U.S. patent application number 11/021320 was filed with the patent office on 2005-06-02 for distributed feedback laser with differential grating.
Invention is credited to Deng, Qing, Giaretta, Giorgio, Lenosky, Thomas.
Application Number | 20050117622 11/021320 |
Document ID | / |
Family ID | 34619179 |
Filed Date | 2005-06-02 |
United States Patent
Application |
20050117622 |
Kind Code |
A1 |
Lenosky, Thomas ; et
al. |
June 2, 2005 |
Distributed feedback laser with differential grating
Abstract
This disclosure concerns distributed feedback ("DFB") lasers. In
one example, a DFB laser includes a body that has first and second
end facets. The DFB laser is implemented in a stack configuration
that includes an active region interposed between a first top layer
and a substrate. A second top layer is disposed on the first top
layer and has an index of refraction different from that of the
first top layer. Additionally, a grating is defined in one of the
top layers and extends from the first end facet to the second end
facet. The grating includes a tooth/gap structure whose
configuration varies between the first end facet and the second end
facet. Finally, an antireflective (AR) coating is disposed on the
first end facet and on the second end facet.
Inventors: |
Lenosky, Thomas; (Mountain
View, CA) ; Giaretta, Giorgio; (Mountain View,
CA) ; Deng, Qing; (San Jose, CA) |
Correspondence
Address: |
ERIC L. MASCHOFF
WORKMAN, NYDEGGER & SEELEY
1000 Eagle Gate Tower
60 East South Temple
Salt Lake City
UT
84111
US
|
Family ID: |
34619179 |
Appl. No.: |
11/021320 |
Filed: |
December 22, 2004 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
11021320 |
Dec 22, 2004 |
|
|
|
10284128 |
Oct 30, 2002 |
|
|
|
Current U.S.
Class: |
372/96 |
Current CPC
Class: |
H01S 5/1212 20130101;
H01S 5/124 20130101; H01S 5/12 20130101; H01S 5/1215 20130101; H01S
5/1225 20130101 |
Class at
Publication: |
372/096 |
International
Class: |
H01S 003/08 |
Claims
What is claimed is:
1. A distributed feedback (DFB) laser, comprising: a body having a
first end facet and a second end facet, and comprising: a
substrate; first and second top layers stacked together, the first
top layer having an index of refraction that is different from an
index of refraction of the second top layer; an active region
interposed between the first top layer and the substrate; and a
grating defined in one of the top layers, the grating extending
from the first end facet to the second end facet and the grating
including a tooth/gap structure whose configuration varies between
the first end facet and the second end facet; and an antireflective
(AR) coating disposed on the first end facet and on the second end
facet.
2. The DFB laser as recited in claim 1, wherein the substrate is
n-doped and the first and second top layers are p-doped.
3. The DFB laser as recited in claim 1, wherein the substrate is
p-doped and the first and second top layers are n-doped.
4. The DFB laser as recited in claim 1, wherein the first and
second top layers are in substantially continuous contact with each
other.
5. The DFB laser as recited in claim 1, wherein the active region
includes a plurality of quantum wells.
6. The DFB laser as recited in claim 1, wherein the grating is
defined in the first top layer.
7. The DFB laser as recited in claim 1, wherein the grating is
configured to have a first value of reflectivity ".kappa."
proximate the first end facet and a second value of reflectivity
".kappa." proximate the second end facet, the first value of
reflectivity ".kappa." being different from the second value of
reflectivity ".kappa.."
8. The DFB laser as recited in claim 1, wherein the grating
comprises first and second portions and is implemented such that a
configuration of the tooth/gap structure in the first portion is
different from a configuration of the tooth/gap structure in the
second portion.
9. The DFB laser as recited in claim 8, wherein the first portion
of the grating extends approximately from the first end facet to a
midpoint of the grating, and the second portion of the grating
extends approximately from the midpoint of the grating to the
second end facet.
10. The DFB laser as recited in claim 1, wherein one portion of the
grating differs from another portion of the grating with regard to
one or more of the following parameters: tooth geometry; and, tooth
spacing.
11. The DFB laser as recited in claim 1, wherein a transition
between differing portions of the grating occurs relatively
abruptly.
12. The DFB laser as recited in claim 1, wherein a transition
between differing portions of the grating occurs relatively
gradually.
