirror 2008 and a vertical axis indicates
slope efficiency. Note that, in the graph, a value of the
reflectivity R.sub.r of the reflection-side
semiconductor-multilayer-mirror 2002 is assumed to be 99.9 percent
or more. In this embodiment, since it is not taken into account
that a laser beam is emitted from the reflection-side
semiconductor-multilayer-mirror 2002 side, it is necessary to set
the reflectivity R.sub.r high. As shown in the graph of FIG. 21, to
improve the slope efficiency S.sub.f, it is preferable that the
reflectivity R.sub.f of the emission-side
semiconductor-multilayer-mirror 2008 is 99.8 percent or less, and
it is necessary to stack p-type DBR layers by a number
corresponding to this reflectivity.
[0204] Next, conditions necessary for reducing a threshold current
to 1 milliampere or less will be examined. A threshold current
density J.sub.th is represented as follows
J.sub.th=(N.sub.wJ.sub.tr.eta..sub.i)
exp{.alpha..sub.i+(.alpha..sub.m/G.sub.0N.sub.w.GAMMA..sub.w)}
(13)
Note that, in expression (13), N.sub.w is the number of QWs,
J.sub.tr is a transparency current density, .eta..sub.i is an
internal quantum efficiency, .alpha..sub.i is an internal loss,
.alpha..sub.m is a mirror loss, G.sub.0 is a gain, and
.GAMMA..sub.w is a coefficient of light confinement in the active
layer 2004. In addition, the mirror loss .alpha..sub.m is
represented as follows
.alpha..sub.m=(1/2)L.times.1n(1/R.sub.fR.sub.r) (14)
By substituting expression (14) in expression (13), it is seen that
the threshold current density J.sub.th is a function of the
reflectivity R.sub.f of the emission-side
semiconductor-multilayer-mirror 2008, and the threshold current
density J.sub.th decreases as the reflectivity R.sub.f increases.
When the reflectivity R.sub.r of the reflection-side
semiconductor-multilayer-mirror 2002 was set to 99.9 percent or
more and the threshold current density was calculated by
substituting specific values for variables other than the
reflectivity R.sub.f of the emission-side
semiconductor-multilayer-mirror 2008, a graph shown in FIG. 22 was
obtained. In the graph shown in FIG. 22, a horizontal axis
indicates the reflectivity R.sub.f and a vertical axis indicates
the threshold current density J.sub.th. Here, since a horizontal
sectional area of a part of the active layer 2004 into which a
current actually flows to contribute to light emission is generally
about 30 .mu.m.sup.2, the threshold current density J.sub.th has to
be reduced to 3 kA/cm.sup.2 or less to reduce a value of the
threshold current to 1 milliampere or less. From this condition and
the graph of FIG. 22, it is preferable that the reflectivity Rf of
the emission-side semiconductor-multilayer-mirror 2008 necessary
for reducing the threshold current to 1 milliampere or less is 99.4
percent or more.
[0205] In other words, as a condition for increasing the slope
efficiency to 0.2 mW/mA or more, a reflectivity of the
emission-side semiconductor-multilayer-mirror 2008 is 99.8 percent
or less, and as a condition for reducing the threshold current to 1
milliampere or less, a reflectivity of the emission-side
semiconductor-multilayer-mirror 2008 is 99.4 percent or more. As a
result, a reflectivity of the emission-side
semiconductor-multilayer-mirror 2008 satisfying both the conditions
is 99.4 percent or more and 99.8 percent or less.
[0206] Next, the number of stacked layers of p-type DBR mirrors and
the number of stacked layers of n-type DBR mirrors necessary for
realizing such a reflectivity is derived. FIG. 23A is a table
showing a relation between the number of stacked layers of the
n-type DBR mirrors and a reflectivity, and FIG. 23B is a table
showing a relation between the number of stacked layers of the
p-type DBR mirrors and a reflectivity. Note that, in FIG. 23A and
FIG. 23B, reflectivities of the n-type DBR mirrors and the p-type
DBR mirrors of the same number of stacked layers are different
because refractive indexes in areas outside the respective DBR
mirrors are different. For example, the n-type substrate 2001, that
is, a semiconductor layer of GaAs or the like is present outside
(below) the n-type DBR mirror, and a reflectivity of the
semiconductor layer is about 3.5. On the other hand, the air with a
refractive index of about 1 is present outside (above) the p-type
DBR mirror. Thus, the reflectivities are different. The inventors
actually created surface emitting lasers with different structures
experimentally and then measured reflectivities to calculate a
maximum reflectivity.
[0207] Referring to the table in FIG. 23A, it is seen that the
number of stacked layers of the n-type DBR mirror necessary for
setting the reflectivity of the reflection-side
semiconductor-multilayer-mirror 2002 provided on the n-type
substrate 2001 side to 99.9 percent or more is thirty or more. In
addition, from FIG. 23B, the number of stacked layers of the p-type
DBR mirror necessary for setting the reflectivity of the
emission-side semiconductor-multilayer-mirror 2008 to 99.4 percent
or more and 99.8 percent or less is twenty or more and twenty-three
or less.
[0208] By forming the reflection-side
semiconductor-multilayer-mirror 2002 and the emission-side
semiconductor-multilayer-mirror 2008 in this way, the reflectivity
of the reflection-side semiconductor-multilayer-mirror 2002 is set
to 99.9 percent, and the reflectivity of the emission-side
semiconductor-multilayer-mirror 2008 is set to 99.4 percent or more
and 99.8 percent or less. This makes it possible to control a
threshold current value to 1 milliampere or less and increase the
slope efficiency to 0.2 mW/mA or more. Therefore, when an injection
current five or more times as large as the threshold current value,
for example, a current of 6 milliamperes is injected, intensity of
a laser beam emitted from the surface emitting laser according to
the seventh embodiment is increased to 1 milliwatt or more. Thus,
the surface emitting laser can be used for applications like a
signal light source.
[0209] Next, a surface emitting laser according to an eighth
embodiment of the invention will be explained. FIG. 24 is a
schematic diagram of a structure of the surface emitting laser
according to the eighth embodiment. In the surface emitting laser
according to the eighth embodiment, an electrode arranged on a
lower surface of a substrate has an opening portion, and a laser
beam is emitted from this opening portion. A structure of the
surface emitting laser according to the eighth embodiment will be
hereinafter explained specifically.
[0210] As shown in FIG. 24, the surface emitting laser according to
the eighth embodiment has a structure in which an emission-side
semiconductor-multilayer-mirror 2017, the lower cladding layer
2003, the active layer 2004, the upper cladding layer 2007, and a
reflection-side semiconductor-multilayer-mirror 2018 are
sequentially stacked on the n-type substrate 2001. In a part of the
area of the reflection-side semiconductor-multilayer-mirror 2018,
the opening portion 2005 and the selectively oxidized layer 2006
around the opening portion 2005 are arranged. An upper area of the
lower cladding layer 2003 and semiconductor layers stacked above
the lower cladding layer 2003 are formed in a mesa-post shape. The
entire mesa-post shaped area and the entire upper surface of the
lower cladding layer 2003 are covered by the protective layer 2010
except a part of an upper surface of the emission-side
semiconductor-multilayer-mirror 2018. The polyimide layer 2011 is
stacked at a periphery of the mesa-post shaped area with the
protective layer 2010 between the periphery of the mesa-post shaped
area and the polyimide layer 2011. A p side electrode 2019 is
arranged on the upper surface of the reflection-side
semiconductor-multilayer-mirror 2018 and on the polyimide layer
2011. The p side electrode 2019 does not have an opening unlike the
seventh embodiment, and a laser beam is never emitted from an area
near the p side electrode 2019. Alternatively, an n side electrode
2020 including an opening portion is arranged on a lower surface of
the n-type substrate 2001, and a laser beam is emitted from the
opening portion included in the n side electrode 2020. In addition,
to prevent reflection on an interface of the n-type substrate 2001
and the air, a non-reflective film 2021 is formed in the opening
portion of the n side electrode 2020, that is, a laser beam
emitting portion. Note that the n side electrode 2020 may be
arranged over the entire lower surface of the n-type substrate 2002
as shown in FIG. 24 or may be formed in an annular shape. In
addition, in the eighth embodiment, portions denoted by reference
numerals identical with those in the seventh embodiment have
equivalent structures and show equivalent functions unless
specifically noted otherwise.
