U.S. patent application number 10/991106 was filed with the patent office on 2005-05-12 for mirror structure for reducing the effect of feedback on a vcsel.
This patent application is currently assigned to Optical Communication Products, Inc.. Invention is credited to Scott, Jeffrey W., Wasserbauer, John.
Application Number | 20050100063 10/991106 |
Document ID | / |
Family ID | 34437090 |
Filed Date | 2005-05-12 |
United States Patent
Application |
20050100063 |
Kind Code |
A1 |
Wasserbauer, John ; et
al. |
May 12, 2005 |
Mirror structure for reducing the effect of feedback on a VCSEL
Abstract
An exemplary embodiment of the present invention integrates an
absorbing layer into the emitting mirror of a VCSEL to reduce the
reflectivity of the emitting mirror as seen by the feedback optical
wave. The absorbing layer may be made of a suitable semiconductor
material, such as a GaAs layer in a laser emitting near 850 nm or
highly doped p-layer, and may disposed epitaxially in a
semiconductor or metamorphic mirror. Alternatively, a metal layer
may be disposed in the dielectric portion of a hybrid mirror or
all-dielectric mirror.
Inventors: |
Wasserbauer, John; (Erie,
CO) ; Scott, Jeffrey W.; (Carpenteria, CA) |
Correspondence
Address: |
BARLOW, JOSEPHS & HOLMES, LTD.
101 DYER STREET
5TH FLOOR
PROVIDENCE
RI
02903
US
|
Assignee: |
Optical Communication Products,
Inc.
Chatsworth
CA
|
Family ID: |
34437090 |
Appl. No.: |
10/991106 |
Filed: |
November 17, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10991106 |
Nov 17, 2004 |
|
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10051510 |
Jan 15, 2002 |
|
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60262261 |
Jan 15, 2001 |
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Current U.S.
Class: |
372/19 ;
372/98 |
Current CPC
Class: |
H01S 5/18308 20130101;
H01S 5/3432 20130101; H01S 5/18316 20130101; H01S 2301/166
20130101; H01S 5/18377 20130101; H01S 5/2022 20130101; H01S 5/18341
20130101; B82Y 20/00 20130101; H01S 5/2063 20130101; H01S 5/18369
20130101 |
Class at
Publication: |
372/019 ;
372/050; 372/098 |
International
Class: |
H01S 003/08 |
Claims
1-24. (canceled)
25. A method for reducing external feedback in a vertical cavity
surface emitting laser (VCSEL), comprising: determining optimum
thickness of at least one of a plurality of high index layers in a
first emitting mirror of a first VCSEL in accordance with air side
reflectivity of said first VCSEL; determining optimum thickness of
an absorbing layer in a second emission mirror of a second VCSEL in
accordance with air side reflectivity of said second VCSEL using
said optimum thickness of said high index layers; and determining
optimum thickness of at least one of a plurality of low index of
refraction layers in a third emission mirror of a third VCSEL in
accordance with air side reflectivity of said third VCSEL using
said optimum thickness of said high index layers and said optimum
thickness of said absorbing layer.
26. A method for reducing external feedback in a vertical cavity
surface emitting laser (VCSEL), comprising: determining optimum
thickness of at least one of a plurality of high-index layers in a
first emitting mirror in accordance with air-side reflectivity of
the VCSEL; determining optimum thickness of an absorbing layer in a
second emission mirror in accordance with air-side reflectivity of
the VCSEL using the optimum thickness of the high-index layers; and
determining optimum thickness of at least one of a plurality of
low-index of refraction layers in a third emission mirror in
accordance with air-side reflectivity of the VCSEL using the
optimum thickness of the high-index layers and the optimum
thickness of the absorbing layer.
27. A method for constructing a vertical cavity surface emitting
laser (VCSEL) with minimal external feedback, comprising: setting
the thickness of an absorbing layer to isolate external and
internal cavities of the VCSEL; reducing air-side reflectivity as a
function of thickness of high-index layers at an emitting facet of
the VCSEL; reducing air-side reflectivity as a function of
thickness of the absorbing layer of the VCSEL; reducing air-side
reflectivity as a function of thickness of the low-index layers at
the emitting facet of the VCSEL.
28. A method of constructing a vertical cavity surface emitting
laser (VCSEL) comprising: depositing a plurality of layers to form
a first mirror; forming a optical cavity adjacent to the first
mirror; depositing a plurality of layers to form a second mirror
adjacent to the optical cavity, wherein one of the layers is an
absorbing layer; said absorbing layer is located at or near a
standing optical wave pattern in closest proximity to an emission
facet so as to minimally interact with transmission light in the
optical cavity, and further so as to strongly interact with
external light reflected back into the optical cavity.
29. The method of claim 28, wherein the layers of the first mirror
are formed of alternating layers having a low index of refraction
and layers having a high index of refraction.
30. The method of claims 28, wherein the layers of the second
mirror, excluding the absorbing layer, are formed of alternating
layers having a low index of refraction and layers having a high
index of refraction.
31. The method of claim 28, wherein the first mirror is a
semiconductor mirror.
32. The method of claim 28, wherein the first mirror is a
dielectric mirror.
33. The method of claim 28, wherein the second mirror is a
semiconductor mirror.
34. The method of claim 28, wherein the second mirror is a
dielectric mirror.
35. The method of claim 28, further comprising the step of forming
an aperture between the optical cavity and the second mirror,
before the layers of the second mirror are deposited.
36. The method of claim 35, wherein the second mirror comprises a
first part and a second part; wherein the first part is formed
adjacent to the aperture and the second part is formed adjacent to
the first part.
37. The method of claim 36, wherein a current constriction having a
constriction aperture is formed within the first part of the second
mirror.
38. The method of claim 37, wherein the absorbing layer is
deposited within the first part of the second mirror at a point
within the confines of the constriction aperture.
39. The method of claim 36, wherein the first part is a
semiconductor mirror.
40. The method of claims 36, wherein the second part is a
dielectric mirror.
41. A method of constructing a vertical cavity surface emitting
laser (VCSEL) comprising: depositing a plurality of layers to form
a first mirror; forming a optical cavity adjacent to the first
mirror; forming an aperture adjacent to the optical cavity;
depositing a plurality of layers to form a second mirror adjacent
to the aperture, wherein one of the layers is an absorbing layer;
said absorbing layer is located at or near a standing optical wave
pattern in closest proximity to an emission facet so as to
minimally interact with transmission light in the optical cavity,
and further so as to strongly interact with external light
reflected back into the optical cavity.
42. The method of claim 41 wherein the layers of the first mirror
are formed of alternating layers having a low index of refraction
and layers having a high index of refraction.