13. The DFB laser as recited in claim 1, wherein the tooth/gap
structure of the grating is substantially symmetric with respect to
a reference point.
14. The DFB laser as recited in claim 1, wherein the tooth/gap
structure of the grating is substantially asymmetric with respect
to a reference point.
15. The DFB laser as recited in claim 1, wherein at least one
parameter of the grating structure varies substantially
continuously between the first end facet and the second end
facet.
16. A distributed feedback (DFB) laser, comprising: a body having a
first end facet and a second end facet, and comprising: a doped
substrate; top and bottom confinement layers, the bottom
confinement layer being disposed on the doped substrate; an active
layer interposed between the top and bottom confinement layers;
first and second top layers stacked together and the first top
layer being disposed on the top confinement layer, each of the top
layers being doped and the first top layer having an index of
refraction that is different from an index of refraction of the
second top layer; and a grating defined in the first top layer, the
grating extending from the first end facet to the second end facet
and the grating including a tooth/gap structure whose configuration
varies between the first end facet and the second end facet; a
contact layer disposed on the second top layer; and an
antireflective (AR) coating disposed on the first end facet and on
the second end facet.
17. The DFB laser as recited in claim 16, wherein a width of each
tooth in at least a portion of the grating is substantially equal
to one-quarter the wavelength of the light waves emitted by the DFB
laser.
18. The DFB laser as recited in claim 16, wherein teeth of the
tooth/gap structure have a substantially square cross-section.
19. The DFB laser as recited in claim 16, wherein teeth of the
tooth/gap structure have a substantially rectangular
cross-section.
20. The DFB laser as recited in claim 16, wherein the first and
second top layers are in substantially continuous contact with each
other.
21. The DFB laser as recited in claim 16, wherein the active region
includes a plurality of quantum wells.
22. The DFB laser as recited in claim 1, wherein the grating is
configured to have a first value of reflectivity ".kappa."
proximate the first end facet and a second value of reflectivity
".kappa." proximate the second end facet, the first value of
reflectivity ".kappa." being different from the second value of
reflectivity ".kappa.."
23. The DFB laser as recited in claim 16, wherein the grating
comprises first and second portions and is implemented such that a
configuration of the tooth/gap structure in the first portion is
different from a configuration of the tooth/gap structure in the
second portion.
24. The DFB laser as recited in claim 23, wherein the first portion
of the grating extends approximately from the first end facet to a
midpoint of the grating, and the second portion of the grating
extends approximately from the midpoint of the grating to the
second end facet.
25. The DFB laser as recited in claim 16, wherein one portion of
the grating differs from another portion of the grating with regard
to one or more of the following parameters: tooth geometry; and,
tooth spacing.
26. The DFB laser as recited in claim 16, wherein a transition
between differing portions of the grating occurs relatively
abruptly.
27. The DFB laser as recited in claim 16, wherein a transition
between differing portions of the grating occurs relatively
gradually.
28. The DFB laser as recited in claim 16, wherein the tooth/gap
structure of the grating is substantially symmetric with respect to
a reference point.
29. The DFB laser as recited in claim 16, wherein the tooth/gap
structure of the grating is substantially asymmetric with respect
to a reference point.
30. The DFB laser as recited in claim 16, wherein at least one
parameter of the grating structure varies substantially
continuously between the first end facet and the second end
facet.
31. The DFB laser as recited in claim 30, wherein the at least one
parameter of the grating structure comprises a tooth period.
32. The DFB laser as recited in claim 16, further comprising at
least one phase shifting tooth portion disposed in the grating.
33. The DFB laser as recited in claim 32, wherein the at least one
phase shifting tooth portion is disposed proximate one of the first
and second end facets.
Description
RELATED APPLICATIONS
[0001] This application is a divisional, and claims the benefit, of
U.S. patent application Ser. No. 10/284,128, entitled DISTRIBUTED
FEEDBACK LASER HAVING A DIFFERENTIAL GRATING, filed Oct. 30, 2002,
incorporated herein in its entirety by this reference.
BACKGROUND OF THE INVENTION
[0002] 1. The Field of the Invention
[0003] The present invention generally relates to semiconductor
laser devices. More particularly, the present invention relates to
a distributed feedback laser device having a structure that
improves both manufacturing yield and operating performance of the
laser device.