[0211] The emission-side semiconductor-multilayer-mirror 2017 has a
conduction type of an n-type and has a structure in which plural
n-type DBR mirrors, which include a high-refractive-index area and
a low-refractive-index area as a pair, are stacked. The
reflection-side semiconductor-multilayer-mirror 2018 has a
conduction type of an p-type and has a structure in which plural
p-type DBR mirrors, which include a high-refractive-index area and
a low-refractive-index area as a pair, are stacked. The
high-refractive-index area is formed of p-type or n-type GaAs, and
the low-refractive-index area is formed of p-type or n-type
Al.sub.0.9Ga.sub.0.1As.
[0212] The surface emitting laser according to the eighth
embodiment has a structure for emitting a laser beam from the lower
surface of the n-type substrate 2001 unlike the seventh embodiment.
In the eighth embodiment, the n-type substrate 2001 is formed of
GaAs, and a forbidden band width of GaAs is about 1.428
electron-volts at a room temperature. When light of a wavelength of
0.8682 .mu.m or less is made incident, the light is absorbed by the
n-type substrate 2001 to generate an electro/hole pair. However, a
laser beam emitted from the surface emitting laser according to the
eighth embodiment has a wavelength of about 1.2 .mu.m to 1.6 .mu.m,
the laser beam can be emitted to the outside without being absorbed
by the n-type substrate 2001.
[0213] Since the surface emitting laser has the structure for
emitting a laser beam from the lower surface of the n-type
substrate 2001, the emission-side semiconductor-multilayer-mirror
2017 and the reflection-side semiconductor-multilayer-mirror 2018
are located in positions opposite to those in the seventh
embodiment. Therefore, to improve slope efficiency while
controlling a threshold current for the surface emitting laser
according to the eighth embodiment, it is necessary to optimize the
structures of the emission-side semiconductor-multilayer-mirror
2017 and the reflection side multilayer reflection mirror 2018
again.
[0214] Expressions (11) to (14) for optimizing a reflectivity are
established for the eighth embodiment as in the seventh embodiment.
Therefore, referring to FIGS. 21 and 22, a reflectivity of the
emission-side semiconductor-multilayer-mirror 2017 is set to 99.4
percent or more and 99.8 percent or less, and a reflectivity of the
reflection-side semiconductor-multilayer-mirror 2018 is set to 99.9
percent or more.
[0215] Next, the number of stacked layers of the n-type DBR mirrors
forming the emission-side semiconductor-multilayer-mirror 2017 and
the number of stacked layers of the p-type DBR mirrors forming the
reflection-side semiconductor-multilayer-mirror 2018 will be
examined. Since the number of stacked layers of the n-type DBR
mirrors and a reflectivity of the emission-side
semiconductor-multilayer-mirror 2017 have the relation shown in
FIG. 23A, to set the reflectivity of the emission-side
semiconductor-multilayer-mirror 2017 to 99.4 percent or more and
99.8 percent or less, twenty-three or more layers and twenty-six or
less layers of the n-type DBR mirrors only have to be stacked.
Similarly, since the number of stacked layers of the p-type DBR
mirror and a reflectivity of the reflection-side
semiconductor-multilayer-mirror 2018 have the relation shown in
FIG. 23B, to set the reflectivity of the reflection-side
semiconductor-multilayer-mirror 2018 to 99.9 percent or more,
twenty-six or more layers only has to be stacked. By forming the
emission-side semiconductor-multilayer-mirror 2017 and the
reflection-side semiconductor-multilayer-mirror 2018 in this way, a
surface emitting laser with a threshold current of 1 milliampere or
less and slope efficiency of 0.2 mW/mA can be realized.
[0216] As described above, the surface emitting laser according to
the invention are explained according to the seventh embodiment and
the eighth embodiment. However, the descriptions and the drawings
forming a part of this disclosure do not limit the invention. Those
skilled in the art would be able to derive various alternative
modes for carrying out the invention, embodiments, and operation
techniques from this disclosure. For example, although the n-type
substrate 1 is explained as being formed of GaAs in the seventh
embodiment and the eighth embodiment, the n-type substrate 1 may be
formed of InP. In addition, for the QW layer forming the active
layer 2004, other than Ga.sub.x3In.sub.1-x3N.sub.y3As.sub.1-y3
(0.3.ltoreq.x.sub.3<1, 0<y.sub.3<1),
Ga.sub.x4In.sub.1-x4As.sub.1-y4-zN.sub.y4Sb.sub.z
(0.3.ltoreq.x.sub.4<1, 0<y.sub.4<0.03,
0.002.ltoreq.z.ltoreq.0.06) may be used. It is confirmed that, when
Ga.sub.x4In.sub.1-x4As.sub.1-y4-zN.sub.y4Sb.sub.z is used,
crystallinity is improved, and a surface emitting laser more
excellent in characteristics can be realized. Similarly,
GaAs.sub.y5Sb.sub.1-y(0<y.sub.5<1) may be used, or the active
layer 4 may be formed of a quantum dot layer instead of the QW
layer.
[0217] The high-refractive-index area forming the n-type DBR mirror
and the p-type DBR mirror is not limited to GaAs, and it is
possible to use Al.sub.x1Ga.sub.1-x1As (0.ltoreq.x1.ltoreq.0.4) as
the high-refractive-index area. Similarly, the low-refractive-index
area is not limited to Al.sub.0.9Ga.sub.0.1As, and it is possible
to use Al.sub.x2Ga.sub.1-x2As (0.6.ltoreq.x.sub.2.ltoreq.0.95) as
the low-refractive-index area. When compositions of the
high-refractive-index area and the low-refractive-index area
change, reflectivities of the n-type DBR mirror and the p-type DBR
mirror may change. However, even in that case, it is possible to
calculate the number of stacked layers realizing appropriate
reflectivities by deriving the tables shown in FIGS. 23A and 23B
experimentally or theoretically.
[0218] Other than AlAs, Al.sub.x6Ga.sub.1-x6As
(0.97.ltoreq.x6<1) may be used for the semiconductor layer
forming the opening portion 2005, that is, the semiconductor layer
before oxidation of the selectively oxidized layer 2006. The
conduction type may be reversed for the semiconductor layer forming
the surface emitting laser. For example, it is possible to stack
semiconductor layers on a p-type substrate.
[0219] Moreover, the active layer 2004 may not be formed in the
structure having a triple QW layer and may be formed in a structure
including a single QW layer or multiple QW layers having about two
to five QW layers.
[0220] Next, an optical transceiver according to a ninth embodiment
of the invention will be explained. FIG. 25 is a block diagram of a
structure of the optical transceiver according to the ninth
embodiment. The optical transceiver according to the ninth
embodiment includes a transceiver 2031 having an optical
transmitting unit 2034 and an optical receiving unit 2035 for
transmitting and receiving an optical signal, a signal multiplexing
circuit 2032 that inputs an electric signal to the transceiver
2031, and a signal separating circuit 2033 that separates an
electric signal obtained from an optical signal received by the
transceiver 2031.
