U.S. patent application number 11/476077 was filed with the patent office on 2006-11-30 for semiconductor laser device and optical fiber amplifier.
Invention is credited to Takeshi Fujita, Yuuhiko Hamada, Tatsuo Hatta, Kiyohide Sakai, Kimio Shigihara, Yasuaki Yoshida.
Application Number | 20060268396 11/476077 |
Document ID | / |
Family ID | 19177218 |
Filed Date | 2006-11-30 |
United States Patent
Application |
20060268396 |
Kind Code |
A1 |
Sakai; Kiyohide ; et
al. |
November 30, 2006 |
Semiconductor laser device and optical fiber amplifier
Abstract
A semiconductor laser device includes an optical fiber having an
optical fiber grating formed therein, a semiconductor laser having
an active layer with a single quantum well, for emitting laser
light, and a coupling optical system for coupling the laser light
emitted out of the semiconductor laser into the optical fiber. The
coupling optical system can include a narrow-band filter for
adjusting an incident angle of the laser light emitted out of the
semiconductor laser. The optical fiber grating can have a
reflection bandwidth wider than or substantially equal to a 3dB
bandwidth of the gain of the semiconductor laser or a spectrum full
width at half maximum of the laser light of the semiconductor
laser.
Inventors: |
Sakai; Kiyohide; (Tokyo,
JP) ; Hamada; Yuuhiko; (Tokyo, JP) ; Fujita;
Takeshi; (Tokyo, JP) ; Yoshida; Yasuaki;
(Tokyo, JP) ; Shigihara; Kimio; (Tokyo, JP)
; Hatta; Tatsuo; (Tokyo, JP) |
Correspondence
Address: |
BUCHANAN, INGERSOLL & ROONEY PC
POST OFFICE BOX 1404
ALEXANDRIA
VA
22313-1404
US
|
Family ID: |
19177218 |
Appl. No.: |
11/476077 |
Filed: |
June 28, 2006 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
10914134 |
Aug 10, 2004 |
|
|
|
11476077 |
Jun 28, 2006 |
|
|
|
10073221 |
Feb 13, 2002 |
6819688 |
|
|
10914134 |
Aug 10, 2004 |
|
|
|
Current U.S.
Class: |
359/341.1 |
Current CPC
Class: |
H01S 5/028 20130101;
H01S 2301/04 20130101; H01S 5/3406 20130101; H01S 5/34353 20130101;
H01S 5/34313 20130101; H01S 5/0617 20130101; H01S 5/3425 20130101;
H01S 3/09415 20130101; H01S 5/141 20130101; H01S 5/02251 20210101;
B82Y 20/00 20130101; H01S 5/146 20130101; H01S 5/34 20130101 |
Class at
Publication: |
359/341.1 |
International
Class: |
H01S 3/00 20060101
H01S003/00; H04B 10/12 20060101 H04B010/12 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 30, 2001 |
JP |
2001-367479 |
Claims
1. An amplifier for an optical fiber comprising: a semiconductor
laser device including an optical fiber having an optical fiber
grating, a laser diode having a plurality of layers including an
active layer with two or more quantum wells formed at intervals
that are close enough to provide quantum coupling, for emitting
pumping light, another laser device for emitting signal light, and
a coupling optical system for coupling the pumping light emitted
out of the laser diode into the optical fiber; a pumping
light-signal light coupling means for coupling the pumping light
emitted out of the semiconductor laser device to the signal light;
and a rare-earth-doped optical fiber that is pumped by the pumping
light so as to amplify the signal light output from the pumping
light-signal light coupling means.
2. The amplifier according to claim 1, wherein the optical fiber
grating having a wavelength characteristic which maintains a
constant emission wavelength of the laser diode.
Description
[0001] This application is a divisional application of application
Ser. No. 10/914,134, filed Aug. 10, 2004, which in turn is a
divisional application of application Ser. No. 10/073,221, filed
Feb. 13, 2002, the entire disclosure of which is expressly
incorporated herein by reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to a semiconductor laser
device for controlling changes in the emission wavelength of laser
light emitted out of a semiconductor laser by means of an optical
fiber grating. The present invention also relates to an optical
fiber amplifier which uses the semiconductor laser device.
[0004] 2. Description of the Prior Art
[0005] FIG. 11 is a diagram showing the structure of a prior art
semiconductor laser device. In FIG. 11, reference numeral 110
denotes a pump laser module that emits laser light, reference
numeral 120 denotes an optical fiber for guiding the laser light
from the pump laser module 110, and reference numeral 130 denotes
an optical fiber grating formed in the optical fiber 120.
[0006] Furthermore, in the pump laser module 110 of FIG. 11,
reference numeral ill denotes a 980-nm band semiconductor laser
(i.e. laser diode), reference numeral 112 denotes a temperature
monitor for monitoring the temperature of the pump laser module
110, reference numeral 113 denotes a cooler for keeping the
temperature of the pump laser module 110 constant according to the
monitoring result of the temperature monitor 112, and reference
numeral 115 denotes a coupling optical system for coupling light
emitted out of the semiconductor laser 111 into an optical fiber
120.
[0007] 980-nm band laser light is used for the excitation of an
erbium-doped fiber amplifier (EDFA). Since the gain-wavelength
characteristic of EDFA changes when the emission wavelength of the
laser light changes during the excitation, an optical fiber grating
130 is disposed at the output of the pump laser module 110 as
measures against changes in the gain-wavelength characteristic.
[0008] FIG. 12 is a diagram showing an example of the structure of
the semiconductor laser 111. In FIG. 12, reference numeral 111a
denotes an n-type electrode, reference numeral 111b denotes a GaAs
substrate, reference numeral 111c denotes an n-type cladding layer,
reference numeral 111d denotes a multiple quantum well (MQW) active
layer, reference numeral 111e denotes a p-type cladding layer, and
reference numeral 111f denotes a p-type electrode. In the prior art
semiconductor laser device, the semiconductor laser 111 having the
MQW active layer 111d is used.
[0009] FIG. 13 is a diagram showing an energy band structure in the
vicinity of the MQW active layer 111d of the semiconductor laser
111. In FIG. 13, reference numeral 142 denotes a conduction band,
reference numeral 143 denotes a valence band, reference numerals
146A and 146B denote quantum wells, respectively, reference numeral
147 denotes a barrier layer, reference numeral 144 denotes a guide
layer, and reference numeral 145 denotes a cladding layer. Each of
the two quantum wells 146A and 146B is composed of InGaAs of In
chemical composition of 0.2. The barrier layer 147 is composed of
AlGaAs of Al chemical composition of 0.2. The guide layer 144 is
composed of AlGaAs of Al chemical composition of 0.2. The cladding
layer 145 is composed of AlGaAs of Al chemical composition of
0.48.
[0010] In general, the number of wells included in the MQW active
layer 111d ranges from 2 to 4. Each of the two quantum wells 146A
and 146B has a thickness Lz ranging from 5 nm to 15 nm, the barrier
layer 147 has a thickness Lb ranging from 10 nm to 50 nm, and the
guide layer 144 has a thickness ranging from 10 nm to 500 nm. The
Al chemical composition of the above-mentioned AlGaAs is adjusted
between 0.0 and 0.5 from the viewpoint of optical confinement.