[0004] 2. The Related Technology
[0005] Semiconductor lasers are currently used in a variety of
technologies and applications, including optical communications
networks. One type of semiconductor laser is the distributed
feedback (DFB) laser. The DFB laser produces a stream of coherent,
monochromatic light by stimulating photon emission from a solid
state material. DFB lasers are commonly used in optical
transmitters, which are responsible for modulating electrical
signals into optical signals for transmission via an optical
communication network.
[0006] Generally, a DFB laser includes a positively or negatively
doped bottom layer or substrate, and a top layer that is oppositely
doped with respect to the bottom layer. An active region, bounded
by confinement regions, is included at the junction of the two
layers. These structures together form the laser body. A coherent
stream of light that is produced in the active region of the DFB
laser can be emitted through either longitudinal end, or facet, of
the laser body. One facet is typically coated with a high
reflective material that redirects photons produced in the active
region toward the other facet in order to maximize the emission of
coherent light from that facet end.
[0007] A grating is included in either the top or bottom layer to
assist in producing a coherent photon beam. For example, the
grating is typically produced in the top layer of the DFB laser
body by depositing a first p-doped top layer having a first index
of refraction atop the active region, then etching evenly spaced
grooves into the first top layer to form a tooth and gap cross
sectional grating structure along the length of the grating. A
second p-doped top layer having a second index of refraction is
deposited atop the first top layer such that it covers and fills in
the grating structure. During operation of the DFB laser, the tooth
and gap structure of the grating, which is overlapped by optical
field patterns created in the active region, provides reflective
surfaces that couple both forward and backward propagating coherent
light waves that are produced in the active region of the laser.
Thus, the grating provides feedback, thereby allowing the active
region to support coherent light wave oscillation. This feedback
occurs along the length of the grating, hence the name of
distributed feedback laser. After reflection is complete, the
amplified light waves are then output via the output end facet as a
coherent light signal. DFB lasers are typically known as single
mode devices as they produce light signals at one of several
distinct wavelengths, such as 1,310 nm or 1,550 nm. Such light
signals are appropriate for use in transmitting information over
great distances via an optical communications network.
[0008] DFB lasers as described above are typically mass produced on
semiconductor wafers. Many DFB laser devices can be formed on a
single wafer. After fabrication, the DFB lasers are separated from
one another by a cleaving process, which cuts each device from the
wafer. This cleaving process creates each end facet of the DFB
device body. Unfortunately, limitations inherent in the cleaving
process do not allow the laser device to be cut such that a
precisely desired distance is established between the end facet and
the nearest adjacent grating tooth.
[0009] The inherent variability of the distance between the end
facet and the adjacent grating tooth created as a result of
cleaving can cause several problems. First, the end facet,
especially an end facet that is coated with a high reflective
coating, may be disposed adjacent the nearest grating tooth such
that the laser during operation will exhibit poor sidemode
suppression, which in turn results in undesired optical frequencies
being amplified within the laser device. These undesired optical
frequencies can spoil the monochromatic nature of the DFB laser
output and result in reduced performance for the apparatus in which
the laser device is disposed.
[0010] Other problems that can arise from the arbitrary cleaving
process include an increased incidence of chirp and low power
output from the DFB laser device. Chirp, or the drifting of the
optical output wavelength over time, is magnified by improper
distances between the grating and the high-reflective end facet
caused by the cleaving process. Similarly, low power output is
evidence of less-than-ideal cleaving of the DFB laser device.
[0011] If one or more of the above-described problems is detected
in a particular DFB laser device after manufacture and testing, it
often must be discarded, thereby lowering the yield of acceptable
DFB laser devices that are produced from a wafer. In some cases,
the percentage of rejected devices suffering from any of the above
problems can exceed 50% per wafer.
[0012] Attempts to mitigate the effects of low precision cleaving
have involved the addition of one or more quarter phase shifts in
the grating. However, the typical DFB grating has a continuous
pattern over the entire wafer. This continuous pattern allows for
the lithography to be simple. Yet, the installation of one or more
quarter phase shifts requires the use of a special lithography
apparatus. Additionally, special techniques are required in order
to add such phase shifts. These special requirements necessarily
increase the cost of production of each DFB device.