43. The method of claims 41 wherein the layers of the second
mirror, excluding the absorbing layer, are formed of alternating
layers having a low index of refraction and layers having a high
index of refraction.
44. The method of claim 41, wherein the first mirror is a
semiconductor mirror.
45. The method of claim 41, wherein the first mirror is a
dielectric mirror.
46. The method of claim 41, wherein the second mirror is a
semiconductor mirror.
47. The method of claim 41, wherein the second mirror is a
dielectric mirror.
48. The method of claim 41, wherein the second mirror comprises a
first part and a second part; wherein the first part is formed
within the aperture and the second part is formed adjacent to the
first part and aperture.
49. The method of claim 48, further comprising the step of forming
a current constriction having a constriction aperture with the
first part of the second mirror.
50. The method of claim 49, wherein the absorbing layer is
deposited within the second mirror at a point within the first part
of the second mirror and within the constriction aperture.
51. The method of claim 48, wherein the first part is a
semiconductor mirror.
52. The method of claims 48, wherein the second part is a
dielectric mirror.
53. A method of constructing a vertical cavity surface emitting
laser (VCSEL) comprising: depositing a plurality of layers to form
a first mirror; forming a optical cavity adjacent to the first
mirror; forming an aperture adjacent to the optical cavity; forming
a current constriction having a constriction aperture adjacent to
the aperture; depositing a plurality of layers to form a second
mirror adjacent to the current constriction, wherein one of the
layers is an absorbing layer; said absorbing layer is located at or
near a standing optical wave pattern in closest proximity to an
emission facet so as to minimally interact with transmission light
in the optical cavity, and further so as to strongly interact with
external light reflected back into the optical cavity.
54. The method of claim 53, wherein the layers of the first mirror
alternate between layers having a low index of refraction and
layers having a high index of refraction.
55. The method of claims 53, wherein the layers of the second
mirror, excluding the absorbing layer, are formed of alternating
layers having a low index of refraction and layers having a high
index of refraction.
56. The method of claim 53, wherein the first mirror is a
semiconductor mirror.
57. The method of claim 53, wherein the first mirror is a
dielectric mirror.
58. The method of claim 53, wherein the second mirror is a
semiconductor mirror.
59. The method of claim 53, wherein the second mirror is a
dielectric mirror.
60. The method of claim 53, wherein the second mirror comprises a
first part and a second part; wherein the first part is formed
within the constriction aperture and the second part is formed
adjacent to the first part and current constriction.
61. The method of claim 60, wherein the absorbing layer is
deposited within the second mirror at a point within the first part
of the second mirror and within the constriction aperture.
62. The method of claim 60, wherein the first part is a
semiconductor mirror.
63. The method of claims 60, wherein the second part is a
dielectric mirror.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] This application claims priority to U.S. Provisional Patent
application Ser. No. 60/262,261, entitled "MIRROR STRUCTURE FOR
REDUCING THE EFFECT OF FEEDBACK ON A VCSEL" filed on Jan. 15, 2001
the content of which is incorporated herein by reference.
FIELD OF THE INVENTION
[0002] The present invention is generally related to optical
communication systems and is more particularly related to optical
transmitters.
BACKGROUND
[0003] In fiber optic systems and certain other applications, an
optical subassembly couples the light from a semiconductor laser
into an end face of an optical fiber. Reflections from anywhere
within the optical sub-assembly, such as the fiber end face,
optical lens element, beam splitter, polarizer or optical isolator,
may provide feedback to the laser. Unfortunately, semiconductor
lasers, including vertical cavity surface emitting lasers (VCSELs),
can be very sensitive to optical feedback. Medium to strong
feedback in the range of about -35 dBm to 0 dBm may give rise to
relative intensity noise (RIN), power modulation, or other coupled
cavity effects.
[0004] Conventionally these effects are addressed through careful
design of the optical package in which the devices are housed.
Current approaches include the use of angled fiber end facets,
fiber anti-reflective (AR) coatings, lens AR coatings, defocusing
along the optical axis, beam splitters, and optical isolators.
However, the cost of adding or modifying external optical elements
is typically higher than the cost of integrated components.
Therefore, these approaches, if used only to address the problem of
optical feedback, may increase the cost of the optical package.
[0005] Conventional device designs further exasperate the problem
of optical feedback. For example, the transmission in a
conventional mirror is roughly the same in each direction. The
reflectivity, however, is typically asymmetric and often is higher
looking from the air toward the cavity, than from cavity to air.
Conventional mirrors may therefore strongly return reflections from
one or more external components and may therefore create a high Q
external cavity when the laser is integrated into an optical
sub-assembly. When fluctuations in laser drive current or
temperature occur, the external cavity acts as a Fabry-Perot etalon
which modulates the output power. In addition, two high Q cavities
in series can cause multiple longitudinal modes to appear, which
can give rise to RIN.
[0006] Referring to FIG. 1, Applicant of the present invention
previously integrated an absorptive layer 8 into the emitting
mirror of a VCSEL 10 to reduce the reflectivity of the emitting
mirror as seen by a feedback optical wave. The VCSEL 10 included a
lower mirror 14 formed above a substrate 12, an optical cavity 16
formed above the lower mirror and an upper mirror formed 18 above
the optical cavity. The upper mirror 18 of this device was a hybrid
mirror, having a semiconductor portion 20 and a dielectric portion
22. The device further included a current confinement ion implant
24 as well as a current constriction 26 for mode control and
defining individual devices on a wafer.
[0007] The dielectric portion 22 of the hybrid mirror comprised
alternating one-quarter wavelength thick layers of a high index of
refraction dielectric material and a low index of refraction
dielectric material. In this approach an absorptive titanium layer
8 was formed at the low-to-high index transition closest to the
emitting facet. In this embodiment the titanium layer 8 was
processed to remove it from within the aperture formed by an upper
ohmic contact 30 to reduce the absorption losses as seen from the
cavity. However, this approach provides less absorption of the
optical feedback as seen from the external cavity. In particular,
the large number of longitudinal modes that appear in the
transmission spectrum due to the external cavity is not
reduced.
SUMMARY
[0008] In one aspect of the present invention a vertical cavity
surface emitting laser includes an optical cavity adjacent a first
mirror, an emitting mirror adjacent the optical cavity, a mode
defining aperture for controlling transverse modes and an absorbing
layer integrated within the emitting mirror, wherein the absorbing
layer is laterally located within at least a portion of said mode
defining aperture.
[0009] In another aspect of the present invention a vertical cavity
surface emitting laser includes an optical cavity adjacent a first
mirror, a semiconductor emitting mirror adjacent the optical
cavity, and an absorbing layer integrated within the emitting
mirror.