[0221] The optical transmitting unit 2034 is a unit for converting
an electric signal inputted from the signal multiplexing circuit
2032 into an optical signal and transmitting the optical signal.
More specifically, the optical transmitting unit 2034 includes a
surface emitting laser 2036 that emits an optical signal, a control
circuit 2037 that controls the surface emitting laser 2036 based on
an inputted electric signal, and an output optical system 2038 for
outputting the optical signal, which is emitted from the surface
emitting laser 2036, to the outside.
[0222] The surface emitting laser according to the seventh
embodiment or the eighth embodiment is used for the surface
emitting laser 2036 included in the optical transmitting unit 2034.
Therefore, the surface emitting laser 2036 has a low threshold
current value and improved slope efficiency and is capable of
outputting an optical signal with intensity of 1 milliwatt or
more.
[0223] The optical receiving unit 2035 is a unit for converting an
optical signal received from the outside into an electric signal
and outputting the electric signal to the signal separating circuit
2033. More specifically, the optical receiving unit 2035 includes a
photoelectric conversion element 2039 for receiving an optical
signal and converting the optical signal into an electric signal,
an input optical system 2040 for guiding an optical signal to the
photoelectric conversion element 2039, and an amplifier circuit
2041 that amplifies an electric signal outputted from the
photoelectric conversion element 2039. The photoelectric conversion
element 2039 outputs an electric signal based on intensity of the
received optical signal. It is possible to use a photoresistor or
the like as the photoelectric conversion element 2039 other than a
photodiode.
[0224] The signal multiplexing circuit 2032 is a circuit for
multiplexing plural electric signals inputted from the outside into
one electric signal. The one electric signal obtained by
multiplexing the electric signals is outputted to the optical
transmitting unit 2034 constituting the transceiver 2031.
[0225] The signal separating circuit 2033 is a circuit for
separating an electric signal obtained from the optical receiving
unit 2035 constituting the transceiver 2031 into plural electric
signals. This is because, since an optical signal received by the
optical receiving unit 2035 originally includes plural signals, to
extract information, it is necessary to separate an electric
signal, which is obtained by subjecting the optical signal to
photoelectric conversion, into plural electric signals.
[0226] Next, operations of the optical transceiver according to the
ninth embodiment will be explained. The optical transceiver
according to the ninth embodiment is an optical transceiver for
transmitting and receiving plural electric signals. A transmission
operation will be explained in the first place.
[0227] First, plural electric signals inputted from the outside is
converted into a single electric signal by the signal multiplexing
circuit 2032. Then, this single electric signal is inputted to the
control circuit 2037 from the signal multiplexing circuit 2032, and
the control circuit 2037 controls a current to be injected into the
surface emitting laser 2036 based on this electric signal. More
specifically, an optical signal having a waveform corresponding to
an electric signal waveform is emitted from the surface emitting
laser 2036 by the control circuit 2037.
[0228] Next, a reception operation will be explained. An optical
signal transmitted from the outside is made incident in the optical
transceiver via the input optical system 2040 and received by the
photoelectric conversion element 2039. The photoelectric conversion
element 2039 has a function of outputting an electric signal having
a waveform corresponding to a change in intensity of the received
optical signal. The converted electric signal is inputted to the
amplifier circuit 2041. Since intensity of the optical signal
inputted from the outside is generally feeble, intensity of the
electric signal outputted from the photoelectric conversion element
2039 is also feeble and is amplified by the amplifier circuit 2041.
Thereafter, the amplified electric signal is inputted to the signal
separating circuit 2033 and separated into plural electric signals.
Here, the reception operation ends.
[0229] Next, an optical communication system according to a tenth
embodiment of the invention will be explained. FIG. 26 is a
schematic diagram showing the optical communication system
according to the tenth embodiment. The optical communication system
according to the tenth embodiment uses the surface emitting laser
according to the seventh embodiment or the eight embodiment as a
signal light source. More specifically, the optical communication
system according to the tenth embodiment includes a signal
multiplexing circuit 2042, a control circuit 2043 connected to the
signal multiplexing circuit 2042, a surface emitting laser 2044
connected to the control circuit 2043, an optical fiber for
transmission 2046, and an optical system 2045 for optically
combining the surface emitting laser 2044 and an end of the optical
fiber for transmission 2046. In addition, the optical communication
system further includes an photoelectric conversion element 2048
optically combined with the other end of the optical fiber for
transmission 2046 via the optical system 2047, an amplifier circuit
2049 connected to the photoelectric conversion element 2048, and a
signal separating circuit 2050 connected to the amplifier circuit
2049.
[0230] The single electric signal obtained by the signal
multiplexing circuit 2042 is inputted to the control circuit 2043.
The control circuit 2043 controls a current, which is injected into
the surface emitting laser 2044, based on this electric signal.
Consequently, an optical signal outputted from the surface emitting
laser 2044 has a waveform corresponding to an electric signal
obtained by the signal multiplexing circuit 2042. The optical
signal outputted from the surface emitting laser 2044 is made
incident in an end of the optical fiber for transmission 2046 via
the optical system 2045 and transmitted through the optical fiber
for transmission 2046.
[0231] Then, the optical signal transmitted through the optical
fiber for transmission 2046 is emitted from the other end of the
optical fiber for transmission 2046 and is made incident in the
photoelectric conversion element 2048 via the optical system 2047.
The photoelectric conversion element 2048 outputs an electric
signal based on the received optical signal. The electric signal is
amplified by the amplifier circuit 2049 and then inputted to the
signal separating circuit 2050.
[0232] The signal separating circuit 2050 separates the inputted
electric signal into individual electric signals before being
multiplexed by the signal multiplexing circuit 2042 and restores
information. In this way, the optical communication system
according to the tenth embodiment transmits the information.
[0233] In the optical communication system according to the tenth
embodiment, the surface emitting laser according to the seventh
embodiment or the eighth embodiment is used as a signal light
source on a transmission side. Therefore, it is possible to use a
surface emitting laser having a low threshold current value and
improved slope efficiency and having intensity of 1 milliwatt or
more. In addition, it is possible to set a fiber length of the
optical fiber for transmission 2046 long in the optical
communication system, which makes it possible to perform long
distance transmission of an optical signal.
[0234] Since the surface emitting laser according to the seventh
embodiment or the eighth embodiment emits a laser beam with a
wavelength of 1.2 .mu.m or more, it is possible to select a
wavelength at which a loss is low in the optical fiber for
transmission 2046. In addition, the surface emitting laser also has
an advantage that an existing optical communication system can be
used in these wavelength bands. For example, it is also possible
that the emission wavelength is set to 1.550 .mu.m and an erbium
doped fiber amplifier (EDFA) is arranged in the optical fiber for
transmission 2046. In this case, since intensity of an optical
signal can be amplified by the EDFA, a transmission distance can be
further extended. Similarly, a thulium doped fiber amplifier
(TDFA), a Raman amplifier, or the like may be used.
[0235] Next, embodiments of the surface emitting laser according to
the invention, and the transceiver, the optical transceiver, and
the optical communication system using the surface emitting laser
will be explained in detail with reference to the drawings. Note
that the invention is not limited by the embodiments.
[0236] First, a surface emitting laser according to an eleventh
embodiment of the invention will be explained. The surface emitting
laser according to the eleventh embodiment is characterized in that
the surface emitting laser oscillates in a bandwidth between 980
nanometers to 1650 nanometers and, in a low-refractive-index layer
and a high-refractive-index layer of a p-type semiconductor
multilayer mirror belonging to a range of a predetermined number of
pairs from an active layer, a predetermined area from an interface
of the low-refractive-index layer and the high-refractive-index
layer is subjected to p-type doping at a high concentration, and
the remaining areas of the low-refractive-index area and the
high-refractive-index area is subjected to p-type doping at a low
concentration equal to or lower than a predetermined value not
affecting laser oscillation.