[0013] In light of the above, it would be desirable to enable the
production of DFB laser devices where the yield per wafer is
substantially increased. Further, a need exists for the DFB laser
to exhibit good sidemode suppression while limiting chirp and
output power loss. Moreover, such a solution should be simply
implemented, thereby limiting production cost increases.
BRIEF SUMMARY OF EXEMPLARY EMBODIMENTS OF THE INVENTION
[0014] Briefly summarized, embodiments of the present invention are
directed to a DFB laser device that overcomes the problems created
by imprecise cleaving operations performed on DFB devices during
their manufacture. Specifically, exemplary embodiments of the
invention are concerned with a DFB laser having a differential
grating configuration suitable for high yield manufacture and
desirable operating characteristics, such as good sidemode
suppression, low chirp, and controlled reflectivity and optical
emission.
[0015] One exemplary DFB laser includes a body that has first and
second end facets. The DFB laser is implemented in a stack
configuration that includes an active region interposed between a
first top layer and a substrate. A second top layer is disposed on
the first top layer and has an index of refraction different from
that of the first top layer. Additionally, a grating is defined in
one of the top layers and extends from the first end facet to the
second end facet. The grating includes a tooth/gap structure whose
configuration varies between the first end facet and the second end
facet. Finally, an antireflective (AR) coating is disposed on the
first end facet and on the second end facet.
[0016] These and other aspects of exemplary embodiments of the
invention will become more fully apparent from the following
description and appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] To further clarify the above and other advantages and
features of the present invention, a more particular description of
the invention will be rendered by reference to specific embodiments
thereof that are illustrated in the appended drawings. It is
appreciated that these drawings depict only typical embodiments of
the invention and are therefore not to be considered limiting of
its scope. The invention will be described and explained with
additional specificity and detail through the use of the
accompanying drawings in which:
[0018] FIG. 1 is a perspective cutaway view of a distributed
feedback laser device manufactured in accordance with one
embodiment of the present invention;
[0019] FIG. 2 is a cross sectional view of the laser of FIG. 1;
[0020] FIG. 3A is a close-up cross sectional view of a portion of
the grating structure shown in FIG. 2 according to one embodiment
thereof;
[0021] FIG. 3B is a close-up cross sectional view of a portion of
the grating structure shown in FIG. 2 according to another
embodiment thereof;
[0022] FIG. 3C is a close-up cross sectional view of a portion of
the grating structure shown in FIG. 2 according to yet another
embodiment thereof;
[0023] FIG. 4 is a cross sectional view of a distributed feedback
laser made in accordance with one embodiment of the present
invention;
[0024] FIG. 5A is a close-up cross sectional view of one portion of
the grating structure shown in FIG. 4;
[0025] FIG. 5B is a close-up cross sectional view of another
portion of the grating structure shown in FIG. 4;
[0026] FIG. 5C is a close-up cross sectional view of yet another
portion of the grating structure shown in FIG. 4;
[0027] FIG. 6A is a close-up cross sectional view of one portion of
the grating structure shown in FIG. 4 according to an alternative
embodiment;
[0028] FIG. 6B is a close-up cross sectional view of another
portion of the grating structure shown in FIG. 4 according to an
alternative embodiment; and
[0029] FIG. 6C is a close-up cross sectional view of yet another
portion of the grating structure shown in FIG. 4 according to an
alternative embodiment.
DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS OF THE INVENTION
[0030] Reference will now be made to figures wherein like
structures will be provided with like reference designations. It is
understood that the drawings are diagrammatic and schematic
representations of presently preferred embodiments of the
invention, and are not limiting of the present invention nor are
they necessarily drawn to scale.
[0031] FIGS. 1-6C depict various features of embodiments of the
present invention, which is generally directed to a distributed
feedback ("DFB") laser that is configured so as to exhibit improved
operating characteristics. The present DFB laser further comprises
a design that enables it to be manufactured such that laser device
yield per wafer is substantially improved.
[0032] Reference is first made to FIG. 1, which shows a cutaway
view of a DFB laser device made in accordance with one embodiment
of the present invention, and which is generally designated at 10.