[0011] Population inversion is formed by an electric current's
flowing in a forward direction between the p-type electrode 111f
and the n-type electrode 111a, and hence injecting electrons and
holes into the MQW active layer 111d. As a result, the
semiconductor laser 111 oscillates at a 980-nm band of emission
wavelengths determined by the bandgap of the MQW active layer 111d,
and emits laser light to the optical fiber 120 by way of the
coupling optical system 115.
[0012] In general, since the semiconductor laser uses interband
transitions, it has a gain over a wide wavelength range (e.g.,
ten-odd nm). The emission wavelength of the semiconductor laser 111
differs and changes according to chip-to-chip variations and change
in temperature. Therefore, the change in the emission wavelength of
the semiconductor laser device is controlled by the optical fiber
grating 130 disposed as an external resonator in the prior art
semiconductor laser device. For example, details of the
semiconductor laser device provided with the optical fiber grating
130 are disclosed in <Reference 1>.
[0013] <Reference 1>: Martin Achtenhagen, et al.:"L-I
Characteristics of Fiber Bragg Grating Stabilized 980-nm Pump
Lasers", IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 13, NO. 5, MAY
2001.
[0014] When the temperature of the pump laser module 110 changes
greatly because of a self heating of the semiconductor laser 111
and change in ambient temperature, the wavelength characteristic of
the threshold gain distribution also changes. On the other hand,
since the wavelength characteristic of the optical fiber grating
130 remains fixed, the semiconductor laser 111 does not oscillate
in external resonance mode and therefore the emission wavelength
cannot be kept constant.
[0015] To avoid this problem, a temperature control mechanism is
disposed in the semiconductor laser device of FIG. 11. In other
words, the prior art semiconductor laser device is so constructed
as to monitor the temperature of the pump laser module 110 by means
of the temperature monitor 112, to control an electric current
flowing through the cooler 113 by means of a temperature controller
not shown in the figure, and to keep the temperature of the pump
laser module 110 constant. Thus, the semiconductor laser device can
stabilize the emission wavelength, and can control the change in
the gain-wavelength characteristic when applied to EDFA. Japanese
patent application publication No. 2000-353856 discloses a prior
art technology associated with the semiconductor laser device
mentioned above, for example.
[0016] A problem with a prior art semiconductor laser device
constructed as mentioned above is that to keep the emission
wavelength constant the semiconductor laser device has to have a
temperature control mechanism that consists of a temperature
monitor, a temperature controller, a cooler, etc., and the
structure of the semiconductor laser device therefore becomes
complex.
SUMMARY OF THE INVENTION
[0017] The present invention is proposed to solve the
above-mentioned problem, and it is therefore an object of the
present invention to provide a semiconductor laser device having a
simple structure and capable of keeping the emission wavelength
constant without having to use a temperature control mechanism.
[0018] It is another object of the present invention to provide a
semiconductor laser device capable of controlling the change in the
emission wavelength by means of a temperature control mechanism
with low control resolution or low control performance.
[0019] It is a further object of the present invention to provide
an optical fiber amplifier provided with such a semiconductor laser
device as a source of pumping light, and capable of controlling the
change in the gain-wavelength characteristic.
[0020] In accordance with an aspect of the present invention, there
is provided a semiconductor laser device, comprising: an optical
fiber having an optical fiber grating; a semiconductor laser having
an active layer with a single quantum well, for emitting laser
light; and a coupling optical system for coupling the laser light
emitted out of the semiconductor laser into the optical fiber.
[0021] In accordance with another aspect of the present invention,
the coupling optical system includes a narrow-band filter for
adjusting an incident angle of the laser light emitted out of the
semiconductor laser.
[0022] In accordance with a further aspect of the present
invention, the optical fiber grating has a reflection bandwidth
wider than or substantially equal to a 3 dB bandwidth of a gain of
the semiconductor laser or a spectrum full width at half maximum of
the laser light of the semiconductor laser.
[0023] In accordance with another aspect of the present invention,
the coupling optical system has a narrow-band filter for adjusting
an incident angle of the laser light emitted out of the
semiconductor laser. Furthermore, the optical fiber grating has a
reflection bandwidth wider than or substantially equal to a 3 dB
bandwidth of a gain of the semiconductor laser or a spectrum full
width at half maximum of the laser light of the semiconductor
laser.
[0024] In accordance with a further aspect of the present
invention, the coupling optical system includes a collimator lens
for collimating the laser light emitted out of the semiconductor
laser and for outputting the collimated laser light to the
narrow-band filter, and a condenser lens for focusing the laser
light output from the narrow-band filter onto the optical
fiber.
[0025] In accordance with another aspect of the present invention,
the semiconductor laser has an anti-reflection coating with a
reflectivity of about 10% or less, which is formed on an emitting
exit face thereof from which the laser light is emitted.
[0026] In accordance with a further aspect of the present
invention, the anti-reflection coating has a reflectivity lower
than that of the optical fiber grating.
[0027] In accordance with another aspect of the present invention,
the semiconductor laser includes a layer having a refraction index
lower than that of an optical guide layer disposed outside the
active layer with the single quantum well, the layer having such a
thickness as to prevent itself from becoming a barrier that keeps
an electric current from flowing through the semiconductor laser
and the layer being disposed outside the optical guide layer.
[0028] In accordance with a further~aspect of the present
invention, the active layer, a barrier layer, and a guide layer of
the semiconductor laser are configured to have a distortion
compensating structure.
[0029] In accordance with another aspect of the present invention,
the optical fiber grating has a reflection bandwidth of 5 nm or
more.
[0030] In accordance with a further aspect of the present
invention, the narrow-band filter includes an incident angle
adjusting mechanism for adjusting the narrow-band filter so that
the incident angle of the laser light incident on the narrow-band
filter approaches 90 degrees with increasing ambient
temperature.
[0031] In accordance with another aspect of the present invention,
the active layer with the single quantum well of the semiconductor
laser has a thickness ranging from 10 nm to 25 nm.
[0032] In accordance with a further aspect of the present
invention, there is provided a semiconductor laser device,
comprising: an optical fiber having an optical fiber grating; a
semiconductor laser having an active layer with two or more quantum
wells formed at intervals that are close enough to provide quantum
coupling, for emitting laser light; and a coupling optical system
for coupling the laser light emitted out of the semiconductor laser
into the optical fiber.
[0033] In accordance with another aspect of the present invention,
the coupling optical system includes a narrow-band filter for
adjusting an incident angle of the laser light emitted out of the
semiconductor laser.
[0034] In accordance with a further aspect of the present
invention, the optical fiber grating has a reflection bandwidth
wider than or substantially equal to a 3 dB bandwidth of a gain of
the semiconductor laser or a spectrum full width at half maximum of
the laser light of the semiconductor laser.
[0035] In accordance with another aspect of the present invention,
the coupling optical system has a narrow-band filter for adjusting
an incident angle of the laser light emitted out of the
semiconductor laser. Furthermore, the optical fiber grating has a
reflection bandwidth wider than or substantially equal to a 3 dB
bandwidth of a gain of the semiconductor laser or a spectrum full
width at half maximum of the laser light of the semiconductor
laser.
[0036] In accordance with a further aspect of the present
invention, the coupling optical system includes a collimator lens
for collimating the laser light emitted out of the semiconductor
laser-and for outputting the collimated laser light to the
narrow-band filter, and a condenser lens for focusing the laser
light output from the narrow-band filter onto the optical
fiber.