[0237] FIG. 27 is a perspective sectional view of the surface
emitting laser according to the eleventh embodiment. FIG. 28 is an
explanatory diagram for explaining a structure of an n-type lower
semiconductor multilayer mirror and a p-type upper semiconductor
multilayer mirror of the surface emitting laser according to the
eleventh embodiment. In particular, the surface emitting laser 3010
shown in FIG. 27 is different from the conventional surface
emitting laser in structures of respective layers of a p-type upper
semiconductor multilayer mirror 3016. Thus, the large difference is
explained in FIG. 28.
[0238] To manufacture the surface emitting laser 3010 shown in FIG.
27, first, an n-type GaAs buffer layer 3012 with a thickness of 0.1
.mu.m is formed at an n-type impurity concentration
1.times.10.sup.18 cm.sup.-3 by the MOCVD method on an n-type GaAs
substrate 3011 of a (100) surface, and an n-type lower
semiconductor multilayer mirror (lower DBR mirror) 3013 is further
formed on this n-type GaAs buffer layer 3012. Here, as shown in
FIG. 28, the n-type lower semiconductor multilayer mirror 3013 is a
layer in which, assuming that a stacked structure of an n-type
high-refractive-index layer 3041 with a thickness of 94 nanometers
and an n-type low-refractive-index layer 3042 with a thickness of
110 nanometers forms one pair, for example, 34.5 pairs are stacked.
Note that the n-type high-refractive-index layer 3041 is formed of
n-type GaAs, and the n-type refractive index layer 3042 is formed
of n-type Al.sub.0.9Ga.sub.0.1As.
[0239] Then, a lower GaAs cladding layer 3031, a multiple QW active
layer 3030, and an upper GaAs cladding layer 3032 are formed in
order on the n-type lower semiconductor multilayer mirror 3013. The
GaAs cladding layers 3031 and 3032 are, for example, 150 nanometers
thick, and the multiple QW active layer 3030 is a triple QW layer
formed of a
Ga.sub.0.63In.sub.0.37N.sub.0.012As.sub.0.972Sb.sub.0.016 well
layer with a thickness of 7.5 nanometers and a GaNAs barrier layer
with a thickness of 30 nanometers at both ends and a thickness of
20 nanometers in other portions.
[0240] The p-type upper semiconductor multilayer mirror 3016 (upper
DBR mirror) is formed on the upper GaAs cladding layer 3032. As
shown in FIG. 28, the p-type upper semiconductor multilayer mirror
3016 is a layer in which, assuming that a stacked structure of a
p-type low-refractive-index layer with a thickness of 110
nanometers and a p-type high-refractive-index layer with a
thickness of 94 nanometers forms one pair, for example, twenty-five
pairs are stacked. Note that the p-type low-refractive-index layer
is formed of p-type GaAs, and the p-type high-refractive-index
layer is formed of p-type Al.sub.0.9Ga.sub.0.1As. Among the pairs
forming the p-type upper semiconductor multilayer mirror 3016, as
shown in FIG. 28, a low-refractive-index layer of a first pair
adjacent to the GaAs cladding layer 3032 is formed of a p-type
Al.sub.0.9Ga.sub.0.1As layer and a p-type AlAs layer 3050 that is
required for forming a current confinement layer in a later
process. For example, the low-refractive-index layer of the first
pair is formed of the p-type Al.sub.0.9Ga.sub.0.1As layer with a
thickness of 90 nanometers and the p-type AlAs layer 3050.
[0241] Here, the invention is characteristic in that, in the p-type
upper semiconductor multilayer mirror 3016, an impurity
concentration of low-refractive-index layers and
high-refractive-index layers for five pairs from the upper GaAs
cladding layer 3032 is different from an impurity concentration of
low-refractive-index layers and high-refractive-index layers for
the remaining twenty pairs. More specifically, as shown in FIG. 28,
in the five pairs from the upper GaAs cladding layer 3032, only
areas of 10 nanometer thickness from interfaces of
low-refractive-index layers 3051 and high-refractive-index layers
3052 are subjected to p-type doping at an impurity concentration of
1 to 2.times.10.sup.19 cm.sup.-3, areas of the low-refractive-index
layers 3051 other than the areas of 10 nanometer thickness are
subjected to p-type doping at an impurity concentration of
7.5.times.10.sup.17 cm.sup.-3, and areas of the
high-refractive-index layers 3052 other than the areas of 10
nanometer thickness are subjected to p-type doping at an impurity
concentration of 2.times.10.sup.17 cm.sup.-3. In addition, in the
remaining twenty pairs, all the areas of low-refractive-index
layers 3053 and high-refractive-index layers 3054 are subjected to
p-type doping at an impurity concentration of 1 to
2.times.10.sup.19 cm.sup.-3. Note that, as an impurity to be
subjected to p-type doping, carbon (C), zinc (Zn), beryllium (Be),
or the like can be used.
[0242] As described above, the p-type AlAs layer 3050 is included
in the five pairs from the upper GaAs cladding layer 3032 as a
first pair of the five pairs. In other words, the surface emitting
laser according to the eleventh embodiment is designed such that,
in the p-type upper semiconductor multilayer mirror 3016, an
impurity concentration of the five pairs from the GaAs cladding
layer 3032 is low compared with an impurity concentration of the
remaining twenty pairs.
[0243] Subsequently, a p-type GaAs contact layer 3017 is formed on
the p-type upper semiconductor multilayer mirror 3016. Then,
through a photolithography process and an etching process (dry
etching or wet etching), an outer edge of a stacked structure,
which consist of the p-type upper semiconductor multilayer mirror
3016, the p-type AlAs layer 3050, the p-type Al0.9Ga0.1As layer
below the p-type AlAs layer 3050, the upper GaAs cladding layer
3032, the multiple QW active layer 3030, and a part of the lower
GaAs cladding layer 3031, is removed. Consequently, for example, a
circular mesa-post with a diameter of 40 .mu.m is formed.
[0244] Next, oxidation treatment is performed at temperature of
about 400.degree. C. in a moisture vapor atmosphere to selectively
oxidize the p-type AlAs layer 3050 from a sidewall of the mesa-post
to form an Al oxide layer 3014. For example, when the Al oxide
layer 3014 is formed in a ring shape with a bandwidth of 17.5
.mu.m, an area of a p-type AlAs layer 3015 in the center, that is,
an area of an aperture to which a current is injected is about 20
.mu.m.sup.2 (with a diameter of 5 .mu.m).
[0245] Then, a silicon nitride film 3019 functioning as a
protective layer is formed on an upper surface and a side surface
of the mesa-post and an exposed upper surface of the lower GaAs
cladding layer 3031. Subsequently, periphery of the mesa-post is
filled with a polyimide 3022. The silicon nitride film 3019 formed
on the upper surface of the mesa-post is removed in a circular
shape with a diameter of 40 .mu.m to further form a p-type
electrode 3018 of a ring shape with an inner diameter of 10 .mu.m
and an outer diameter of 40 .mu.m on the p-type GaAs contact layer
3017 exposed by the removal. After grinding the substrate to have a
thickness of, for example, 200 .mu.m, an n-type electrode 3021 is
formed on the back of the n-type GaAs substrate 3011. An electrode
pad 3020, on which a wire is bonded, is formed on the polyimide
3022 to come into contact with the p-type electrode 3018.