[0010] In a further aspect of the present invention a method for
reducing external feedback in a vertical cavity surface emitting
laser includes determining optimum thickness of at least one of a
plurality of high index layers in a first emitting mirror of a
first VCSEL in accordance with the air side reflectivity of the
first VCSEL, determining the optimum thickness of an absorbing
layer in a second emission mirror of a second VCSEL in accordance
with the air side reflectivity of the second VCSEL using the
optimum thickness of the high index layers, and determining optimum
thickness of at least one of a plurality of low index of refraction
layers in a third emission mirror of a third VCSEL in accordance
with the air side reflectivity of the third VCSEL using the optimum
thickness of the high index layers and the optimum thickness of the
absorbing layer.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] These and other features, aspects, and advantages of the
present invention will become better understood with regard to the
following description, appended claims, and accompanying drawings
where:
[0012] FIG. 1 is a cross sectional view of a prior art VCSEL
developed by the Applicant of the present application having an
absorbing layer integrated in the emission mirror outside the
aperture formed by the upper ohmic contact;
[0013] FIG. 2 is a cross sectional view of a VCSEL having an
absorbing layer integrated within the mode-defining aperture in
accordance with an exemplary embodiment of the present
invention;
[0014] FIG. 3 is a cross sectional view of an exemplary
multi-quantum well optical cavity;
[0015] FIG. 4 graphically illustrates the index and standing wave
profiles (not to scale) of a hybrid mirror structure having an
absorbing layer integrated at a null in the standing wave intensity
pattern in accordance with an exemplary embodiment of the present
invention;
[0016] FIG. 5 graphically illustrates the index and standing wave
profiles (to scale) of a VCSEL with hybrid mirror and no
absorber;
[0017] FIG. 6 is an expanded view of the index and standing wave
profiles (to scale) of the dielectric mirror portion of the VCSEL
of FIG. 5;
[0018] FIG. 7 is an expanded view of the index and standing wave
profiles (to scale) of the dielectric mirror portion of a VCSEL
with a Ti absorber in accordance with an exemplary embodiment of
the present invention;
[0019] FIG. 8 graphically illustrates the air side reflectivity
spectra of a hybrid mirror with and without a Ti absorber;
[0020] FIG. 9 is a flow chart illustrating an exemplary process for
optimizing the design of an emission mirror having an absorbing
layer in accordance with an exemplary embodiment;
[0021] FIG. 10 is an optimization curve for the thickness of the
SiN.sub.x layer deposited on top of a 100 nm Ti absorber layer in
accordance with an exemplary embodiment of the present
invention;
[0022] FIG. 11 is an optimization curve for the thickness of the Ti
absorber layer with a SiNx layer thickness of 0.200.lambda. in
accordance with an exemplary embodiment of the present
invention;
[0023] FIG. 12 is an optimization curve for the thickness of the
SiO.sub.2 layer with a SiN.sub.x layer thickness of 0.200.lambda.
and a Ti layer thickness of 19.6 nm in accordance with an exemplary
embodiment of the present invention;
[0024] FIG. 13 graphically illustrates the air side reflectivity
spectra of an optimized hybrid mirror with and without a Ti
absorber in accordance with an exemplary embodiment of the present
invention;
[0025] FIG. 14 is a graphical illustration of a simulation of the
index profile and standing wave pattern of an external cavity
formed from a VCSEL emitting mirror without an integrated absorber
with a 1.lambda. air gap and a 4% reflector that simulates the
reflection from a fiber facet;
[0026] FIG. 15 is a graphical illustration of a simulation of the
index profile and standing wave pattern for the VCSEL structure of
FIG. 14 with an optimized Ti absorber structure substituted for the
last three dielectric mirror pairs in accordance with an exemplary
embodiment of the present invention;
[0027] FIG. 16 is a graphical illustration of a simulation of the
transmission through the external cavities of FIGS. 14 and 15 when
the thickness of the air gap varies from 6000 to 6001 wavelengths
in accordance with an exemplary embodiment of the present
invention;
[0028] FIG. 17 is a graphical illustration of a simulation of the
transmission through the external cavities FIGS. 14 and 15 when the
wavelength of the transmitted light is varied slightly in
accordance with an exemplary embodiment of the present
invention;
[0029] FIG. 18 is a graphical illustration of a simulation of the
index and standing wave profiles of a VCSEL with hybrid mirror, no
absorber and a 1.lambda. external cavity with a feedback DBR mirror
in accordance with an exemplary embodiment of the present
invention;
[0030] FIG. 19 is a graphical illustration of a simulation of the
transmission spectra of an external cavity formed when the emitting
mirror of the VCSEL structure of FIG. 18, without the internal
VCSEL cavity and lower mirror, is coupled with a 6000.lambda. air
gap 520 and a 51/2 pair reflector 530 to provide feedback in
accordance with an exemplary embodiment of the present
invention;
[0031] FIG. 20 is a graphical illustration of a simulation of the
transmission spectra of a VCSEL with hybrid mirror, with and
without Ti absorber, with and without a 6000.lambda. external
cavity with DBR mirror in accordance with an exemplary embodiment
of the present invention;
[0032] FIG. 21 is a graphical illustration of a simulation of the
transmission spectra of two VCSELs each with a hybrid mirror
without Ti absorber and with a 6000.lambda. external cavity with a
feedback DBR mirror, wherein there is a 0.12 nm difference in the
optical thickness of the two devices;
[0033] FIG. 22 is a simulation of the transmission spectra of two
VCSELs each with a hybrid mirror with a Ti absorbing layer and with
a 6000.lambda. external cavity with a feedback DBR mirror, wherein
there is a 0.12 nm difference in the optical thickness of the two
devices in accordance with an exemplary embodiment of the present
invention;
[0034] FIG. 23 is a cross-sectional view of a VCSEL having a
dielectric emitting mirror with an integrated absorber layer in
accordance with an exemplary embodiment of the present invention;
and
[0035] FIG. 24 is a cross-sectional view of a VCSEL having a
semiconductor emitting mirror with an integrated absorber layer in
accordance with an exemplary embodiment of the present
invention.
DETAILED DESCRIPTION
[0036] An exemplary embodiment of the present invention may
integrate an absorbing layer into the emitting mirror of a VCSEL to
reduce the reflectivity of the emitting mirror as seen by a
feedback optical wave. The absorbing layer may be made of a
suitable semiconductor material. For example, a narrow bandgap GaAs
layer may be used as an absorbing layer in a laser emitting near
850 nm or a highly doped p-type semiconductor layer. The absorbing
layer may be disposed epitaxially in a semiconductor or metamorphic
mirror. Alternatively, a metal layer may be disposed in the
dielectric portion of a hybrid mirror or an all-dielectric
mirror.