[0037] In accordance with another aspect of the present invention,
the semiconductor laser has an anti-reflection coating with a
reflectivity of about 10% or less, which is formed on an emitting
exit face thereof from which the laser light is emitted.
[0038] In accordance with a further aspect of the present
invention, the anti-reflection coating has a reflectivity lower
than that of the optical fiber grating.
[0039] In accordance with another aspect of the present invention,
the active layer, a barrier layer, and a guide layer of the
semiconductor laser are configured to have a distortion
compensating structure.
[0040] In accordance with a further aspect of the present
invention, the two or more quantum wells are formed at intervals of
8 nm or less.
[0041] In accordance with another aspect of the present invention,
the optical fiber grating has a reflection bandwidth of 5 nm or
more.
[0042] In accordance with a further aspect of the present
invention, the narrow-band filter includes an incident angle
adjusting mechanism for adjusting the narrow-band filter so that
the incident angle of the laser light incident on the narrow-band
filter approaches 90 degrees with increasing ambient
temperature.
[0043] In accordance with another aspect of the present invention,
there is provided an optical fiber amplifier comprising: a
semiconductor laser device including an optical fiber having an
optical fiber grating, a semiconductor laser having an active layer
with a single quantum well, for emitting pumping light, and a
coupling optical system for coupling the pumping light emitted out
of the semiconductor laser into the optical fiber; a pumping
light-signal light coupling unit for coupling the pumping light
emitted out of the semiconductor laser device to signal light; and
a rare-earth-doped optical fiber that is pumped by the pumping
light so as to amplify the signal light output from the pumping
light-signal light coupling unit.
[0044] In accordance with a further aspect of the present
invention, there is provided an optical fiber amplifier comprising:
a semiconductor laser device including an optical fiber having an
optical fiber grating, a semiconductor laser having an active layer
with two or more quantum wells formed at intervals that are close
enough to provide quantum coupling, for emitting pumping light, and
a coupling optical system for coupling the pumping light emitted
out of the semiconductor laser into the optical fiber; a pumping
light-signal light coupling unit for coupling the pumping light
emitted out of the semiconductor laser device to signal light; and
a rare-earth-doped optical fiber that is pumped by the pumping
light so as to amplify the signal light output from the pumping
light-signal light coupling unit.
[0045] Accordingly, in accordance with the present invention, the
semiconductor laser device can keep the emission wavelength of the
laser light constant with a simple structure and without any
temperature control mechanism. In addition, by using a temperature
control mechanism which is so simply structured that its
temperature control resolution or its temperature control
performance is reduced, the semiconductor laser device can control
changes in the emission wavelength.
[0046] Further objects and advantages of the present invention will
be apparent from the following description of the preferred
embodiments of the invention as illustrated in the accompanying
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0047] FIG. 1 is a diagram showing the structure of a semiconductor
laser device according to a first embodiment of the present
invention;
[0048] FIG. 2 is a diagram showing an energy band structure in the
vicinity of an SQW active layer of a semiconductor laser of the
semiconductor laser device according to the first embodiment;
[0049] FIGS. 3(a) and 3(b) ate diagrams showing that the emission
wavelength of the semiconductor laser is kept constant by means of
an optical fiber grating;
[0050] FIGS. 4(a) and 4(b) are diagrams showing an advantage of the
semiconductor laser device according to the first embodiment of the
present invention;
[0051] FIG. 5 is a diagram showing the structure of a semiconductor
laser device according to a second embodiment of the present
invention;
[0052] FIG. 6 is a diagram showing an energy band structure in the
vicinity of a coupling MQW active layer of a semiconductor laser of
the semiconductor laser device according to the second
embodiment;
[0053] FIGS. 7(a) and 7(b) are diagrams showing an advantage of the
semiconductor laser device according to the second embodiment of
the present invention;
[0054] FIG. 8 is a diagram showing the structure of a semiconductor
laser device according to a third embodiment of the present
invention;
[0055] FIG. 9 is a diagram showing the structure of a semiconductor
laser device according to a fourth embodiment of the present
invention;
[0056] FIG. 10 is a diagram showing an energy band structure in the
vicinity of an active layer of a semiconductor laser with a
distortion compensation structure of the semiconductor laser device
according to a fourth embodiment;
[0057] FIG. 11 is a diagram showing the structure of a prior art
semiconductor laser device;
[0058] FIG. 12 is a diagram showing an example of the structure of
a semiconductor laser of the prior art semiconductor laser
device;
[0059] FIG. 13 is a diagram showing an energy band structure in the
vicinity of an MQW active layer of the semiconductor laser; and
[0060] FIG. 14 is a diagram showing the structure of a
semiconductor laser device according to another embodiment of the
present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0061] Hereafter, preferred embodiments of the present invention
will be explained.
Embodiment 1
[0062] FIG. 1 is a diagram showing the structure of a semiconductor
laser device according to a first embodiment f the present
invention. In FIG. 1, reference numeral 10 denotes a pump laser
module that emits laser light, reference numeral 20 denotes an
optical fiber for guiding the laser light from the pump laser
module 10, and reference numeral 30 denotes an optical fiber
grating formed in the optical fiber 20.
[0063] Furthermore, in the pump laser module 10 of FIG. 1,
reference numeral 11 denotes a semiconductor laser (i.e. laser
diode) having a single quantum well (SQW) active layer, reference
numeral 14 denote an anti-reflection coating formed on a laser
light emitting exit face of the semiconductor laser 11, and
reference numeral 15 denote a coupling optical system for coupling
the laser light emitted out of the laser light emitting exit face
of the semiconductor laser 11 into the optical fiber 20.
[0064] The semiconductor laser 11 with the SQW active layer is used
in the pump laser module 10 of FIG. 1 as can be seen compared with
a prior art semiconductor laser device. In contrast, no temperature
monitor for monitoring the temperature of the pump laser module 10
and no temperature control mechanism, such as a cooler, for keeping
the temperature of the pump laser module 10 constant are
disposed.
[0065] FIG. 2 is a diagram showing an energy band structure in the
vicinity of the SQW active layer of the semiconductor laser 11. In
FIG. 2,reference numeral 41 denotes the SQW active layer, reference
numeral 42 denotes a conduction band, reference numeral 43 denotes
a valence band, reference numeral 44 denotes a guide layer, and
reference numeral 45 denotes a cladding layer. The SQW active layer
41 is composed of InGaAs of In chemical composition of 0.2, the
guide layer 44 is composed of AlGaAs of Al chemical composition of
0.2, and the cladding layer 45 is composed of AlGaAs of Al chemical
composition of 0.48.
[0066] The SQW active layer 41 of the semiconductor laser 11 can
have a thickness Lz ranging from 10 nm to 25 nm. For example, the
SQW active layer 41 of the semiconductor laser 11 has a thickness
Lz of 18 nm. The guide layer 44 can have a thickness ranging from
10 nm to 500 nm. The Al chemical composition of AlGaAs is adjusted
so that it falls within a range of 0.0 to 0.5 from the viewpoint of
optical confinement.
[0067] The semiconductor laser device of the present invention has
the following two features.
[0068] <Feature 1>: When the emission wavelength of the
semiconductor laser 11 differs or changes from its original value,
the semiconductor laser device can keep the emission wavelength
constant by means of the optical fiber grating 30.
[0069] <Feature 2>: When the temperature of the pump laser
module 10 changes greatly, the semiconductor laser device can keep
the emission wavelength constant over a wide range of change in the
temperature without having to use a temperature control mechanism.