[0246] Characteristic points of the invention, that is, an effect
realized by reducing an impurity concentration of the five pairs
from the GaAs cladding layer 3032 and selection of a value of the
impurity concentration will be hereinafter explained.
[0247] First, to create a surface emitting laser with satisfactory
characteristics of an oscillation threshold current and slope
efficiency and realize high-frequency direct modulation serial
transmission of 10 Gbps or the like, it is known that an impurity
concentration of the p-type upper semiconductor multilayer mirror
3016 is important. To realize a stable direct modulation operation
at high frequency, it is necessary to decrease an electric
resistance of the p-type upper semiconductor multilayer mirror
3016. Therefore, it is necessary to subject impurities to p-type
doping at a high concentration of about 1.times.10.sup.19 cm.sup.-3
in a predominant area determining an electric resistance, that is,
an interface of a high-refractive-index layer and a
low-refractive-index layer forming one pair to decrease an electric
resistance in that part. This is often performed. In addition, it
is preferable to subject impurities of a certain amount to p-type
doping in portions other than the interface to decrease an electric
resistance of the entire high-refractive-index layer and
low-refractive-index layer.
[0248] However, it is well known that areas close to the p-type
upper semiconductor multilayer mirror 3016 and the multiple QW
active layer 3030 have a high optical density and are susceptible
to intervalence band absorption or free carrier absorption. Thus,
in areas other than an interface of a high-refractive-index layer
and a low-refractive-index layer in a pair belonging to the area,
it is necessary to reduce an impurity concentration to a certain
degree or less and avoid development of intervalence band
absorption or free carrier absorption to secure stable light
generation in the multiple QW active layer 3030.
[0249] Thus, through a trial calculation of the threshold current
density J.sub.th necessary for laser oscillation, the inventors
found an optimum impurity concentration for high-refractive-index
layers and low-refractive-index layers of the p-type upper
semiconductor multilayer mirror 3016 for five pairs from the upper
GaAs cladding layer 3032. Here, a result of the trial calculation
is described. A surface emitting laser manufactured on trial for
the trial calculation has the structure shown in FIG. 27 with a
1.3-.mu.m-band as an oscillation wavelength, in which areas other
than interfaces of high-refractive-index layers and
low-refractive-index layers of the p-type upper semiconductor
multilayer mirror 3016 for five pairs from the upper GaAs cladding
layer 3032 are subjected to p-type doping at an impurity
concentration of 4.times.10.sup.18 cm.sup.-3, and the interfaces
areas of the high-refractive-index layers and the
low-refractive-index layers and all the areas of the
high-refractive-index layers and the low-refractive-index layers of
the p-type upper semiconductor multilayer mirror 3016 for the
remaining twenty pairs are subjected to p-type doping at an
impurity concentration of 1 to 2.times.10.sup.19 cm.sup.-3.
[0250] FIG. 29 is a graph in which a relation between a threshold
current density J.sub.th in a vertical direction and a mirror loss
with an absorption loss in a p-type semiconductor multilayer mirror
in an effective cavity portion, where light seeps out, as a
parameter using laser parameters extracted by an edge emitting type
laser. More specifically, the graph shown in FIG. 29 is a result
that is obtained when an absorption loss (.alpha..sub.p) of a
p-type semiconductor multilayer mirror portion is changed to 20
cm.sup.-1, 40 cm.sup.-1, 60 cm.sup.-1, 80 cm.sup.-1, and 100
cm.sup.-1 using the following expression (21) and laser parameters
shown in FIG. 30.
Jth = Nw Jtr .eta. exp [ .xi. w Nw .alpha. a + ( 1 - .xi. w )
.alpha. c + .alpha. m .xi. w NwG 0 ] ( 21 ) ##EQU00001##
[0251] Here, .alpha..sub.m indicates a mirror loss, N.sub.W
indicates the number of wells, .GAMMA. indicates a light
confinement coefficient, .alpha..sub.i indicates an absorption
loss, J.sub.tr is a transparency current density, .eta. indicates a
current injection ratio of spontaneous emission at a threshold
value, and G.sub.0 indicates a gain constant. Note that F is
calculated as 2 percent taking a periodic gain into account. M
indicates a ratio of spread of a current with respect to an
aperture area of a current confinement layer. In addition,
.alpha..sub.i is represented by the following expression.
.alpha. i = ( L eff - 0.45 m ) 10 cm - 1 + ( 0.45 m ) .alpha. p L
eff ( 22 ) ##EQU00002##
[0252] Here, L.sub.eff (.mu.m) indicates an effective cavity length
taking into account seeping-out of light to DBR. An average
internal loss of an effective cavity portion other than a
seeping-out portion to a p-DBR portion is calculated as 10
cm.sup.-1, and a seeping-out length of DBR is calculated as 0.45
.mu.m.
[0253] In addition, the mirror loss .alpha..sub.m can be
represented by the following expression using a front facet
reflectivity (R.sub.f), a rear facet reflectivity (R.sub.f), and a
cavity length (L).
.alpha. m = 1 2 L ln ( 1 RfRr ) ( 23 ) ##EQU00003##
[0254] External differential quantum efficiency (.eta..sub.d),
external differential quantum efficiency (.eta..sub.f) on the front
facet side, and slope efficiency (S.sub.f) on the front facet side
can be represented by the following expression.
.eta. d = .eta. i .alpha. m .alpha. i + .alpha. m ( 24 )
##EQU00004##
where, .eta..sub.i indicates internal efficiency
.eta. f = .eta. d ( 1 1 + Rf Rr ( 1 - Rr 1 - Rf ) ) ( 25 ) S f =
.eta. f 1.24 .lamda. [ m ] [ W / A ] ( 26 ) ##EQU00005##
[0255] Note that, as a cavity length of a surface emitting laser,
an effective cavity length (L) taking into account seeping-out of
light (0.45 .mu.m on one side) to a semiconductor multilayer mirror
is set in a design value for a cavity itself (2.lamda.) (in this
case, L=1.6 .mu.m). FIG. 30 shows the parameters as a list.
[0256] In this trial manufacturing, since .alpha..sub.m is 5
cm.sup.-1 and Jth as VCSEL is 3.6 kA/cm.sup.2, an absorption loss
(.alpha..sub.c) of the p-type semiconductor multilayer mirror is
estimated as about 80 cm.sup.-1 from the graph shown in FIG. 29. In
this case, .alpha..sub.i is 27 cm.sup.-1. Note that, in a surface
emitting laser in the 850-nanometer-band, an absorption loss is
estimated as .alpha..sub.p=20 cm.sup.-1 by the same method. A
surface emitting laser in the 1.3-.mu.m-band has a larger
absorption loss of the p-type semiconductor multilayer mirror than
that of the surface emitting laser in the 850-nanometer-band. It is
surmised that this is because intervalence band absorption or free
carrier absorption are large in a long wavelength band in a p-type
upper semiconductor multilayer mirror.
[0257] FIG. 31 is a publicly-known graph in which a relation
between a doping concentration and an absorption coefficient for
p-type GaAs is arranged with respect to incident rays of 1.3 .mu.m
and 1.55 .mu.m [D.I. Babic, UCSB technical report 95-20, p 96,
August 1995]. From FIG. 31, since an absorption coefficient is 100
cm.sup.-1 at an impurity concentration of 4.times.10.sup.18
cm.sup.-3, and a GaAs layer and AlGaAs other than an interface of
this time is subjected to p-type doping at an impurity
concentration of 4.times.10.sup.18 cm.sup.-3, this analysis is
considered to be relatively correct.