[0037] In an exemplary embodiment of the present invention, the
absorber layer, such as for example 200 .ANG. of Ti, may be
integrated over at least a portion of the mode defining aperture,
directly in the path of the exit beam. In this embodiment, the
absorbing layer not only reduces the Q of the external cavity,
which reduces power modulation, but also reduces the number of
external cavity-generated longitudinal modes, and may therefore
also reduce RIN.
[0038] Conventionally, absorptive layers located in the optical
path of the beam are generally avoided because it reduces the
efficiency of the device. However, in an exemplary embodiment of
the present invention the absorbing layer may be selectively
located and tuned to substantially reduce absorption of the
transmit optical beam. Therefore the described exemplary embodiment
may substantially increase absorption as seen from the external
cavity with a relatively small increase in absorption as seen from
the VCSEL cavity. Furthermore, in the case of partial reflectors
such as thin metal layers, the small increase in absorption may be
accompanied by a reduction in the number of mirror pairs, which
eases fabrication requirements.
[0039] One of skill in the art will appreciate that the present
invention is not limited to particular material systems or emission
wavelengths. Rather, the compound semiconductor layers of the
described exemplary light-emitting devices may be formed from a
variety of group III-V or II-VI compound semiconductors, such as,
for example, GaAs/AlGaAs, InGaAs/AlGaAs or InP/InGaAsP or other
direct bandgap semiconductor materials.
[0040] However the advantages of the present invention may be best
understood in the context of an exemplary VCSEL 40 as illustrated
in FIG. 2 where like reference numerals may be used to designate
like features. An exemplary VCSEL may include a substrate 12, a
lower mirror 14, an optical cavity 16 and an upper mirror 42. A
conventional VCSEL portion of an exemplary embodiment of the
present invention is disclosed in U.S. patent application Ser. No.
09/237,580, the contents of which are hereby incorporated by
reference.
[0041] As is commonly known in the art, an exemplary substrate may
comprise gallium arsenide (GaAs) or any other suitable material.
The lower mirror 14 may comprise a multi-layered distributed Bragg
reflector, (DBR) as is conventional in the art. An exemplary
embodiment of the present invention may include a semiconductor
upper mirror, a dielectric upper mirror or a hybrid upper mirror
having semiconductor mirror layers 44 and dielectric mirror layers
46 as illustrated in FIG. 2.
[0042] An exemplary VCSEL may be formed into discrete lasers by a
combination of current confinement and ohmic contacts. Current
constriction may be achieved by implanting ions at multiple
energies as is known in the art. Implantation regions 24(a) and
24(b) convert the semiconductor mirror layers 44 of the hybrid
mirror to a high resistivity. The encircling high resistance region
forms a funnel to provide current confinement as is known in the
art. Other techniques for current constriction, such as selective
AlAs oxidation, are also applicable.
[0043] The upper ohmic contact 30 is disposed above the optical
cavity and in an exemplary embodiment may be formed adjacent the
upper semiconductor mirror layers 42. The top ohmic contact 30
creates an ohmic aperture inside the aperture formed by the ion
implant regions 24(a) and 24(b), to provide a ring contact. In the
described exemplary embodiment the top ohmic contact 30 may be a
p-type ohmic contact and may be, for example, gold with 2%
beryllium added or a layered structure of titanium/platinum/gold,
preferably deposited by electron beam evaporation.
[0044] In accordance with an exemplary embodiment, the upper ohmic
contact 30 may also function as a mode-defining aperture
restricting emission to a single transverse mode. One of skill in
the art will appreciate however that a current constriction in the
form of an ion implant or oxide aperture may also be utilized as a
mode-defining aperture to provide single or multi-mode
emission.
[0045] The bottom of the substrate may include a contact
metalization, forming a lower ohmic contact 50. In one embodiment
the lower ohmic contact may be an n-type ohmic contact and may be,
for example, eutectic gold germanium deposited by electron beam
evaporation or sputtering.
[0046] In the described exemplary embodiment, current flows from
the upper ohmic contact 30 down through the current funnel, into
the optical cavity where it recombines with opposite conductivity
type carriers flowing up from the lower ohmic contact. The
recombination in the optical cavity is a composite of spontaneous
and stimulated emission, the stimulated emission exits the device
out the top surface via the aperture in the upper ohmic
contact.
[0047] One of skill in the art will appreciate that there are a
plurality of suitable VCSEL structures which may be used to
practice the present invention. Therefore the disclosed VCSEL
structure is by way of example only and not by way of
limitation.
[0048] Referring to FIG. 3, the optical cavity 16 in accordance
with an exemplary embodiment of the present invention may include
an active region surrounded by first and second cladding regions 52
and 54. In the described exemplary embodiment the first and second
cladding regions 52 and 54 may comprise AlGaAs. The active region
may comprise a plurality of quantum wells 56, 58, and 60, with
barrier layers 62 and 64 there between. In the described exemplary
embodiment the quantum wells 56, 58, and 60 may comprise GaAs and
the adjacent barrier layers 62 and 64 may comprise
Al.sub.xGa.sub.1-xAs.
[0049] However, as is generally understood in the art, the
materials forming the quantum wells and surrounding barrier layers
may be varied depending on the design. Therefore, the disclosed
optical cavity is by way of example and not by way of
limitation.
[0050] Referring back to FIG. 2, in an exemplary embodiment, the
lower mirror 14 and the semiconductor mirror layers 44 of the
hybrid upper mirror may comprise alternating layers of aluminum
gallium arsenide (AlGaAs) and aluminum arsenide (AlAs), with
varying concentrations of aluminum. In an exemplary embodiment the
upper and lower semiconductor mirror layers may be doped of
opposite conductivity types. The optical thickness of each mirror
layer is typically designed to be a quarter wavelength of the
emitted light of the laser where the optical thickness is given by
the product of the physical thickness and the index of
refraction.
[0051] The dielectric mirror layers 46 of the hybrid mirror may
comprise alternating one-quarter wavelength layers of silicon
nitride and silicon dioxide or other suitable dielectric materials.
The alternating layers of the dielectric mirror 46 may be patterned
either by etching or liftoff processes known to those skilled in
the art.
[0052] In the described exemplary embodiment an absorber layer 80
may be integrated into the dielectric layers of the hybrid upper or
emitting mirror. The described exemplary absorbing layer extends
across the entire ohmic aperture to provide maximum reflectivity as
measured from the cavity side of the upper mirror and maximum
absorption as measured from the air side of the upper mirror.