This is because the width of the gain spectrum of the semiconductor
laser 11 is wide as described later, and therefore the optical
fiber grating 30 is so constructed that its wavelength locking
cannot be released easily.
[0070] First of all, the principle of <Feature 1> will be
explained generally.
[0071] FIGS. 3(a) and 3(b) are diagrams for explaining an operation
of the semiconductor laser device when the emission wavelength is
kept constant by means of the optical fiber grating. FIG. 3(a)
shows changes in the gain spectrum with changes in an electric
current passing through the active layer of the semiconductor
laser, and FIG. 3(b) shows changes in the gain spectrum with
changes in the temperature of the pump laser module. The horizontal
axis represents the emission wavelength (nm) and the vertical axis
represents the gain (cm.sup.-1).
[0072] In FIGS. 3(a) and 3(b), S.sub.1 to S.sub.7 denote gain
spectrums of the semiconductor laser. As shown in FIG. 3(a), the
gain spectrum S.sub.1 at a certain electric current value in turn
changes into the gain spectrum S.sub.2 and then the gain spectrum
S.sub.3 with increasing electric current value. Although each of
the gain spectrums S.sub.1 to S.sub.7 actually has a complex shape,
as illustrated in FIGS. 4(a) and 4(b), the shapes of the gain
spectrums S.sub.1 to S.sub.7 are simply illustrated in FIGS. 3(a)
and 3(b) for the sake of simplicity.
[0073] Furthermore, a straight line indicated by a reference
character G of FIGS. 3(a) and 3(b) is an optical gain in the case
of a single semiconductor laser. Here, the single semiconductor
laser means a case where no optical fiber grating is disposed at
the output of the semiconductor laser device. When a loss in the
semiconductor laser is a, the length of cavity is L, the
reflectivity of a front facet of the semiconductor laser is
R.sub.f, the reflectivity of a back facet of the semiconductor
laser is R.sub.r, and the optical confinement coefficient is
.GAMMA., the optical gain G is given by the following equation (1).
.GAMMA.G=.alpha.+(0.5/L)ln[1/(R.sub.fR.sub.r)] (1)
[0074] In addition, a bent segment of the optical gain G indicated
by a reference string G.sub.fg of FIGS. 3(a) and 3(b) is a mode
gain when the optical fiber grating is disposed at the output of
the semiconductor laser device. By using an equivalent front
reflectivity R.sub.eff of the semiconductor laser when the optical
fiber grating is disposed at the output of the semiconductor laser
device, the mode gain G.sub.fg is given by the following equation
(2). .GAMMA.G.sub.fg=.alpha.+(0.5/L)ln[1(R.sub.ffR.sub.f)] (2)
[0075] In the above equations (1) and (2), the loss in the
semiconductor laser a ranges from 1 to 20 cm.sup.-1, the length L
of the cavity ranges from 500 .mu.m to 3000 .mu.m, the front
reflectivity R.sub.f is 10% or less, the equivalent front
reflectivity R.sub.eff is 20% or less, the back reflectivity
R.sub.r is 80% or more, and the optical confinement coefficient
.GAMMA. ranges from 0.001 to 0.1. In this example, .alpha.=4
cm.sup.-1, L=900 .mu.m, R.sub.f=1%, R.sub.eff=4%, R.sub.r=98%,
F=0.0175, and therefore G=1700 cm.sup.-1 and G.sub.fg=1260
cm.sup.-1.
[0076] When no optical fiber grating is disposed in the
semiconductor laser device, the semiconductor laser oscillates at a
wavelength .lamda..sub.1 at which the maximum value of the gain
spectrum S.sub.2 agrees with the mode gain G. On the other hand,
when the optical fiber grating is disposed in the semiconductor
laser device, the semiconductor laser oscillates at an injection
current at which the gain spectrum S.sub.1 at an electric current
less than that for the gain spectrum S.sub.2 agrees with the mode
gain G.sub.fg, and the emission wavelength becomes equal to a
reflection wavelength .lamda..sub.fg of the optical fiber grating,
e.g., 980 nm. In the first and second embodiments, a 980-nm band
pump laser module is illustrated as an example. As an alternative,
the first embodiment can be applied to a 1480-nm band pump laser
module.
[0077] As the temperature of the pump laser module rises, the gain
spectrum S.sub.1 changes into the gain spectrum S.sub.4 and then
the gain spectrum S.sub.5, as shown in FIG. 3(b), and the
semiconductor laser enters a state at which its emission wavelength
is locked to the constant wavelength .lamda..sub.fg by the optical
fiber grating. However, at the gain spectrum S.sub.5, the
semiconductor laser enters a state in which the two emission
wavelengths .lamda..sub.fg and .lamda..sub.2 go into competition
with each other.
[0078] In addition, when the temperature of the pump laser module
further rises and the semiconductor laser then enters a state of
the gain spectrum S.sub.6 with a maximum value which exceeds the
mode gain G, the wavelength locking by the optical fiber grating is
released because the electric current for the gain spectrum S.sub.7
with a maximum value equal to the optical gain G is less than that
for the gain spectrum S.sub.6 and the semiconductor laser
oscillates at a wavelength .lamda..sub.3 at which the gain spectrum
S.sub.7 agrees with the optical gain G.
[0079] As can be seen from the above-mentioned calculation, since
by reducing the reflectivity R.sub.f of the anti-reflection coating
disposed on the laser light emitting exit face of the semiconductor
laser to a low one (particularly 10% or less), the difference
between the mode gain G in the case of a single semiconductor laser
and the mode gain G.sub.fg in the case of having an optical fiber
grating can be increased, it is possible to maintain the wavelength
locking by the optical fiber grating over a wider temperature
range.
[0080] By the way, as can be seen from the explanation about FIG.
3(b), the wider the width of the gain spectrum in the vicinity of
980 nm and the flatter the peak of the gain spectrum, the wider the
range of change in the temperature for the wavelength locking can
be made to become. In general, while a semiconductor laser with an
MQW active layer has an advantage of achieving a single mode
oscillation easily because its gain spectrum has a narrow full
width at half maximum (FWHM), the semiconductor laser has a
disadvantage resulting from the above-mentioned reason from the
viewpoint of the wavelength locking by the optical fiber
grating.
[0081] Next, based upon the above-mentioned fact a description will
be made as to the reason why the temperature range in which the
wavelength locking can be carried out is extended, i.e.,
<Feature 2> in the semiconductor laser device according to
the first embodiment provided with the semiconductor laser 11
having an SQW active layer.
[0082] FIGS. 4(a) and 4(b) are diagrams for explaining the
advantage of the semiconductor laser device according to the first
embodiment of the present invention, and the horizontal axis
represents the wavelength (nm) and the vertical axis represents the
gain (cm.sup.-1). FIG. 4(a) shows an example of calculation of the
gain spectrum when the thickness of the well is 8 nm, which
corresponds to the gain spectrum of the prior art MQW active layer
111d, and FIG. 4(b) shows an example of calculation of the gain
spectrum when the thickness of the well is 18 nm, which corresponds
to the gain spectrum of the SQW active layer 41 of the present
invention.
[0083] Six curves of FIGS. 4(a) and 4(b) show gain spectrums when
changing the carrier density from 1.times.10.sup.17cm.sup.-3 to
3.1.times.10.sup.18 cm.sup.-3 in steps of 6.0.times.10 cm.sup.-3,
which correspond to the ones when the semiconductor laser is placed
in an oscillation state in a case where the gain at a wavelength of
980 nm becomes 1600 cm.sup.-1, for example (in the case of FIGS.