The DFB laser 10 includes an n-doped bottom layer or substrate 12
on which a bottom confinement layer 14 is disposed. An active layer
16, comprising a plurality of quantum wells or other similar
structure, is disposed atop the confinement layer 14 and is covered
by a top confinement layer 18. A p-doped first top layer 20
overlies the confinement layer 18. A grating 22 is defined in the
first top layer 20. A p-doped second top layer 24 is disposed atop
the grating 22. Alternatively, the first and second top layers 20
and 24 can be n-doped, while the substrate 12 is p-doped. A contact
layer 26 for providing an electrical signal to the DFB laser 10 is
disposed atop the second top layer 24. The various layers described
above extend between a first end facet 28 and a second end facet
30, partially shown in FIG. 1. FIG. 1 illustrates several basic
components that generally comprise the DFB laser device 10. It is
appreciated that additional or alternative layers or structures can
be incorporated into the present laser device as will be understood
by those of skill in the art.
[0033] Reference is now made to FIGS. 2 and 3A, which show the DFB
laser device 10 of FIG. 1 in cross section. In the illustrated
embodiment, the first and second end facets 28 and 30 are shown
having an anti-reflective ("AR") coating 32 disposed thereon. The
AR coating 32, which is typically applied after cleaving, reduces
reflection of internal light waves off of the end facets 28 and 30
during laser operation and instead allows the light to exit the
laser device 10 through the end facets. Though it allows light
waves to exit out of both end facets 28 and 30, the present DFB
laser device 10 is configured to direct a majority of the coherent
light produced by the laser through only one end facet, as will be
described.
[0034] As mentioned, the grating 22 is disposed on a top surface of
the first top layer 20. In greater detail, the grating 22 comprises
a periodic series of closely-spaced grooves that are etched or
otherwise defined in the first top layer 20. As will be seen, the
grooves, when seen in cross section, define a series of teeth 34
protruding from the first top layer 20, and gaps 36 between
adjacent teeth. The gaps 36 are filled with the second top layer 24
such that continuous contact is established between the first top
layer 20 and the second top layer. Definition of the grating 22 on
the first top layer 20 can be accomplished using known grating
techniques, including electron beam lithography.
[0035] Though both the first top layer 20 and second top layer 24
are similarly doped, each has a distinct index of refraction with
respect to one another. As seen in FIG. 3A, which is magnified to
show grating detail, the first top layer 20 has an index of
refraction n1, while the second top layer has an index of
refraction n2. This relative refractive index disparity is required
to enable each tooth 34 to act as a feedback surface for reflecting
light waves and enabling their coherent propagation within the DFB
laser 10, as is known in the art. Thus, the grating serves as a
boundary between two similarly doped materials having distinct
indices of refraction. Further details concerning the grating 22
are discussed further below.
[0036] As already described, both the first and second end facets
28 and 30 are coated with AR coating 32. By making each end facet
light transmissive via the AR coating 32, problems that otherwise
arise with respect to the imprecise distance between a respective
end facet and the last tooth 34 adjacent thereto is eliminated. In
other words, lightwaves that would otherwise reflect off the high
reflective end facet (as in the prior art) that is potentially not
properly positioned with respect to adjacent teeth 34 of the
grating 22 are not, in fact, reflected to undesirably interact with
the coherent light waves within the laser device, but are instead
allowed to pass through the anti-reflective facet and exit the
device. In this way, any problems normally created as a result of
the inherent randomness in the cleaving process that defines the
end facets 28 and 30 of the DFB device 10 are eliminated by making
each end facet anti-reflective via the AR coating 32. This in turn
improves the yield of DFB laser devices from a given wafer while
improving the sidemode suppression characteristics of each such
device.
[0037] Notwithstanding the improvements in light emission integrity
made possible by the above AR-coated end facets 28 and 30, this by
itself is insufficient to optimize coherent light emission from the
DFB laser 10. Without further modification, a laser device having
AR-coated end facets will emit approximately one-half of its
coherent light through either facet. This results in a significant
waste of light emission.