However, the lateral extent of the absorbing layer may be limited
to extend across only a portion of the ohmic aperture.
[0053] FIG. 4 graphically illustrates the index of refraction of
the upper mirror overlayed with the optical standing wave intensity
as a functional of vertical position within the device. Typically,
the indexes of refraction of the semiconductor layers 44 are
greater than the indexes of refraction of the dielectric layers 46,
as illustrated by the index profile 100.
[0054] In the described exemplary embodiment a dielectric spacer
layer 110 may be integrated between the semiconductor mirror layers
44 and the dielectric mirror layers 46 to maintain the correct
phase between the two portions of the mirror. The optical thickness
of the dielectric spacer layer may be chosen such that the maxima
of the standing wave pattern 120, in both the semiconductor and
dielectric portions of the mirror, appear at the high-to-low index
transitions as seen from the optical cavity. This also ensures that
the nulls in the standing wave pattern are located at the
low-to-high index transitions as seen from the cavity.
[0055] In the described exemplary embodiment the absorbing layer 80
may be integrated at the standing wave null 150 nearest the
emitting facet of the mirror 160. The axial standing wave intensity
corresponds to the intensity of the light in the VCSEL relative to
the vertical position within the device. Hence, the standing wave
maxima are where the light is most intense, and the standing wave
minima are where the light is least intense. Therefore, optical
loss and optical scattering may be reduced by placing absorptive
materials and or scattering sources at or near an axial
standing-wave null.
[0056] If the absorbing layer is formed from Ti or other reflective
material, the total reflectivity of the mirror as seen from the
cavity will increase. Advantageously, this allows the number of
dielectric mirror pairs to be reduced, easing processing
requirements and reducing the total strain due to the dielectric
layers.
[0057] FIG. 5 graphically illustrates the simulated index profile
200 and standing wave (near field) pattern 210 of a VCSEL with a
hybrid upper mirror and no absorber layer. In the illustrated
embodiment the VCSEL has been designed to emit at a nominal
wavelength of 850 nm. The near field standing wave is most intense
at or near the optical cavity 220 and decays exponentially as a
function of the distance from the optical cavity. FIG. 6 is an
expanded view of the index profile and near field standing wave
intensity pattern 250 within the hybrid upper mirror of FIG. 5 near
the emitting facet 240.
[0058] FIG. 7 graphically illustrates the simulated index profile
265 and near field standing wave pattern 260 for a VCSEL structure
with a 20 nm Ti absorbing layer 270 integrated into the dielectric
portion of the upper mirror. In the described exemplary embodiment
the absorbing layer is integrated at or near the null in the near
field standing wave intensity pattern that is nearest the emitting
facet 275. In this embodiment the number of mirror pairs in the
dielectric portion of the hybrid upper mirror has been reduced by
two to keep the overall reflectivity approximately constant.
[0059] The performance of the upper mirror of the described
exemplary VCSEL structure may be further characterized as a
function of the calculated reflectivity, R, transmission, T, and
absorption, A, viewed from the cavity and the air as summarized in
Table 1. Without an absorbing layer the operating performance of a
hybrid upper or emitting mirror is substantially symmetric, having
approximately equal reflection, transmission and absorption as
viewed from the internal cavity or external, air side of the
mirror.
[0060] The reflection and transmission as viewed from the cavity
remains substantially equal for the described exemplary VCSEL
structure having an absorbing layer integrated into the dielectric
portion of the upper mirror and two fewer dielectric mirror pairs
as illustrated in FIG. 7. However, the integration of the absorbing
layer increases the absorption of the output as viewed from the
cavity from 0% to approximately 0.1405%. The magnitude of the
increase is relatively minimal because the absorbing layer is
integrated at or near a null in the standing wave intensity pattern
at a maximum distance from the internal VCSEL cavity. Therefore,
this relatively small increase in absorption may cause only a
relatively minor reduction in the efficiency of the laser.
[0061] The reflectivity of the described exemplary upper mirror
with an absorbing layer greatly decreases when viewed from the air
toward the internal VCSEL cavity. Referring again to Table 1, in
the described exemplary embodiment the reflectivity decreases from
99.8668% to 3.5519%, due to an increase in absorption from 0% to
96.3045%. In effect, the described exemplary mirror with an
absorbing layer performs like a one-way mirror. FIG. 8 graphically
illustrates the air side reflectivity spectrum of the upper mirror
with an absorbing Ti layer 280 and without an absorbing Ti layer
290.
1 TABLE 1 Parameter Without Ti With Ti R.sub.cavity 99.8668%
99.7159% T.sub.cavity 0.1332% 0.1436% A.sub.cavity 0% 0.1405%
R.sub.air 99.8668% 3.5519% T.sub.air 0.1332% 0.1436% A.sub.air 0%
96.3045%
[0062] In accordance with an exemplary embodiment the integration
of an absorbing layer into the emitting mirror of a VCSEL structure
may be further optimized to provide further reductions in the
external reflectivity of a VCSEL. FIG. 9 is flow chart illustrating
an exemplary process for optimizing the design of an emitting
mirror having an integrated absorbing layer. In accordance with an
exemplary embodiment an optimum emission mirror may be developed
for a particular application using the transfer matrix formalism to
optimize the thickness of the absorbing layer, as well as the
optical thickness of the mirror layers that form the last mirror
pair.
[0063] In accordance with an exemplary process a user may first
define the constitutive parameters of the emitting mirror and the
absorbing layer 300. In accordance with an exemplary embodiment of
the present invention, it is the reflectivity as seen from the
external cavity that is reduced or minimized. In an exemplary
embodiment a Ti absorbing layer with a refractive index of
approximately 3.3 and an absorption constant of about 5.7 may be
integrated into a dielectric upper mirror formed from a plurality
of alternating layers of SiO.sub.2 and SiN.sub.x for emission at a
nominal wavelength of 850 nm. In the described exemplary embodiment
the Ti absorbing layer is integrated at the low-to-high index
transition (the SiO.sub.2/SiN.sub.x interface) in the last mirror
pair. The variables in the optimization procedure are therefore the
thickness of the Ti layer and the optical thickness of the
SiN.sub.x and SiO.sub.2 layers that form the last mirror pair.
[0064] In accordance with the described exemplary design process a
user may simplify the design process by selecting starting layer
thicknesses that allow for the independent optimization of each of
the layers that form the final mirror pair. For example, if the Ti
absorbing layer is sufficiently thick, say 100 nm, all light
incident from the air side will be absorbed or reflected. This
isolates the external cavity from the rest of the emitting mirror.