3(a) and 3(b)). As can be seen from a comparison of FIG. 4(a) and
FIG. 4(b), while the gain spectrum of the MQW active layer 111d in
the vicinity of 980 nm has a narrow width and varies abruptly with
wavelength when the semiconductor laser is placed in the
oscillation state, the gain spectrum of the SQW active layer 41 in
the vicinity of 980 nm has a wide width and is flat because of an
influence of a e2-hh2 transition. Therefore, due to the fact that
the wider the width of the gain spectrum in the vicinity of 980 nm
and the flatter the peak of the gain spectrum, the wider the range
of change in the temperature for the wavelength locking can be made
to become, the semiconductor laser device of FIG. 1 can keep the
emission wavelength of the laser light constant over a wide
temperature range.
[0084] In general, a semiconductor laser with an MQW active layer
has an advantage that it easily enters a single-mode of operation
because its gain spectrum has a narrow FWHM, whereas a
semiconductor laser with an SQW active layer has a disadvantage
that it does not easily enter a single-mode of operation. However,
there is a trade-off between the single-mode of operation and the
wavelength locking using the optical fiber grating 30. In
accordance with the first embodiment of the present invention, the
range of the wavelength locking using the optical fiber grating 30
can be extended based on the fact that the gain spectrum of the SQW
active layer has a wide FWHM and is flat, which usually becomes a
drawback.
[0085] Thus the semiconductor laser device of the first embodiment
with a simple structure can reduce changes in the emission
wavelength over a wide temperature range without any temperature
control mechanism, unlike a prior art semiconductor laser device.
As an alternative, the semiconductor laser device of the first
embodiment can be provided with a temperature control mechanism. In
this case, the temperature control mechanism for reducing changes
in the emission wavelength is so simply structured that its
temperature control resolution or its temperature control
performance is reduced as compared with that required for a prior
art semiconductor laser device. In addition, the first embodiment
offers an advantage of being able to permit easing of management of
combinations of variations in the threshold gain band of the
semiconductor laser 11, which occur during manufacturing, and the
wavelength characteristic of the optical fiber grating 30.
[0086] As mentioned above, in accordance with the first embodiment
of the present invention, the semiconductor laser device comprises
an optical fiber 20 having an optical fiber grating 30 formed
therein, for guiding laser light, and a pump laser module 10
including a semiconductor laser 11 having an SQW active layer 41,
for emitting laser light, and a coupling optical system 15 for
coupling the laser light emitted out of the semiconductor laser 11
into the optical fiber 20. Accordingly, the semiconductor laser
device can keep the emission wavelength of the laser light constant
over a wider temperature range compared with prior art
semiconductor laser devices. The semiconductor laser device with a
simple structure can thus reduce changes in the emission wavelength
without any temperature control mechanism. The semiconductor laser
device of the first embodiment can be provided with a temperature
control mechanism which is so simply structured that its
temperature control resolution or its temperature control
performance is reduced so as to control changes in the emission
wavelength. In addition, the first embodiment offers an advantage
of being able to permit easing of management of combinations of
variations in the threshold gain band of the semiconductor laser
11, which occur during manufacturing, and the wavelength
characteristic of the optical fiber grating 30.
[0087] Furthermore, in accordance with the first embodiment, the
semiconductor laser 11 has an anti-reflection coating 14 with a
reflectivity of about 10% or less which is formed on laser light
emitting exit face thereof. Accordingly, the semiconductor laser
device can maintain the wavelength locking by using the optical
fiber grating 30 over a wider temperature range.
[0088] In addition, in accordance with the first embodiment, the
anti-reflection coating 14 has a reflectivity lower than that of
the optical fiber grating 30. Accordingly, the semiconductor laser
device can maintain the wavelength locking by using the optical
fiber grating 30 over a wider temperature range.
[0089] In addition, in accordance with the first embodiment, the
semiconductor laser 11 includes a single quantum well with an
active layer having a thickness ranging from 10 nm to 25 nm.
Accordingly, the gain spectrum of the SQW -active layer can have a
wide FWHM and can be flat, and therefore the range of the
wavelength locking using the optical fiber grating 30 can be
extended.
Embodiment 2
[0090] FIG. 5 is a diagram showing the structure of a semiconductor
laser device according to a second embodiment of the present
invention. The same reference numerals as shown in FIG. 1 denote
the same components as those of the first embodiment or like
components. In FIG. 5, reference numeral 16 denotes a semiconductor
laser having a coupling multiple quantum well (coupling MQW) active
layer.
[0091] FIG. 6 is a diagram showing an energy band structure in the
vicinity of the coupling MQW active layer of the semiconductor
laser 16. The same reference numerals as shown in FIG. 2 denote the
same components as those of the first embodiment or like
components. In FIG. 6, reference numerals 46A and 46B denote
quantum wells each of which is composed of InGaAs of In chemical
composition of 0.2, reference numeral 47 denotes a barrier layer
that is composed of AlGaAs of Al chemical composition of 0.2, and
reference numeral 48 denotes a coupling MQW active layer in which
the two quantum wells 46A and 46B are coupled to each other.
[0092] Each of the quantum wells 46A and 46B of the coupling MQW
active layer 48 has a thickness Lz ranging from 5 nm to 15 nm, the
barrier layer 47 has a thickness Lb ranging from 0.1 nm to 8 nm,
and the guide layer 44 has a thickness ranging from 10 nm to 500
nm. In this example, Lz=8 nm, the thickness Lb of the barrier layer
47=3 nm, and the thickness of the guide layer 44=50 nm. Other
conditions, such as Al chemical composition, are equal to those
shown in FIG. 2.
[0093] The semiconductor laser device according to second
embodiment has <Feature 1>, like the above-mentioned first
embodiment, because it is provided with an optical fiber grating
30. In addition, since the width of the barrier layer 47 of the
semiconductor laser 16 is narrowed to 3 nm to provide quantum
coupling, the levels are divided by the interaction between the
levels according to the tunnel effect, and the gain spectrum of the
semiconductor laser 16 is extended. In other words, the
semiconductor laser device of the second embodiment has also
<Feature 2>, like the first embodiment.
[0094] Furthermore, since the coupling MQW active layer 48 is used,
it is possible to easily provide a sufficient optical confinement
coefficient even if the thickness Lz of each of the two quantum
wells 46A and 46B is reduced to below its critical thickness, and
it is therefore possible to improve the degree of freedom of the
design. Here, the critical thickness is the limit of an active
layer thickness at which no crystal defect is caused even if the
lattice constant of the active layer differs from that of the
substrate. A crystal defect is caused when the active layer has a
thickness equal to or greater than the critical thickness.
Therefore when using a material, such as InGaAs, with a lattice
constant different from that of GaAs, as the active layer, the
critical thickness is an important index.
[0095] Since the semiconductor laser according to the second
embodiment includes the two quantum wells 46A and 46B each having a
thickness equal to or less than the critical thickness, the
semiconductor laser device does not suffer from a problem of
crystal defect generation due to a mechanical stress, and failures
such as degradation of the initial performance, decrease in the
light output, and quenching, and therefore the reliability of the
semiconductor laser device can be improved.