[0258] Here, it is seen from FIG. 29 that .alpha..sub.p is required
to be 30 cm.sup.-1 or less to realize a sufficiently low
oscillation threshold value by reducing J.sub.th in the vertical
direction to about 1.5 kA/cm.sup.2 or less. In FIG. 31, it is seen
that it is necessary to set an upper limit of an impurity
concentration to 2.times.10.sup.18 cm.sup.-3 for areas other than
interface areas of high-refractive-index layers and
low-refractive-index layers in pairs near an active layer of the
p-type upper semiconductor multilayer mirror. In addition, if
.alpha..sub.p is set to 30 cm.sup.-1, it is possible to increase
the slope efficiency S.sub.f to a high value of 0.1 W/A or more,
and it is possible to obtain optical output power of 0.5 milliwatt
or more at 5 milliamperes.
[0259] Conversely, in pairs near an active layer of the p-type
upper semiconductor multilayer mirror, a lower limit of the
impurity concentration for areas other than interface areas of
high-refractive-index layers and low-refractive-index layers is
calculated on trial as described below. In a surface emitting
laser, a diameter of about 5 .mu.m (an area of about 30
.mu.m.sup.2) is necessary as a current confinement area to realize
oscillation of single lateral mode in a long wavelength band. In
addition, a working voltage of a C-MOS driver, which drives the
surface emitting laser, is 3.3 volts, and an upper limit of 2 volts
is set for the surface emitting laser itself. A threshold voltage
of the surface emitting laser in the 1.3-.mu.m-band is 1.2 volts.
It is necessary to set a bias current to ten times as large as
I.sub.th (=0.5 microampere), that is, 5 microamperes to perform
high-frequency direct modulation at 10 Gbps. Therefore, a
differential resistance (R.sub.d) of the surface emitting laser is
required to satisfy the following inequality.
V.sub.th[=1.2V]+I.sub.op.times.R.sub.d.ltoreq.2[V] (27)
From inequality (27), R.sub.d.ltoreq.160 ohms. In addition, by a
simulation of a circuit by the inventors, it was found that it is
optimum to set the differential resistance to 50 ohms for an
operation at 10 Gbps taking into account impedance matching of the
circuit, and the operation at 10 Gbps is possible up to about 120
ohms. From these findings, it is preferable to set the differential
resistance of the surface emitting laser to 120 ohms or less.
[0260] A structure of the surface emitting laser in the
1.3-.mu.m-band according to the eleventh embodiment is
substantially identical with that of an oxidation confinement type
surface emitting optical laser element in the 850-nanometer-band,
which uses an AlGaAs semiconductor multilayer mirror on a GaAs
substrate, except a structure other than an active layer. Since a
differential resistance of the oxidation confinement type surface
emitting laser in the 850-nanometer-band having a current
confinement area with a diameter of 5 .mu.m (an area of 20
.mu.m.sup.2) is about 65 ohms, it is necessary to control a
resistance increase to be about 60 ohms for the oxidation
confinement type surface emitting laser in the 850-nanometer-band
to reduce the differential resistance of the surface emitting laser
in the 1.3-.mu.m-band to 120 ohms or less.
[0261] FIG. 32 is a table showing an increase in resistance
(.DELTA.R) with respect to the oxidation confinement type surface
emitting laser in the 850-nanometer-band at the time when an
impurity concentration of areas other than interface areas of
low-refractive-index layers and high-refractive-index layers is
changed for five pairs from the GaAs cladding layer 3032 in the
p-type upper semiconductor multilayer mirror.
[0262] From the table shown in FIG. 32, to control the increase in
resistance to about 60 ohms or less in total, it is necessary to
set an impurity concentration of p-type Al.sub.0.9Ga.sub.0.1As
forming the low-refractive-index layer 3051 to 7.5.times.10.sup.17
cm.sup.-3 (=40 ohms increase) or more and set an impurity
concentration of p-type GaAs forming the high-refractive-index
layer 3052 to 2.times.10.sup.17 cm.sup.-3 (=19 ohms increase) or
more, or set an impurity concentration of p-type
Al.sub.0.9Ga.sub.0.1As forming the low-refractive-index layer 3051
to 5.times.10.sup.17 cm.sup.-3 (=53 ohms increase) or more and set
an impurity concentration of p-type GaAs forming the
high-refractive-index layer 3052 to 5.times.10.sup.17 cm.sup.-3
(=6.4 ohms increase) or more.
[0263] Note that, as an example of other combinations, it is also
possible that an impurity concentration of p-type
Al.sub.0.9Ga.sub.0.1As forming the low-refractive-index layer 3051
is set to 1.times.10.sup.18 cm.sup.-3 (=27 ohms increase) or more
and an impurity concentration of p-type GaAs forming the
high-refractive-index layer 3052 is set to 1.times.10.sup.18
cm.sup.-3 (=2.5 ohms increase) or more. In addition, it is also
possible that an impurity concentration of p-type
Al.sub.0.9Ga.sub.0.1As forming the low-refractive-index layer 3051
is set to 2.times.10.sup.18 cm.sup.-3 (=14 ohms increase) or more
and an impurity concentration of p-type GaAs forming the
high-refractive-index layer 3052 is set to 5.times.10.sup.17
cm.sup.-3 (=6.4 ohms increase) or more.
[0264] Summarizing the upper limits and the lower limits explained
above, in the pairs near the active layer of the p-type upper
semiconductor multilayer mirror, an impurity concentration in the
areas other than the interface areas of the high-refractive-index
layers and the low-refractive-index layers is required to satisfy
the following conditions:
low-refractive-index layer: 2.times.10.sup.18
cm.sup.-3.gtoreq.impurity
concentration.gtoreq.7.5.times.10.sup.17 cm.sup.-3
high-refractive-index layer: 2.times.10.sup.18
cm.sup.-3.gtoreq.impurity
concentration.gtoreq.2.times.10.sup.17 cm.sup.-3
or
low-refractive-index layer: 2.times.10.sup.18
cm.sup.-3.gtoreq.impurity
concentration.gtoreq.5.times.10.sup.17 cm.sup.-3
high-refractive-index layer: 2.times.10.sup.18
cm.sup.-3.gtoreq.impurity
concentration.gtoreq.5.times.10.sup.17 cm.sup.-3.
Note that an example adopting the former impurity concentration is
described in the explanation of the structure shown in FIG. 27.
[0265] By adopting the structure according to such design
parameters, CW oscillation at a threshold value of 0.5 milliampere,
slope efficiency of 0.25 W/A, and 100.degree. C. or more was
obtained in the surface emitting laser according to the eleventh
embodiment. Note that a differential resistance was 120 ohms, a
working voltage at 5 milliamperes was 1.8 volts, and an eye pattern
after transmission of 15 kilometers at 10 Gbps was also obtained as
a satisfactory observation result.
[0266] As explained above, according to the surface emitting laser
according to the eleventh embodiment, in low-refractive-index
layers and high-refractive-index layers of a p-type semiconductor
multilayer mirror belonging to a range of the predetermined number
of pairs from an active layer, predetermined areas from interfaces
of the low-refractive-index layers and the high-refractive-index
layers are subjected to p-type doping at a high impurity
concentration, and the remaining areas of the low-refractive-index
layer and the high-refractive-index layer is subjected to p-type
doping at a low impurity concentration equal to or lower than a
predetermined value to reduce a mirror loss. This makes it possible
to perform stable oscillation in a long wavelength band such as the
1.3-.mu.m-band that realizes a low oscillation threshold value, a
low resistance, a low working voltage, high slope efficiency, and
high-frequency direct modulation.