Thus, an exemplary design process may first substantially reduce or
minimize the air side reflectivity as a function of the thickness
of the high index of refraction SiN.sub.x layer 310 with an Ti
absorbing layer on the order of about 100 nm. FIG. 10 graphically
illustrates the air side reflectivity as a function of the optical
thickness of the high index SiN.sub.x layer. In the described
exemplary embodiment, a minimum occurs in the air side reflectivity
when the optical thickness of the SiN.sub.x layer is approximately
0.200.lambda..
[0065] In accordance with an exemplary design process the air side
reflectivity of the emission mirror may now be minimized or
substantially reduced as a function of the thickness of the Ti
absorbing layer using the previously optimized SiN.sub.x high index
layer. FIG. 11 graphically illustrates the air side reflectivity as
a function of the thickness of the Ti absorbing layer. In the
described exemplary embodiment a minimum in the air side
reflectivity occurs when the Ti absorbing layer is 0.0196 .mu.m
thick.
[0066] In accordance with an exemplary design process the optical
thickness of the SiO.sub.2 layer may now be optimized by minimizing
or substantially reducing the air side reflectivity as a function
of the optical thickness of the SiO.sub.2 layer using the
previously optimized SiN.sub.x high index and Ti absorbing layers.
FIG. 12 graphically illustrates the air side reflectivity as a
function of the optical thickness of the SiO.sub.2 layer. In the
described exemplary embodiment a minimum occurs in the air side
reflectivity when the SiO.sub.2 layer is approximately
0.210.lambda. thick.
[0067] An optimized air side reflectivity spectrum may now be
calculated using the optimized values for the thickness of the
absorbing layer and the high and low index layers of the last
mirror period. FIG. 13 graphically illustrates the air side
reflectivity spectrum for a VCSEL having an emission mirror with an
integrated absorbing layer 400 and without an integrated absorbing
layer 410. The optimized air side reflectivity spectrum of the
emission mirror having an integrated absorbing layer 400 is
significantly reduced as compared to the un-optimized spectrum 290
illustrated in FIG. 8. The minimum reflectivity of the optimized
structure is R.sub.air=0.0125%.
[0068] The advantages of the present invention may be better
demonstrated when the described exemplary VCSEL having an
integrated absorbing layer is coupled to an external cavity. For
the purposes of illustration FIG. 14 graphically illustrates the
index profile 420 and standing wave pattern 430 of an external
cavity formed from the emitting mirror of the described exemplary
VCSEL structure with a 1.lambda. air gap 440 and a 4% reflector 450
to provide external feedback.
[0069] In this example, the 4% reflector simulates typical
reflections from a fiber facet. In addition, the 1.lambda. air gap,
while smaller than the cavity in a typical optical subassembly,
provides a clear illustration of the standing wave structure. In
practice, the air gap or external cavity may be many thousands of
wavelengths thick. FIG. 15 shows the index profile 460 and standing
wave pattern 470 for the same structure with an optimized Ti
absorber structure substituted for the last three dielectric mirror
pairs.
[0070] In a conventional optical sub-assembly (OSA) the external
cavity may be on the order of 5 mm, which corresponds to about 6000
wavelengths. The Q, or transmissivity, of such an external cavity
becomes very sensitive to the precise thickness of the air gap
and/or wavelength of light that is being transmitted through it.
For example, FIG. 16 is a graphical illustration of a simulation of
the transmission through the external cavities of FIGS. 14 and 15
when the thickness of the cavity or air gap varies from 6000 to
6001 wavelengths.
[0071] Transmission through the external cavity 480 without an
integrated absorbing layer is significantly modulated. In the
illustrated embodiment, the transmission undergoes a reduction of
approximately 57% from peak to valley. In this instance the entire
structure may be thought of as the variable-reflectivity emitting
mirror of a VCSEL. As the length of the external cavity fluctuates
due to thermal expansion or contraction, the output power will be
modulated by approximately the aforementioned amount. This will
result in unstable light output coupled to the fiber.
[0072] However, the integration of an exemplary absorbing layer
into the emitting mirror as illustrated in FIG. 15 reduces the
modulation of the transmission spectra 485 as a function of the
cavity size. In fact the power modulation of the described
exemplary embodiment with an optimized Ti absorbing layer is
reduced to approximately 0.8%.
[0073] Similarly, FIG. 17 is a graphical illustration of a
simulation of the transmission through the external cavities of
FIGS. 14 and 15 when the wavelength of the transmitted light is
varied slightly. The transmission spectra 490 without an absorbing
layer again experiences significant modulation with a reduction of
approximately 57% from peak to valley. In operation as the bias
current in the VCSEL is modulated, carrier induced index changes
may cause the resonant wavelength of the VCSEL to change, resulting
in output power modulation by approximately the aforementioned
amount. Similarly, as the temperature in the VCSEL changes due to a
change in ambient temperature or bias point, Temperature induced
index changes may cause the resonant wavelength of the VCSEL to
change, resulting in output power modulation by approximately the
aforementioned amount.
[0074] However, the integration of an exemplary absorbing layer
into the emitting mirror as illustrated in FIG. 15 reduces the
modulation of the transmission spectra 495 as a function of the
wavelength of the emitted light. In fact the power modulation of
the described exemplary embodiment with the optimized Ti absorber
present is reduced to approximately 0.8%. Thus the present
invention may significantly reduce the output power modulation of a
laser transmitter due to coupled cavity effects.
[0075] In addition to power modulation, optical feedback in the
range -35 dBm to -15 dBm may cause a significant increase in RIN.
RIN degrades the signal to noise ratio of the modulated beam of the
VCSEL and increases the bit error rate (BER) of a digitally
modulated signal. Thus it would be beneficial if coherent external
feedback could be reduced or eliminated, thus reducing RIN.
[0076] The advantages of the present invention may be further
demonstrated by examining the transmission spectra of an exemplary
VCSEL having an integrated absorbing layer coupled to an external
cavity. For purposes of illustration the described exemplary VCSEL
may be coupled to an external feedback reflector. For example, FIG.
18 graphically illustrates the index profile 500 and standing wave
pattern 510 of the described exemplary VCSEL structure coupled to a
51/2 pair dielectric DBR 530 via a one wavelength air gap. The one
wavelength air gap is again included here to provide a clear
illustration of the standing wave structure.
[0077] For the purposes of illustration FIG. 19 is a graphical
illustration of a simulation of the transmission spectra of an
external cavity formed when the emitting mirror of the described
exemplary VCSEL structure of FIG. 18 is coupled with a
100,000.lambda. air gap and a 51/2 pair reflector to provide
feedback. In this illustration the VCSEL structure does not include
the internal VCSEL cavity or the lower VCSEL mirror to clearly
illustrate the effects of the external cavity. In addition, an
exaggerated air gap has been used so that many modes will appear in
the simulated spectrum.