[0096] Next, the reason why the temperature range where the
wavelength locking can be carried out is extended when the
semiconductor laser 16 with the coupling MQW active layer 48
according to the second embodiment is used will be explained.
[0097] FIGS. 7(a) and 7(b) are diagrams for explaining advantages
of the semiconductor laser device according to the second
embodiment of the present invention. FIG. 7(a) shows a band
structure of a single quantum well having a thickness Lz of 8 nm,
and FIG. 7(b) shows a band structure when coupling according to the
tunnel effect is provided between the two quantum wells. FIG. 7(a)
and FIG. 7(b) show the structure of the active layer 111d of the
prior art multiple quantum well semiconductor laser shown in FIG.
13 and that of the coupling MQW active layer 48 of the
semiconductor laser 16 of the second embodiment of the present
invention shown in the FIG. 6, respectively. In FIG. 7(b), the
thickness Lz of each of the two wells is narrowed to 8 nm and the
thickness Lb of the barrier layer is narrowed to 3 nm.
[0098] In FIGS. 7(a) and 7(b), reference numeral 51 denotes a band
for electron, reference numeral 52H denotes a band for heavy hole,
reference numeral 52L denotes a band for light hole, reference
numerals 53 and 54 denote a first sub-band and a second sub-band
for electron, respectively, reference numerals 55 and 56 denote a
first sub-band and a second sub-band for heavy hole, respectively,
reference numerals 57 and 58 denote a first sub-band and a second
sub-band for light hole, respectively, and reference numeral 59
denotes a barrier layer.
[0099] As explained in <Reference 2> mentioned below, in FIG.
7(a), a transition (e1-hh1) from the first sub-band 55 for heavy
hole to the first sub-band 53 for electron and a transition
(e1-1h1) from the first sub-band 57 for light hole to the first
sub-band 53 for electron are acceptable. On the other hand, in FIG.
7(b), since the width of the barrier layer is narrowed to 3 nm, the
levels are divided by the interaction among the levels due to the
tunnel effect and a transition (e2-hh2) from the second sub-band 56
for heavy hole to the second sub-band 54 for electron and a
transition (e2-1h2) from the second sub-band 58 for light hole to
the second sub-band 54 for electron are acceptable in addition to
the transition (e1-hh1) and the transition (e1-1h1).
[0100] Therefore, as in the case of FIG. 4(b), the range of the
wavelength locking using the optical fiber grating 30 can be
extended because the gain spectrum in the vicinity of the e1-hh1
transition becomes flat under the influence of other
transitions.
[0101] <Reference 2>: R. Dingle:"Confined Carrier Quantum
State Ultrathin Semiconductor Heterostructure", Festkoerperplobleme
XV, Advances in Solid-State Physics, pp. 21 to 48 (1975).
[0102] Since the gain spectrum of the coupling MQW active layer has
a wide FWHM, and the semiconductor laser with the coupling MQW
active layer does not oscillate easily in single mode, the coupling
MQW active layer is not generally used in the prior art. In
contrast, in accordance with the second embodiment, through the use
of the wide FWHM of the gain spectrum of the coupling MQW active
layer, which is a drawback usually, the range of the wavelength
locking using the optical fiber grating 30 is extended.
[0103] As mentioned above, in accordance with the second embodiment
of the present invention, the semiconductor laser device comprises
an optical fiber 20,having an optical fiber grating 30 formed
therein, for guiding laser light, and a pump laser module 10
including a semiconductor laser 16 having a coupling MQW active
layer, for emitting laser light, and a coupling optical system 15
for coupling the laser light emitted out of the semiconductor laser
16 into the optical fiber 20. Accordingly, the semiconductor laser
device can widen the threshold gain band of the semiconductor
laser. Like the semiconductor laser of the first embodiment, the
semiconductor laser device with a simple structure can also keep
the emission wavelength of-the laser light constant over a wider
temperature range without any temperature control mechanism. The
semiconductor laser device of the second embodiment can be provided
with a temperature control mechanism which is so simply structured
that its temperature control resolution or its temperature control
performance is reduced so as to control changes in the emission
wavelength. In addition, the second embodiment offers an advantage
of being able to permit easing of management of combinations of
variations in the threshold gain band of the semiconductor laser
16, which occur during manufacturing, and the wavelength
characteristic of the optical fiber grating 30. The second
embodiment offers another advantage of being able to reduce lattice
distortions, which can occur between the active layer and the
substrate, thereby improving the reliability of the semiconductor
laser.
Embodiment 3
[0104] FIG. 8 is a diagram showing the structure of a semiconductor
laser device according to a third embodiment of the present
invention. The same reference numerals as shown in FIG. 1 denote
the same components as those of the first embodiment or like
components. In FIG. 8, reference numeral 17 denotes a narrow-band
filter.
[0105] The narrow-band filter 17 is disposed between a collimator
lens 15A and a condenser lens 15B which constitute a coupling
optical system 15, and has a structure that makes it possible to
arbitrarily set an incident angle of laser light from a
semiconductor laser 11 upon the narrow-band filter 17. Furthermore,
in accordance with the third embodiment, an optical fiber grating
30 formed in an optical fiber 20 has a reflection bandwidth that is
set so that it is wider than or substantially equal to a 3 dB
bandwidth of the gain of the semiconductor laser 11 or a spectrum
FWHM of the semiconductor laser 11.
[0106] The laser light, which is emitted out by way of an
anti-reflection coating 14 with a reflectivity of 10% or less which
is formed on laser light emitting exit face of the semiconductor
laser 11, is transformed into parallel light by the collimator lens
15A and is then incident upon the narrow-band filter 17. The laser
light, which penetrates the narrow-band filter 17, is focused onto
the optical fiber 20 by the condenser lens 15B.
[0107] The reflection characteristic of the optical fiber grating
30 cannot easily be changed because it is determined when the
grating pattern of the optical fiber grating is produced.
Therefore, the reflection bandwidth of the optical fiber grating 30
is set so that it is wider than or substantially equal to the 3 dB
bandwidth of the gain of the semiconductor laser 11 or the spectrum
FWHM of the semiconductor laser 11, and the incident angle of the
laser light with the narrow-band filter 17 can be changed when
assembling or adjusting the module.
[0108] The incident angle of the laser light with the narrow-band
filter 17 can be adjusted and fixed when assembling or adjusting
the module and the wavelength of light that can transmit through
the narrow-band filter 17 can be set to a desired one so that the
optical fiber grating 30 can reflect light having a wavelength
suitable for the threshold gain band even if variations in the
threshold gain band are caused due to variations in the
manufacturing of the semiconductor laser 11.
[0109] As mentioned above, in accordance with the third embodiment
of the present invention, the coupling optical system 15 comprises
a narrow-band filter 17 for adjusting an incident angle of the
laser light emitted out of the semiconductor laser 11. Accordingly,
the third embodiment offers an advantage of making it possible for
the optical fiber grating 30 to reflect light having a wavelength
suitable for the threshold gain band even if variations in the
threshold gain band are caused due to variations in the
manufacturing of the semiconductor laser 11.
[0110] Furthermore, in accordance with the third embodiment, since
the reflection bandwidth of the optical fiber grating 30 is set so
that it is wider than or substantially equal to the 3 dB bandwidth
of the gain of the semiconductor laser 11 or the spectrum FWHM of
the semiconductor laser 11, the optical fiber grating 30 can
reflect light having a wavelength suitable for the threshold gain
band even if variations in the threshold gain band are caused due
to variations in the manufacturing of the semiconductor laser
11.