[0267] Note that, in the eleventh embodiment explained above, it is
explained that the high-refractive-index layer forming the p-type
upper semiconductor multilayer mirror 3016 and the n-type lower
semiconductor multilayer mirror 3013 is formed of GaAs. However,
the high-refractive-index layer may be formed of
Al.sub.xGa.sub.1-xAs (0<x.ltoreq.0.2). In addition, it is
explained that the low-refractive-index layer is formed of
Al.sub.0.9Ga.sub.0.1As. However, the low-refractive-index layer may
be formed of Al.sub.xGa.sub.1-xAs (0.5.ltoreq.x.ltoreq.0.97). An
inclined composition layer for easing a refractive index difference
between the low-refractive-index layer and the
high-refractive-index layer may be arranged near a boundary surface
of both the layers.
[0268] In the eleventh embodiment, it is explained that, in the
low-refractive-index layers 3051 and the high-refractive-index
layers 3052 for five pairs from the GaAs cladding layer 3032 of the
p-type upper semiconductor multilayer mirror 3016, areas of 10
nanometer thickness from the interfaces of the low-refractive-index
layers 3051 and the high-refractive-index layers 3052 are subjected
to p-type doping at an impurity concentration of 1 to
2.times.10.sup.19 cm.sup.-3. However, the areas may be areas of 1
nanometer to 40 nanometers thickness, and an impurity concentration
thereof may be set to 2.times.10.sup.19 cm.sup.-3 to
1.times.10.sup.20 cm.sup.-3.
[0269] In the explanation of eleventh embodiment, an impurity
concentration of the low-refractive-index layers 3051 and the
high-refractive-index layers 3052 for five pairs form the GaAs
cladding layer 3032 of the p-type upper semiconductor multilayer
mirror 3016 is limited. However, an impurity concentration of the
low-refractive-index layers 3051 and the high-refractive-index
layers 3052 for one to ten pairs may be limited. In addition, the
p-type AlAs layer 50 forming the oxidation confinement area may be
formed of Al.sub.xGa.sub.1-xAs (0.97.ltoreq.x<0.1).
[0270] In the eleventh embodiment, the surface emitting laser using
the GaInNAsSb QW on the GaAs substrate in the 1300-nanometer-band
is described. However, to create a 1300 nm bad surface emitting
laser on the GaAs substrate, various semiconductor materials like a
GaInNAs QW, a GaAsSb QW, and a (Ga)InAs quantum dot can be used.
Further, the eleventh embodiment is not limited to the surface
emitting laser in the 1300-nanometer-band and can be applied to a
GaAs surface emitting laser in a long wavelength band with a
wavelength of 850 nanometers or more, that is, a
980-nanometer-band, a 1200-nanometer-band, a 1480-nanometer-band,
and a 1550-nanometer-band, a 1650-nanometer-band.
[0271] In the eleventh embodiment, the surface emitting laser using
the AlGaAs semiconductor multilayer mirror on the GaAs substrate is
described as an example. However, the invention can be applied to a
long wavelength band surface emitting laser (1200-nanometer-band to
1650-nanometer-band) using an AlGaAsSb semiconductor multilayer
mirror on an InP substrate. In this case, various semiconductor
materials like AlGaInAs, GaINAsP, and GaInNAs(Sb) can be adopted
for the active layer. In addition, a GaAs substrate, an InP
substrate, a GaInAs ternary substrate can be used as the
substrate.
[0272] Note that, in the eleventh embodiment, a semiconductor
multilayer mirror can also be manufactured by the MOCVD method.
[0273] Next, an optical transceiver according to a twelfth
embodiment of the invention will be explained. FIG. 33 is a block
diagram of a schematic structure of the optical transceiver
according to the twelfth embodiment. In FIG. 33, an optical
transceiver 3070 according to the twelfth embodiment includes a
transceiver 3071 that has an optical transmitting unit 3074 and an
optical receiving unit 3075 for transmitting and receiving an
optical signal, a signal multiplexing circuit 3072 that inputs an
electric signal to the transceiver 3071, and a signal separating
circuit 3073 that separates an electric signal obtained from an
optical signal received by the transceiver 3071.
[0274] The optical transmitting unit 3074 is a unit for converting
an electric signal inputted from the signal multiplexing circuit
3072 into an optical signal and transmitting the optical signal.
More specifically, the optical transmitting unit 3074 includes a
surface emitting laser 3076 that emits an optical signal, a derive
circuit 3077 that controls the surface emitting laser 3076 based on
the inputted electric signal, and an output optical system 3078 for
outputting the optical signal emitted from the surface emitting
laser 3076 to the outside.
[0275] As the surface emitting laser 3076 included in the optical
transmitting unit 3074, the surface emitting laser according to the
eleventh embodiment is used. Therefore, the surface emitting laser
3076 has a low threshold current and high reliability and can
perform single lateral mode oscillation.
[0276] The optical receiving unit 3075 is a unit for converting an
optical signal received from the outside into an electric signal
and outputting the electric signal to the signal separating circuit
3073. More specifically, the optical receiving unit 3075 includes a
photoelectric conversion element 3079 for receiving an optical
signal and converting the optical signal into an electric signal,
an input optical system 3080 for guiding the optical signal to the
photoelectric conversion element 3079, and an amplifier circuit
3081 that amplifies the electric signal outputted from the
photoelectric conversion element 3079. The photoelectric conversion
element 3079 outputs an electric signal based on intensity of the
received optical signal. As the photoelectric conversion element
3079, it is possible to use a photoresistor and the like other than
a photodiode.
[0277] The signal multiplexing circuit 3072 is a circuit for
multiplexing plural electric signals inputted from the outside into
one electric signal. The one electric signal obtained by
multiplexing the electric signals is outputted to the optical
transmitting unit 3074 constituting the transceiver 3071.
[0278] The signal separating circuit 3073 is a circuit for
separating an electric signal obtained from the optical receiving
unit 3075 constituting the transceiver 3071 into plural electric
signals. This is because, since an optical signal received by the
optical receiving unit 3075 originally includes plural signals, to
extract information, it is necessary to separate an electric
signal, which is obtained by subjecting the optical signal to
photoelectric conversion, into plural electric signals.
[0279] Operations of the optical transceiver according to the
twelfth embodiment will be explained. The optical transceiver
according to the twelfth embodiment is an optical transceiver for
transmitting and receiving plural electric signals. A transmission
operation will be explained in the first place.
[0280] First, plural electric signals inputted from the outside is
converted into a single electric signal by the signal multiplexing
circuit 3072. Then, this single electric signal is inputted to the
drive circuit 3077 from the signal multiplexing circuit 3072, and
the control circuit 3077 controls a current to be injected into the
surface emitting laser 3076 based on this electric signal. More
specifically, an optical signal having a waveform corresponding to
an electric signal waveform is emitted from the surface emitting
laser 3076 by the drive circuit 3077. Note that, since the surface
emitting laser element 3076 consists of the surface emitting laser
according to the eleventh embodiment, direct optical modulation is
possible at 10 Gbit/s at the maximum. Therefore, it is possible to
add a large amount of information to an optical signal to transmit
the optical signal. The optical signal outputted from the surface
emitting laser 3076 is outputted to the outside via the output
optical system 3078. Here, the transmission operation ends.
[0281] Next, a reception operation will be explained. An optical
signal transmitted from the outside is made incident in the optical
transceiver via the input optical system 3080 and received by the
photoelectric conversion element 3079. The photoelectric conversion
element 3079 has a function of outputting an electric signal having
a waveform corresponding to a change in intensity of the received
optical signal. The converted electric signal is inputted to the
amplifier circuit 3081. Since intensity of the optical signal
inputted from the outside is generally feeble, intensity of the
electric signal outputted from the photoelectric conversion element
3079 is also feeble and is amplified by the amplifier circuit 3081.
Thereafter, the amplified electric signal is inputted to the signal
separating circuit 3073 and separated into plural electric signals.