[0078] The transmission spectra 560 for the structure having an
emitting mirror without an integrated absorber layer includes
numerous resonant peaks that result from external feedback.
However, the transmission spectra 580 for the structure when the
emitting mirror includes an integrated absorbing layer is nearly
equal to zero. The described exemplary Ti absorbing layer
significantly reduces the resonances of the external cavity. A
similar effect can be expected with the external cavity is coupled
to a complete VCSEL structure.
[0079] In a conventional optical sub-assembly (OSA) the external
cavity may be on the order of 5 mm, which corresponds to about 6000
wavelengths. The inter-modal spacing of such an external cavity is
relatively small and may be calculated in accordance with equation
1 as follows:
.DELTA..lambda.=.lambda..sup.2/2nL (1)
[0080] Thus, for .lambda.=850 nm, n=1 and L=5.1 mm, the inter-modal
spacing .DELTA..lambda. is approximately equal to 0.07 nm.
Typically, the stop band of a VCSEL mirror is several hundred nm
wide. Therefore, many thousands of longitudinal modes of the
external cavity may appear inside the stop band of the VCSEL mirror
and the transmission characteristics of the VCSEL may be altered by
feedback from the external cavity.
[0081] For example, FIG. 20 is a simulation of the transmission
spectra of an exemplary VCSEL having a hybrid upper or emission
mirror, without a Ti absorbing layer integrated with a 6000.lambda.
external cavity and feedback DBR 600 and without an external cavity
610. Without the external cavity 610 there is one longitudinal mode
allowed in the stop band of the mirrors. With the addition of the
6000.lambda. external cavity 600, however, a non-uniform "comb"
function with multiple transmission peaks 620(a) and 620(b) has
been introduced into the transmission spectra. In addition, the
original transmission peak that was evident without the external
cavity has been significantly reduced.
[0082] In operation, the gain of the described exemplary device is
typically broadband and relatively flat over a wide wavelength
range. Therefore, a large fraction of the modes within the stopband
of the mirror see essentially the same amount of gain. The
competition for amplification (i.e. spectral hole burning) among
these modes may cause mode hopping that may lead to relative
intensity noise (RIN).
[0083] FIG. 20 also illustrates the transmission spectra of the
described exemplary VCSEL with a Ti absorbing layer integrated with
a 6000.lambda. external cavity and a feedback DBR 630 and without
an external cavity 640. In operation the integration of a Ti
absorbing layer into the emission mirror reduces the transmission
as compared to the VCSEL without an absorbing layer. More
importantly however, when an external cavity is present only one
resonance mode is present. Thus, the described exemplary emission
mirror with an absorbing layer may substantially reduce or
eliminate longitudinal mode hopping.
[0084] In addition to the mode hopping that may result from gain
competition and spectral hole burning, relatively small
perturbations to the VCSEL cavity, such as carrier or temperature
induced index fluctuations may significantly affect the
transmission spectra of a VCSEL. For example, FIG. 21 is a
graphical illustration of a simulation of the transmission spectra
of a VCSEL without an absorbing layers and with a 6000.lambda.
external cavity and feedback DBR. The two spectra represent a VCSEL
cavity optical thickness perturbation of 0.12 nm. The perturbation
of the cavity alters the amplitude of the transmission spectra
peaks, some of which increase, while others decrease. Such
instability may contribute to longitudinal mode hopping.
[0085] Small perturbations to the cavity of a VCSEL having an
absorbing layer integrated into the emission mirror may also alter
the VCSEL transmission spectra. For example, FIG. 22 is a graphical
illustration of a simulation of the transmission spectra of a VCSEL
with a Ti absorbing layer integrated into the emission mirror and
coupled to a 6000.lambda. external cavity and a feedback DBR
mirror. The two spectra represent a 0.12 nm perturbation of the
optical thickness of the VCSEL cavity. The perturbation of the
cavity again alters the peak transmission. However, the laser
retains a single longitudinal mode and mode hopping is again
avoided.
[0086] Although an exemplary embodiment of the present invention
has been described, it should not be construed to limit the scope
of the appended claims. Those skilled in the art will understand
that various modifications may be made to the described embodiment.
For example, the present invention is not limited to hybrid
emitting mirrors having a semiconductor portion and a dielectric
portion. Rather the present invention may be readily integrated
into a dielectric or semiconductor emitting mirror.
[0087] For example, FIG. 23 illustrates an exemplary VCSEL having a
lower mirror 1112 adjacent a substrate 1110, a dielectric upper
mirror 1114 above the lower mirror and an optical cavity 1116,
sandwiched between the upper and lower mirrors.
[0088] The lower mirror of the VCSEL may comprise a plurality of
epitaxially grown compound semiconductor mirror periods. As is
known in the art, the mirror periods may comprise one-quarter
wavelength thick alternating layers of a high index of refraction
material and a low index of refraction material. The lower mirror
of such a device may often be doped n-type with a reflectivity of
at least about 99%.
[0089] In operation, electrical current is conducted through an
intra-cavity contact 1118 (also referred to as an upper contact)
into the optical cavity 1116 so that the upper mirror 1114 need not
be conductive. Advantageously the dielectric upper mirror may
reduce the series resistance and optical loss that may otherwise be
associated with a semiconductor upper mirror.
[0090] In the described exemplary embodiment the compound
semiconductor optical cavity 1116 may be epitaxially grown on the
lower mirror 1112. The optical cavity 1116 may have an optical
thickness that is an integer multiple of one-half the wavelength of
the light generated within the optical cavity. The optical cavity
may include an active region having, for example, one or more
quantum-wells 1122 surrounded by barrier layers (not explicitly
shown) as may be preferable for the formation of the VCSEL device
1100. The quantum-wells provide quantum confinement of electrons
and holes therein to enhance recombination for the generation of
the light, and may also include semiconductor layers comprising a
plurality of quantum wires, quantum dots or other suitable gain
material.
[0091] In the described exemplary embodiment the optical cavity
1116 may further comprise a delta doped upper cladding layer or
contact layer 1130 for conducting holes into the active region to
cause lasing. The upper cladding layer 1130 may be bulk doped with
a suitable p-type dopant, such as, for example, Si or Be at a
relatively low density to reduce absorption of light therein.
[0092] In one embodiment, the delta doped upper cladding layer 1130
may therefore include one or more p-type doping spikes 1132(a) and
1132(b) located at or near nulls in the optical standing wave
pattern. In the described exemplary embodiment the p-type doping
spikes 1132(a) and 1132(b) have a thickness equal to or less than
30 nm, and are separated by a thickness of .lambda./2n, where n is
the index of refraction of the cladding material. Advantageously,
the placement of the p-type doping spikes, at or near the standing
wave nodes where the optical losses are near zero, provides lateral
conduction from the intra-cavity contact with reasonable
resistance, without significantly compromising the optical
efficiency.