[0111] In addition, in accordance with the third embodiment, since
the coupling optical system,15 comprises a narrow-band filter 17
for adjusting an incident angle of the laser light emitted out of
the semiconductor laser 11, and the reflection bandwidth of the
optical fiber grating 30 is set so that it is wider than or
substantially equal to the 3 dB bandwidth of the gain of the
semiconductor laser 11 or the spectrum FWHM of the semiconductor
laser 11, the optical fiber grating 30 can reflect light having a
wavelength suitable for the threshold gain band even if variations
in the threshold gain band are caused due to variations in the
manufacturing of the semiconductor laser 11.
[0112] In addition, in accordance with the third embodiment, the
coupling optical system 15 includes a collimator lens 15A for
collimating the laser light emitted out of the semiconductor laser
and for outputting the collimated laser light to the narrow-band
filter 17, and a condenser lens 15B for focusing the laser light
output from the narrow-band filter 17 onto the optical fiber.
Accordingly, the third embodiment offers another advantage of being
able to couple the laser light from the semiconductor laser 11 into
the optical fiber 20 by way of the narrow-band filter 17 with a
high degree of efficiency.
Embodiment 4
[0113] The narrow-band filter 17 shown in the above-mentioned third
embodiment can be applied to the semiconductor laser 16 with a
coupling MQW active layer shown in the above-mentioned second
embodiment. In this case, the same advantages as offered by the
third -embodiment are provided. FIG. 9 is a diagram showing the
structure of a semiconductor laser device according to a fourth
embodiment of the present invention. The same reference numerals as
shown in FIGS. 5 and 8 denote the same components as those of the
second and third embodiments or like components.
[0114] A narrow-band filter 17 is disposed between a collimator
lens 15A and a condenser lens 15B which constitute a coupling
optical system 15, and has a structure that makes it possible to
set an incident angle of laser light from a semiconductor laser 16
with an MQW active layer to an arbitrary one.
[0115] Furthermore, an optical fiber grating 30 formed in an
optical fiber 20 has a reflection bandwidth that is set so that it
is wider than or substantially equal to a 3 dB bandwidth of the
gain of the semiconductor laser 11 or a spectrum FWHM of the
semiconductor laser 11, as in the above-mentioned third
embodiment.
[0116] The laser light, which is emitted by way of an
anti-reflection coating 14 with a reflectivity of 10% or less which
is formed on a laser light emitting exit face of the semiconductor
laser 16, is transformed into parallel light by the collimator lens
15A and is then incident upon the narrow-band filter 17. The laser
light, which penetrates the narrow-band filter 17, is focused onto
and is incident upon the optical fiber 20 by the condenser lens
15B.
[0117] The reflection characteristic of the optical fiber grating
30 cannot easily be changed because it is determined when the
grating pattern of the optical fiber grating is produced.
Therefore, the reflection bandwidth of the optical fiber grating 30
is set so that it is wider than or substantially equal to the 3 dB
bandwidth of the gain of the semiconductor laser 11 or the spectrum
FWHM of the semiconductor laser 11, and the incident angle of the
laser light with the narrow-band filter 17 can be changed when
assembling or adjusting the module.
[0118] Thus, the incident angle of the laser light with the
narrow-band filter 17 can be adjusted and fixed when assembling or
adjusting the module and the wavelength of light that can transmit
through the narrow-band filter 17 can be set to a desired one so
that the optical fiber grating 30 can reflect light having a
wavelength suitable for the threshold gain band even if variations
in the threshold gain band are caused due to variations in the
manufacturing of the semiconductor laser 16, as in the
above-mentioned third embodiment.
Embodiment 5
[0119] By disposing a layer for relieving a distortion due to
grid-interval mismatching outside the SQW active layer of the
semiconductor laser 11 of the above-mentioned first embodiment or
the coupling MQW active layer of the semiconductor laser 16 of the
above-mentioned first embodiment (e.g., by disposing a layer having
a refractive index lower than that of a light guide layer disposed
outside the SQW active layer and such a thickness as to prevent
itself from becoming a barrier that keeps an electric current from
flowing through the semiconductor laser outside the light guide
layer), or by making the active layer, the barrier layer, and the
guide layer be of distortion compensation structure, the frequency
of occurrence of crystal defects can be reduced and the rate of
accidental failure can be reduced.
[0120] FIG. 10 is a diagram showing an energy band structure in the
vicinity of an active layer of a semiconductor laser of distortion
compensation structure. In FIG. 10, reference numeral 61 denotes a
conduction band, reference numeral 62 denotes a valence band,
reference numerals 63A and 63B denote quantum wells, respectively,
reference numeral 64 denotes a barrier layer, reference numeral 65
denotes a guide layer, and reference numeral 66 denotes a cladding
layer. Each of the two quantum wells 63A and 63B is composed of
InGaAs of In chemical composition of 0.2, the barrier layer 64 is
composed of GaAsP, the guide layer 65 is composed of
Ga0.8In0.2As0.62P0.38, and the cladding layer 66 is composed of
Ga0.51In0.49P. The quantum well 63 has a thickness of 8 nm, the
barrier layer 64 has a thickness of 20 nm, and the guide layer 65
has a thickness of 80 nm.
[0121] As explained in the above-mentioned second embodiment, a
distortion is caused in the crystal because InGaAs has a lattice
constant different from that of a GaAs substrate. Between two
materials, there can be provided a ratio of a difference between
the lattice constant of one of them and that of the other material
to the lattice constant of one of them, which is called the amount
of distortion. In this-example, by setting the amount of distortion
between the barrier layer and the GaAs substrate to be -1.0%, and
setting the amount of distortion between each quantum well and the
GaAs substrate to be +1.4%, the average amount of distortion
between the substrate and the vicinity of the active layer can be
reduced.
[0122] By using a semiconductor laser with an SQW active layer or a
coupling MQW active layer, in which a layer for relieving a
distortion due to grid-interval mismatching is disposed outside the
SQW active layer or the coupling MQW active layer, or the active
layer, the barrier layer, and the guide layer are made to be of
distortion compensation structure, as the semiconductor laser 11 of
the above-mentioned first embodiment or the semiconductor laser 16
of the above-mentioned second embodiment, the fifth embodiment
offers an advantage of being able to generate laser light output
having a stable emission wavelength over a wide temperature range
without any temperature control, like the above-mentioned first or
second embodiment. The fifth embodiment also offers an advantage of
being able to permit easing of management of combinations of
variations in the threshold gain band of the semiconductor laser,
which occur during manufacturing, and the wavelength characteristic
of the optical fiber grating 30 even when a temperature monitor and
a cooler are provided.
[0123] As mentioned above, in according to the fifth embodiment of
the present invention, the semiconductor laser 11 or 16 includes a
layer having a refraction index lower than that of the optical
guide layer disposed outside the active layer with the single
quantum well, the layer having such a thickness as to prevent
itself from becoming a barrier that keeps an electric current from
flowing through the semiconductor laser and the layer being
disposed outside the optical guide layer. Accordingly, the fifth
embodiment offers an advantage of being able to reduce the
frequency of occurrence of crystal defects and-the rate of
accidental failure.