[0038] To alleviate the above situation, the grating 22 is modified
according to principles taught according to the present invention
so as to direct the majority of coherent light emission through
only one of the end facets. This is accomplished by anisotropically
altering the physical configuration of the grating 22 as a function
of position along the grating length. For example, FIG. 3A, which
is a close-up view of the circled portion in FIG. 2, shows a
portion of the grating 22 near a longitudinal mid-point 38 of the
grating length. The grating 22 is bifurcated by an imaginary
bifurcation line at the mid-point 38 wherein each half of the
grating length defines a distinct tooth and gap structure. On the
left side of the mid-point 38 as seen in FIG. 3, a first half 22A
of the grating structure, comprising periodic teeth 34A and gaps
36A, is substantially uniform. In contrast to this, a second half
22B of the grating 22, beginning at and continuing to the right of
the mid-point 38, is characterized by a second order tooth
structure, wherein every second tooth 34B that would otherwise be
present (shown in dashes) is missing, and in its place a gap 36B
having twice the normal length is disposed. This second order
structure shown in FIG. 3A continues from the mid-point 38 along
the entire length of the second grating half 22B to the second end
facet 30 shown in FIG. 2. Thus, a non-uniform tooth and gap
structure is established along the length of the grating 22.
[0039] Because of the non-uniform grating structure along the
length of the grating 22, the reflectivity per unit length of the
grating, or kappa (".kappa."), which is related to the particular
configuration of the grating, is also non-uniform along the grating
length. In the present embodiment, .kappa. is high on the uniform
first grating half 22A, and relatively lower on the second grating
half 22B. Because .kappa. is directly related to the number of
times a light wave will be reflected from the surfaces of the
grating teeth, a lower .kappa. number associated with the second
grating half 22B indicates that light waves will be reflected by
the grating less than those waves traveling through the first
grating half 22A, which possesses a higher .kappa. value. This is
so because of the particular tooth and gap structure of each
grating half 22A and 22B. In the first grating half 22A, for
instance, a propagating light wave created in the active region 16
will encounter a tooth/gap interface, and thus a reflective
opportunity, at every interval "a," corresponding to the repetitive
period of the teeth 34 and gaps 36. On the other hand, a light wave
propagating through the second order structure of the second
grating half 22B encounters a tooth/gap interface only half as many
times as in first half 22A. Thus the light wave is reflected fewer
times, which in turn increases the number of light signals that are
able to progress to and pass through the second end facet 30.
Correspondingly, because a light signal passing though the higher
.kappa. value first grating half 22A encounters more reflective
opportunities, relatively fewer signals are able to reach and pass
through the first end facet 28. Consequently, a substantial
majority of light waves pass through the second end facet 30 when
the DFB laser 10 is configured with a grating as shown in FIGS. 2
and 3A.
[0040] In addition to the second order tooth and gap configuration
shown in the second grating half 22B, the grating could be modified
to alternatively include a third, fourth, or higher order tooth and
gap configuration, if desired. For instance, in a third order
configuration, every third tooth is missing from the grating
structure. Such grating configurations can be designed so as to
achieve the desired reduction or increase in the .kappa. value for
the particular grating portion involved.
[0041] It is noted that the bifurcated grating structure in FIG. 3A
is separated by the imaginary bifurcation line located at the
mid-point 38. However, it is not necessary that the bifurcation
occur at the mid-point 38. Indeed, and in agreement with the
teachings herein, the division of grating structure topology can be
defined at any appropriate point along the length of the grating
22. Thus, in the present example the second order grating structure
can alternatively occupy one-third of the length of the grating 22
nearest the second end facet 30, while the uniform grating
structure portion is defined along the remainder of the grating
length. Moreover, the transition from uniform grating structure to
second order structure is seen in FIG. 3A occurring abruptly at the
mid-point 38. However, the present invention is not restricted to
such a configuration. Indeed, the transition from one grating
structure to another can occur abruptly or gradually, as may be
desired for a particular application. These principles explained
here also apply to the following additional embodiments as
well.
[0042] FIG. 3B illustrates how the circled portion of the grating
22 in FIG. 2 would look if modified in accordance with another
embodiment of the present invention. As in the previous embodiment,
the grating length here is virtually bifurcated about the mid-point
38 to define first and second grating halves 22A and 22B. In this
embodiment, as in the previous embodiment, the first half 22A of
the grating 22 has a uniformly periodic length and tooth and gap
configuration, wherein each tooth 34A' has a substantially similar
width w1. The second half 22B of the grating, however, is modified
in its per-tooth duty cycle such that each substantially similar
tooth 34B' has a width w2 that is less than the width w1. This
correspondingly increases the length of each gap 36B' disposed
between the teeth 34B'.