[0093] One of skill in the art will appreciate that the p-type
doping spikes in the delta doped upper cladding may not be
necessary in some designs where device resistance is not a limiting
constraint. In addition, in an exemplary embodiment there may be a
region (not specifically illustrated) of the delta doped upper
cladding layer 1130 near the quantum wells that is not doped.
[0094] In accordance with an exemplary embodiment, the optical
cavity may further comprise an n-type lower cladding layer (not
explicitly shown) opposite the p-type upper cladding layer. In the
described exemplary embodiment the lower cladding layer has no
doping near the active region, but may have some continuous n-type
doping closer to the lower mirror.
[0095] The intra-cavity contact 1118 (also termed upper contact)
may be disposed on the topmost surface of the delta-doped optical
cavity 1116 and inside a high resistance region formed by an ion
implant 1142. The bottom of the substrate 1110 may include a
contact metalization, forming an n-type ohmic contact 1146. In one
embodiment, the n-type ohmic contact 1146 may form an annular
aperture for backside monitoring of the output power. The n-type
ohmic contact may be formed from AuGe/Ni/Ge or the like and may be
deposited by electron beam evaporation or sputtering. The
intra-cavity contact 1118 may be, for example, gold with 2%
beryllium (Au/Be) added or a layered structure of
titanium/platinum/gold (Ti/Pt/Au), preferably deposited by electron
beam evaporation.
[0096] In the described exemplary embodiment, a contact layer 1152
may be deposited on the uppermost surface of the delta doped
cladding layer 1130. In the described exemplary embodiment the
contact layer is highly conductive and the intra-cavity contact
1118 may then be deposited on the more heavily doped contact layer
1152 to provide a lower resistance. In accordance with an exemplary
embodiment, the contact layer may also act as a current spreading
layer to provide a more uniform current distribution across the
ohmic aperture, improving the uniformity of current injection into
the active region of the optical cavity.
[0097] In the described exemplary embodiment the upper mirror 1114
may comprise a dielectric DBR formed from alternating one-quarter
wavelength thick layers of a high index of refraction dielectric
material and a low index of refraction dielectric material. For
example, the upper mirror may comprise alternating layers of
silicon nitride and silicon dioxide or other suitable dielectric
materials. In accordance with an exemplary embodiment an absorbing
layer 1200 may be integrated at the low-to-high index transition
(SiO.sub.2/SiN.sub.x interface) in the last mirror pair. In
accordance with an exemplary embodiment the thickness of the Ti
layer and the optical thickness of the SiN.sub.x and SiO.sub.2
layers that form the last mirror pair may again be optimized as
previously described with respect to FIG. 9.
[0098] One of skill in the art will appreciate that the present
invention is not limited to VCSELs having a dielectric or hybrid
emission mirror. Rather, an absorbing layer may also be integrated
into a VCSEL 2000 having a semiconductor emission mirror as
illustrated in FIG. 24 wherein like reference numbers may be used
to represent like features. The VCSEL may again comprise a layered
structure epitaxially-grown on a semiconductor substrate 12. An
exemplary light emitting device may comprise a lower mirror 14
formed above the semiconductor substrate 12, an optical cavity 16
formed above the lower mirror stack and a second or upper mirror
2010 formed above the optical cavity.
[0099] In the described exemplary embodiment an upper ohmic contact
2020 is disposed above the optical cavity and in an exemplary
embodiment may be formed on the upper mirror 2010 with an aperture
inside the proton blocking aperture 22. The bottom of the substrate
may again include a contact metalization, forming a lower ohmic
contact 50. In one embodiment the lower ohmic contact may be an
n-type ohmic contact and may be, for example, eutectic gold
germanium deposited by electron beam evaporation or sputtering.
[0100] The top ohmic contact 2020 creates an ohmic aperture inside
the proton blocking aperture, to provide a ring contact. In the
described exemplary embodiment the top ohmic contact 2020 may be a
p-type ohmic contact and may be, for example, gold with 2%
beryllium added or a layered structure of titanium/platinum/gold,
preferably deposited by electron beam evaporation.
[0101] In the described exemplary embodiment, the upper mirror may
comprise a semiconductor mirror formed from a plurality of
alternating mirror layers. In one embodiment the semiconductor
mirror layers may comprise, aluminum gallium arsenide (AlGaAs) and
aluminum arsenide (AlAs), with varying concentrations of aluminum
for the desired emission wavelength. In an exemplary embodiment the
upper and lower semiconductor mirror layers may be doped of
opposite conductivity types. In the described exemplary embodiment
the semiconductor mirror layers in the upper mirror may be p-type,
doped with a suitable concentration of carbon or other dopants
known to those skilled in the art.
[0102] In the described exemplary embodiment an absorbing layer
2040 may be integrated at the low to high index interface (i.e.
standing wave null) nearest the emitting facet of the mirror. In
accordance with an exemplary embodiment the absorbing layer 2040
may comprise for example, a GaAs layer in a laser emitting at a
nominal wavelength of 850 nm or a highly doped p-type semiconductor
layer. In accordance with an exemplary embodiment the thickness of
the absorbing layer and the optical thickness of the high and low
index layers that form the last mirror pair may again be optimized
as previously described with respect to FIG. 9.
[0103] The previous description of the disclosed embodiments is
provided to enable any person skilled in the art to make or use the
present invention. Various modifications to these embodiments will
be readily apparent to those skilled in the art, and the generic
principles defined herein may be applied to other embodiments
without departing from the spirit or scope of the invention.
[0104] For example, the described exemplary light emitting devices
may be manufactured in the form of arrays, wherein the devices are
epitaxially grown on a single substrate, processed and auto-tested
as a whole wafer. Conventionally, individual devices within an
array may be defined by implanting protons in the form of an
annular isolation region that confines current flow within the
individual VCSEL devices, while also electrically isolating each
device from the other VCSEL devices in the array. However, in the
case of long wavelength VCSELs the thickness of the upper mirror
may exceed the maximum depth to which hydrogen or other ions may be
reasonably implanted. In these instances mesas may be formed to
isolate individual devices on the wafer.
[0105] The invention itself herein will further suggest solutions
to other tasks and adaptations for other applications to those
skilled in the art. It is therefore desired that the present
embodiments be considered in all respects as illustrative and not
restrictive, reference being made to the appended claims rather
than the foregoing description to indicate the scope of the
invention.
* * * * *