[0124] Furthermore, in accordance with the fifth embodiment, the
active layer, the barrier layer, and the guide layer of the
semiconductor laser 11 or 16 are configured to have a distortion
compensating structure. Even in this case, the fifth embodiment
offers an advantage of being able to reduce the frequency of
occurrence of crystal defects and the rate of accidental
failure.
Embodiment 6
[0125] By setting intervals at which two or more quantum wells are
formed in a semiconductor laser 16 with a coupling MQW active layer
shown in the above-mentioned second embodiment to be 8 nm or less,
the two quantum wells can be coupled to each other with a high
degree of efficiency. Therefore, since the semiconductor laser 16
can provide a wide threshold gain band and high efficiency, and can
generate laser light having a stable emission wavelength over a
wide temperature range, like the second above-mentioned embodiment,
the semiconductor laser device can provide laser light having a
stable emission wavelength over a wide temperature range without
any temperature control by forming an external resonator by
optically coupling the semiconductor laser 16 to an optical fiber
20 having an optical fiber grating 30.
[0126] Furthermore, like the above-mentioned fourth embodiment, by
arranging a narrow-band filter 17 located on an emitting exit face
side of the semiconductor laser 16, an optical fiber 20 into which
laser light from the semiconductor laser 16 is coupled by way of
the narrow-band filter 17, and an optical fiber grating 30 formed
in the optical fiber 20i and having a reflection bandwidth wider
than or substantially equal to a 3 dB bandwidth of the gain of the
semiconductor laser 16 or a spectrum FWHM of the semiconductor
laser 16, laser light having a stable emission wavelength can be
generated over a wide temperature range without any temperature
control.
[0127] The sixth embodiment offers an advantage of being able to
permit easing of management of combinations of variations in the
threshold gain band of the semiconductor laser 16, which occur
during manufacturing, and the wavelength characteristic of the
optical fiber grating 30 even when the semiconductor laser device
is provided with a temperature monitor and a cooler.
[0128] As mentioned above, in accordance with the sixth embodiment
of the present invention, since the semiconductor laser 16 has two
or more quantum wells formed at intervals of 8 nm or less, quantum
coupling can be provided with a high degree of efficiency.
Embodiment 7
[0129] An optical fiber grating having a reflection bandwidth of 5
nm or more can be used as an optical fiber grating 30 shown in the
above-mentioned third or fourth embodiment. Through the use of the
optical fiber grating 30 an external resonator is formed and laser
light having a stable emission wavelength can be produced over a
wide temperature range. Furthermore, the semiconductor laser device
can accommodate to variations in the threshold gain band of the
semiconductor laser with the single kind of optical fiber grating
30.
[0130] As mentioned above, in accordance with the seventh
embodiment of the present invention, since the semiconductor laser
device includes the optical fiber grating 30 having a reflection
bandwidth of 5 nm or more, the optical fiber grating 30 can form an
external resonator, and the semiconductor laser device can generate
laser light having a stable emission wavelength over a wide
temperature range and can accommodate to variations in the
threshold gain band of the semiconductor laser.
Embodiment 8
[0131] The semiconductor laser device according to the
above-mentioned first or second embodiment can be provided with a
semiconductor laser 11 or 16 having an anti-reflection coating 14
with a reflectivity of 10% or less formed on a laser light emitting
exit face thereof, and an optical fiber 20 containing an optical
fiber grating 30 with a reflectivity higher than that of the
anti-reflection coating 14. The semiconductor laser can therefore
oscillate at the same wavelength as the reflection wavelength of
the optical fiber grating 30 over a wide temperature range.
[0132] Furthermore, the semiconductor laser device according to the
above-mentioned third or fourth embodiment can be provided with a
semiconductor laser 11 or 16 having an anti-reflection coating 14
with a reflectivity of 10% or less formed on a laser light emitting
exit face thereof, and an optical fiber 20 containing an optical
fiber grating 30 with a reflectivity higher than that of the
anti-reflection coating 14. The semiconductor laser can therefore
oscillate at a wavelength associated with the wavelength
characteristic of a narrow-band filter 17 used in the
above-mentioned third or fourth embodiment.
Embodiment 9
[0133] There can be provided an incident angle adjustment mechanism
for adjusting the incident angle of laser light incident upon a
narrow-band filter 17 so that it approaches 90.sup.0at high ambient
temperature. For example, as shown in FIG. 9, this incident angle
adjustment mechanism 40 includes a temperature monitor 41 for
monitoring the temperature of the semiconductor laser device, and a
control unit 42 for storing a relationship between the incident
angle of the laser light with the narrow-band filter 17 and the
transmission property of the narrow-band filter 17 while making a
function and table associated with the relationship, and adjusting
the incident angle of the laser light with the narrow-band filter
17 by referring to the temperature (i.e. thermistor 43) monitored
by the temperature monitor and the function and table associated
with the relationship. The incident angle of the laser light with
the narrow-band filter-17 can be varied according to the
temperature of the semiconductor laser device so that light having
a certain wavelength always passes through the narrow-band filter
17.
[0134] Thus, the provision of the incident angle adjustment
mechanism for adjusting the incident angle of laser light incident
upon the narrow-band filter 17 so that it approaches 90.degree. at
high ambient temperature makes it possible to make the reflection
bandwidth of the optical fiber grating 30 change into a longer one
with increasing ambient temperature, and make the semiconductor
laser device oscillate in external resonator mode over a wide
temperature range, thereby maintaining the emission wavelength so
that it falls within the bandwidth of the gain of an optical fiber
amplifier including the semiconductor laser device.
[0135] There can be provided an optical fiber amplifier that
includes the semiconductor laser device according to any one of the
above-mentioned first through ninth embodiments as an excitation
light source. The optical fiber amplifier can comprise a pumping
light-signal light coupling unit 50 for coupling laser light
emitted, as pumping light, by the semiconductor laser device and
another laser light provided as signal light 51, and a
rare-earth-doped optical fiber 52 which is pumped by the pumping
light from the pumping light-signal light coupling unit so as to
amplify the signal light from the pumping light-signal light
coupling unit. The optical fiber amplifier can thus control the
change in the gain-wavelength characteristic of the semiconductor
laser device.
[0136] As mentioned above, in accordance with the ninth embodiment
of the present invention, since the narrow-band filter 17 is
provided with an incident angle adjustment mechanism for adjusting
the incident angle of laser light incident thereupon so that it
approaches 90.degree. at high ambient temperature, the
semiconductor laser device makes it possible to make the reflection
bandwidth of the optical fiber grating 30 change into a longer one
with increasing ambient temperature, and make the semiconductor
laser oscillate in external resonator mode over a wide temperature
range, thereby maintaining the emission wavelength so that it falls
within the gain bandwidth of an optical fiber amplifier including
the semiconductor laser device.
[0137] Furthermore, in accordance with the ninth embodiment, there
can be provided an optical fiber amplifier that includes a
semiconductor laser device according to the present invention, a
pumping light-signal light coupling unit for coupling laser light
emitted, as pumping light, by the semiconductor laser device and
other laser light provided as signal light, and a rare-earth-doped
optical fiber which is pumped by the pumping light from the pumping
light-signal light coupling unit so as to amplify the signal light
from the pumping light-signal light coupling unit, thereby
controlling the change in the gain-wavelength characteristic of the
semiconductor laser device.
[0138] Many widely different embodiments of the present invention
may be constructed without departing from the spirit and scope of
the present invention. It should be understood that the present
invention is not limited to the specific embodiments described in
the specification, except as defined in the appended claims.
* * * * *