[0043] In a similar manner to the previous embodiment, the grating
configuration shown in FIG. 3B features a reduced .kappa. value on
the second grating half 22B in comparison with the .kappa. value of
the first grating half 22A. Specifically, the relatively skinnier
teeth 34B' of the second grating half 22B having width w2 are less
effective at creating reflections of light waves, and therefore
allow a relatively greater number of coherent light waves to
proceed without significant reflection to exit through the second
end facet 30. Correspondingly, the relatively wider teeth 34A' of
the first grating half 22A having width w1 cause substantially more
light wave reflection, thereby reducing the overall light emission
from the first end facet 28, as desired.
[0044] The differences in width between the teeth 34A' and 34B' in
FIG. 3B are relative. Accordingly, the width w1 can vary relative
to the width w2 in a variety of possible configurations, in
addition to those described here.
[0045] FIG. 3C depicts another embodiment of the present invention,
wherein the amplitude or height of the grating teeth is modified in
order to alter the .kappa. value on the grating 22. Here, the first
grating half 22A features teeth 34A" having a height h1 and
periodic length a. The second grating half 22B, disposed to the
right of mid-point 38, possesses teeth 34B" having the same
periodic length a, but with a relatively lower height h2.
[0046] During operation of the DFB laser 10, the grating 22 shown
in FIG. 3C operates in a similar manner to previous embodiments in
biasing coherent light emission toward the second end facet 30. In
particular, the relatively tall teeth 34A" of the first grating
half 22A possess a relatively high K value in comparison with the
relatively lower-height teeth 34B" of the second grating half 22B.
This differential in .kappa. values biases light emission toward
the second end facet 30 nearest the second grating half 22B, as in
previous embodiments.
[0047] It should be noted that in each of the embodiments depicted
in FIGS. 3A-3C, the periodic length of adjacent tooth/gap pairs
remains substantially constant within the respective grating
halves. For example, in FIG. 3B, each tooth/gap pair has the same
period both on the first grating half 22A and the second grating
half 22B. In FIG. 3A, the tooth/gap pairs that are present on the
second grating half 22B have the same period as those teeth on the
first grating half 22A if the missing teeth of the second grating
half are considered. The same principle applies to the tooth/gap
pairs in FIG. 3C.
[0048] It should also be noted that the designation of a particular
end facet for transmission of the majority of coherent light waves
is not limited to that described in the accompanying figures. The
DFB laser 10 could be alternatively configured such that the
majority of coherent light waves exit to the left through the first
end facet 28.
[0049] It is appreciated that the shape of the teeth 34 in the
various embodiments discussed herein can also vary from that
depicted. For instance, instead of a square shape, the teeth could
have rounded tops or comprise triangular shapes. Notwithstanding
the shape of the grating teeth, the present invention can be
practiced as described herein.
[0050] Reference is now made to FIGS. 4-6C, which depict additional
alternative embodiments of the present DFB laser. As cross
sectionally seen in FIG. 4, a DFB laser device is generally
depicted at 110 and comprises a similar structure to the DFB laser
10 shown in FIGS. 1 and 2. Specifically, the DFB laser 110
comprises an n-doped substrate 112, a bottom confinement layer 114,
an active layer 116, and a top confinement layer 118 disposed atop
one another in a sandwich fashion. Overlying the top confinement
layer 118 is a p-doped first top layer 120 having a grating 122
comprised of closely-spaced grooves defined thereon, and a p-doped
second top layer 124 overlying the grating 122. The first and
second top layers 120 and 124 each possess differing indices of
refraction. A contact layer 126 is disposed atop the second top
layer 124. First and second end facets 128 and 130, respectively,
are shown having AR coatings 132 applied thereon. Though the
details of the grating 122 are not apparent in FIG. 4, they will be
further described below in connection with FIGS. 5-6C.