Here, the reception operation ends.
[0282] As described above, according to the optical transceiver
according to the twelfth embodiment, since the optical transceiver
is constituted using the surface emitting laser according to the
eleventh embodiment, effects of a low oscillation threshold value,
high slope efficiency, and high-frequency direct modulation can be
realized for the surface emitting laser 3076. It is possible to
output an optical signal having a large amount of information with
high reliability. Moreover, when an outputted optical signal is
transmitted by an optical fiber, a transmittable distance is
increased to 15 kilometers or more, which makes it possible to
perform long distance transmission.
[0283] Next, an optical communication system according to a
thirteenth embodiment of the invention will be explained. FIG. 34
is a schematic diagram of a schematic structure of the optical
communication system according to the thirteenth embodiment. The
optical communication system according to the thirteenth embodiment
uses the surface emitting laser according to the eleventh
embodiment as a signal light source. More specifically, the optical
communication system according to the thirteenth embodiment
includes a signal multiplexing circuit 3091, a drive circuit 3092
connected to the signal multiplexing circuit 3091, a surface
emitting laser 3093 connected to the drive circuit 3092, an optical
fiber for transmission 3095, and an optical system 3094 for
optically combining the surface emitting laser 3093 and an end of
the optical fiber for transmission 3095. In addition, the optical
communication system further includes an photoelectric conversion
element 3097 optically combined with the other end of the optical
fiber for transmission 3095 via the optical system 3096, an
amplifier circuit 3098 connected to the photoelectric conversion
element 3097, and a signal separating circuit 3099 connected to the
amplifier circuit 3098.
[0284] The single electric signal obtained by the signal
multiplexing circuit 3091 is inputted to the drive circuit 3092.
The drive circuit 3092 injects a current into the surface emitting
laser 3093 and drives the surface emitting laser 3093 based on this
electric signal. Consequently, an optical signal outputted from the
surface emitting laser 3093 has a waveform corresponding to an
electric signal obtained by the signal multiplexing circuit 3091.
The optical signal outputted from the surface emitting laser 3093
is made incident in an end of the optical fiber for transmission
3095 via the optical system 3094 and transmitted through the
optical fiber for transmission 3095.
[0285] Then, the optical signal transmitted through the optical
fiber for transmission 3095 is emitted from the other end of the
optical fiber for transmission 3095 and is made incident in the
photoelectric conversion element 3097 via the optical system 3096.
The photoelectric conversion element 3097 outputs an electric
signal based on the received optical signal. The electric signal is
amplified by the amplifier circuit 3098 and then inputted to the
signal separating circuit 3099.
[0286] The signal separating circuit 3099 separates the inputted
electric signal into individual electric signals before being
multiplexed by the signal multiplexing circuit 3091 and restores
information. In this way, the optical communication system
according to the thirteenth embodiment transmits the
information.
[0287] As described above, according to the optical communication
system according to the thirteenth embodiment, since the surface
emitting laser according to the eleventh embodiment is used,
effects of a low oscillation threshold value, high slope
efficiency, and high-frequency direct modulation can be realized.
An optical signal can be transmitted surely with high reliability.
More specifically, it is possible to transmit an optical signal
directly modulated at 10 Gbit/s even if a fiber length of the
optical fiber for transmission 3095 is extended to 15 kilometers or
more.
[0288] Since the surface emitting laser according to the eleventh
embodiment can change an emission wavelength in a range of 980
nanometers to 1650 nanometers, it is possible to select a
wavelength at which a loss is low in the optical fiber for
transmission 3095. In addition, the surface emitting laser also has
an advantage that an existing optical communication system can be
used in these wavelength bands. For example, it is also possible
that the emission wavelength is set to 980 nanometers and an erbium
doped fiber amplifier (EDFA) is arranged in the optical fiber for
transmission 3095. In this case, since intensity of an optical
signal can be amplified by the EDFA, a transmission distance can be
further extended. Similarly, a thulium doped fiber amplifier
(TDFA), a Raman amplifier, or the like may be used.
[0289] As described above, according to the surface emitting laser
according to the invention, since the AlAs layer is included in any
one of the lower semiconductor multilayer mirror and the upper
semiconductor multilayer mirror or both, characteristics of a low
refractive index and high thermal conductivity inherent in the AlAs
layer can be adopted in the semiconductor multilayer mirrors. As a
result, there is an effect that a reflectivity and temperature
characteristics are improved and stable high power laser
oscillation is made possible.
[0290] According to the surface emitting laser according to the
invention, since the inclined composition layer is arranged between
the Al.sub.yGa.sub.1-yAs (x<y<1) layer of the
low-refractive-index area and the Al.sub.xGa.sub.1-xAs
(0.ltoreq.x<1) layer of the high-refractive-index area, there is
an effect that an electric resistance of the semiconductor
multilayer mirror can be further reduced and higher power laser
oscillation is made possible.
[0291] According to the invention, since the structure of the
selectively oxidized area is optimized, there is an effect that a
surface emitting laser, which controls a threshold current to be
low, has high reliability, and is capable of performing single
lateral mode oscillation, performing direct modulation at 10
Gbit/s, and performing long distance transmission, can be
provided.
[0292] According to the invention, since the surface emitting
laser, in which the structure of the selectively oxidized area is
optimized, is used, there is an effect that a transceiver, an
optical transceiver, and an optical communication system, which are
capable of performing single lateral mode oscillation and
performing long distance transmission, can be provided.
[0293] According to the invention, since a reflectivity of the
emission side reflective surface is set to 99.4 percent or more and
99.8 percent or less and a reflectivity of the reflection side
reflective surface is set to 99.9 percent, there is an effect that
a threshold current value can be controlled to be 1 milliampere or
less and slope efficiency can be increased to 0.2 mW/mA or more.
Therefore, there is an effect that, when a current of a value five
or more times as large as the threshold current value, for example,
6 milliamperes is injected, intensity of an emitted laser beam is 1
milliwatt or more and the surface emitting laser can be used as a
signal light source and the like.
[0294] According to the invention, since the above-mentioned
surface emitting laser is used, a light source operating at a low
threshold value and having high optical output power can be used,
and a transceiver, an optical transceiver, and an optical
communication system, which are capable of transmitting an optical
signal a long distance, can be realized.
[0295] According to the surface emitting laser according to the
invention, in the low-refractive-index layers and the
high-refractive-index layer of the p-type semiconductor multilayer
mirror belonging to a range of a predetermined number of pairs from
the active layer, predetermined areas from interfaces of the
low-refractive-index layers and the high-refractive-index layers
are subjected to doping at a high impurity concentration, and the
remaining areas of the low-refractive-index layers and the
high-refractive-index layers are subjected to doping at a low
impurity concentration equal to or lower than a predetermined value
to reduce a mirror loss. Thus, there is an effect that stable
oscillation in a long wavelength band such as the 1.3-.mu.m-band,
which realizes a low oscillation threshold value, a low resistance,
a low working voltage, high slope efficiency, and high-frequency
direct modulation, is made possible.
[0296] According to the transceiver, the optical transceiver, and
the optical communication system according to the invention, since
the above-mentioned surface emitting laser is mounted, there is an
effect that the effects realized by the surface emitting laser can
be enjoyed and stable long distance transmission is made
possible.
INDUSTRIAL APPLICABILITY
[0297] As described above, the invention is suitable for the
surface emitting laser that is capable of performing single lateral
mode oscillation even in a long wavelength, makes long distance
transmission possible, and realizes a low oscillation threshold
value, high slope efficiency, and high-frequency direct modulation,
and the transceiver, the optical transceiver, and the optical
communication system using the surface emitting laser.
* * * * *