[0051] Reference is now made to FIGS. 5A, 5B, and 5C, together with
FIG. 4, in describing details of the grating 122. FIGS. 5A, 5B, and
5C are close-up cross sectional views of the designated circled
portions A, B, and C, respectively, of the grating 122 shown in
FIG. 4. The grating 122, as before, comprises a tooth and gap
configuration defined in the first top layer 120 having a plurality
of teeth 134 and gaps 136 disposed between adjacent teeth. In
contrast to the previous embodiments depicted in FIGS. 2-3C, the
tooth period, or distance between the beginning of one tooth/gap
pair and the beginning of a succeeding tooth/gap pair, designated
as "a," is varied along the grating length. For example, it can be
seen in FIGS. 5A-5C that the period a1 of the grating teeth 134A
disposed near the first end facet 128 is substantially less than
the period a2 of the teeth 134B disposed near the mid-point of the
grating 122, indicated at 138. Similarly, the period a3 of the
grating teeth 134C near the second end facet 128 is substantially
less than those teeth 134B disposed near the mid-point 138. Thus,
the period of the teeth disposed along the length of the grating
122 in the present embodiment continuously increases toward the mid
point 138 of the grating, and continuously decreases toward the end
facets 128 and 130. The total magnitude of period change shown in
these and in the foregoing and following figures is merely
exemplary; indeed, the magnitude of change can be varied as desired
for a particular application. The continuously shaped tooth period
of the grating 122 shown in FIGS. 5A-5C serves to improve the
power, frequency response, and chirp characteristics of the DFB
laser 110 by reducing spatial hole burning (i.e., non-uniform light
intensity within the laser). In particular, reduced hole burning
can enhance the power output, as well as frequency response, in a
single-wavelength laser device. Additionally, because spatial hole
burning can influence the wavelength of the coherent light waves
emitted by the laser, reduction of such hole burning via the
alteration of the grating period as described above can reduce the
amount of chirp produced by the laser during modulation. Because
the effects due to spatial hole burning are most evident in lasers
having a large product of K and L (the length of the laser), it is
in such lasers where the most significant reductions in chirp are
anticipated. These principles can therefore be used to improve the
operating characteristics of the laser device.
[0052] FIGS. 6A, 6B, and 6C depict yet another alternative
embodiment of the present invention. FIGS. 6A-6C depict the three
regions A, B, and C, of the grating 122 shown in FIG. 4 according
to the present embodiment. As illustrated, the period of the
grating teeth 134 is continuously varied along the length of the
grating 122: the grating teeth 134A' disposed near the first end
facet 128 have a relatively large period, as indicated by a1 in
FIG. 6A, while the teeth 134B' disposed near the mid-point 138 are
intermediately sized, as indicated by a2. The teeth 134C', disposed
near the second end facet 130, possess a relatively small period,
as indicated by a3. This configuration of the grating 122 enables
not only the chirp and frequency response of the DFB laser 110 to
be improved, but also biases the coherent light signal toward the
first end facet 128, given the higher .kappa. value possessed by
the grating portion having a period equal to, or nearly equal to,
a1. The embodiment illustrated in FIGS. 6A-6C therefore combines
principles taught in connection with the variation of the grating
teeth period with those relating to the variation of the K value
along the grating length. This produces a DFB laser device having
desirable operating characteristics that minimize problems, such as
yield or sidemode suppression, that are associated with imprecise
cleaving of the laser device end facets during fabrication.
[0053] In light of the present embodiment, it is generally
appreciated that the teachings of the various embodiments as
disclosed herein can be combined to produce grating configurations
not explicitly illustrated here. For example, FIGS. 5A-6C
illustrate a grating having teeth that continuously vary in period
along the length of the grating. However, it is also possible to
configure the grating such that the tooth period abruptly changes
at a specified point along the grating length, as seen in FIGS.
3A-3C. Thus, these and other modifications are contemplated as
falling under the present invention.
[0054] If desired, it is also possible for a phase shift, such as a
quarter wavelength phase shift, to be added to the grating
structure to further enhance coherent light output from the DFB
laser. Such a phase shift can be added at any appropriate location
along the grating length, such as near either end facet, or at the
mid-point.
[0055] Finally, it is noted that, though the gratings discussed
herein have been shown as primarily disposed above the active
region, it is also possible to dispose a grating made in accordance
with the principles taught herein under the active region, such as
in the laser substrate.
[0056] The present invention may be embodied in other specific
forms without departing from its spirit or essential
characteristics. The described embodiments are to be considered in
all respects only as illustrative, not restrictive. The scope of
the invention is, therefore, indicated by the appended claims
rather than by the foregoing description. All changes that come
within the meaning and range of equivalency of the claims are to be
embraced within their scope.
* * * * *