U.S. patent application number 16/993953 was filed with the patent office on 2021-02-18 for tunable vcsel with combined gain and dbr mirror.
The applicant listed for this patent is Axsun Technologies, Inc.. Invention is credited to Bartley C. Johnson, Mark E. Kuznetsov, Peter S. Whitney.
Application Number | 20210050712 16/993953 |
Document ID | / |
Family ID | 1000005033141 |
Filed Date | 2021-02-18 |
United States Patent
Application |
20210050712 |
Kind Code |
A1 |
Johnson; Bartley C. ; et
al. |
February 18, 2021 |
Tunable VCSEL with combined gain and DBR mirror
Abstract
A vertical cavity surface emitting laser (VCSEL) has a shortened
overall laser cavity by combining the gain section with a
distributed Bragg reflector (DBR). The overall cavity length can be
contracted by placing gain structures inside the DBR. This
generally applies to a number of semiconductor material systems and
wavelength bands, but this scheme is very well suited to the
AlGaAs/GaAs material system with strained InGaAs quantum wells as a
gain medium, for example.
Inventors: |
Johnson; Bartley C.; (North
Andover, MA) ; Kuznetsov; Mark E.; (Lexington,
MA) ; Whitney; Peter S.; (Lexington, MA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Axsun Technologies, Inc. |
Billerica |
MA |
US |
|
|
Family ID: |
1000005033141 |
Appl. No.: |
16/993953 |
Filed: |
August 14, 2020 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
62887332 |
Aug 15, 2019 |
|
|
|
62962367 |
Jan 17, 2020 |
|
|
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01S 5/18369 20130101;
H01S 5/34306 20130101; H01S 5/18366 20130101; H01S 5/041
20130101 |
International
Class: |
H01S 5/183 20060101
H01S005/183; H01S 5/04 20060101 H01S005/04; H01S 5/343 20060101
H01S005/343 |
Claims
1. A vertical surface emitting laser, comprising: a distributed
Bragg reflector; and quantum wells located in the distributed Bragg
reflector.
2. The laser of claim 1, further comprising a deflectable membrane
carrying a mirror defining an optical cavity of the laser.
3. The laser of claim 1, wherein the quantum wells are located in
shallow layers of the distributed Bragg reflector.
4. The laser of claim 1, wherein the quantum wells located in the
distributed Bragg reflector are fabricated in AlGaAs/GaAs.
5. The laser of claim 1, wherein the quantum wells are located
between high index layers and low index layers of the distributed
Bragg reflector.
6. The laser of claim 1, wherein the quantum wells are located in
high index layers of the distributed Bragg reflector.
7. The laser of claim 1, wherein the quantum wells are placed at
antinodes of standing wave patterns in the laser.
8. The laser of claim 1, further comprising eight or more quantum
wells.
9. The laser of claim 1, wherein the high index layers of the
distributed Bragg reflector are thinner than the low index
layers.
10. The laser of claim 1, wherein the quantum wells are optically
pumped.
11. The laser of claim 1, wherein the quantum wells are
electrically pumped.
12. A vertical surface emitting laser system, comprising: a
vertical surface emitting laser, including a distributed Bragg
reflector and quantum wells located in the distributed Bragg
reflector; and a pump laser for optically pumping the quantum
wells.
13. The system of claim 12, wherein the laser further includes a
deflectable membrane carrying a mirror defining an optical cavity
of the laser.
14. The system of claim 12, wherein the quantum wells are located
in shallow layers of the distributed Bragg reflector.
15. The system of claim 12, wherein the quantum wells located in
the distributed Bragg reflector are fabricated in AGaAs/GaAs.
Description
RELATED APPLICATIONS
[0001] This application claims the benefit under 35 USC 119(e) of
U.S. Provisional Application No. 62/887,332, filed on Aug. 15, 2019
and U.S. Provisional Application No. 62/962,367, filed on Jan. 17,
2020, which are incorporated herein by reference in their
entirety.
BACKGROUND OF THE INVENTION
[0002] Tunable VCSELs with microelectromechanical (MEMS) movable
mirrors have found application in telecommunications, Matsui, Y.,
Vakhshoori, D., Wang, P., Chen, P., Lu, C.-C., Jiang, M., Knopp,
K., Burroughs, S., and Tayebati, P., "Complete polarization mode
control of long-wavelength tunable vertical-cavity surface-emitting
lasers over 65-nm tuning, up to 14-mw output power," IEEE J.
Quantum Electronics 39, 1037-1048 (2003) and Knopp, K., Vakhshoori,
D., Wang, P., Azimi, M., Jiang, M., Chen, P., Matsui, Y.,
McCallion, K., Baliga, A., Sakhitab, F., Letsch, M., Johnson, B.,
Huang, R., Jean, A., DeLargy, B., Pinzone, C., Fan, F., Liu, J.,
Lu, C., Zhou, J., Zhu, H., Gurjar, R., Tayebati, P., MacDaniel, D.,
Baorui, R., Waterson, R., and Van der Rhodes, G., "High power
MEMs-tunable vertical-cavity surface-emitting lasers," in [2001
Digest of LEOS Summer Topical Meetings], TuA1.3, 31-32 (2001). They
have also found application in optical coherence tomography (OCT).
See for example B. Johnson, W. Atia, S. Woo, C. Melendez, M.
Kuznetsov, T. Ford, N. Kemp, J. Jabbour, E. Mallon, P. Whitney,
"Tunable 1060 nm VCSEL co-packaged with pump and SOA for OCT and
LiDAR," SPIE Photonics West BiOS Proceedings, 10867 (2019) and
Flanders, D. C., Kuznetsov, M. E., Atia, W. A., and Johnson, B. C.,
"OCT system with bonded MEMS tunable mirror VCSEL swept source." US
Patent application Pub. No. US2016/0329682 A1 (10 Nov. 2016).
[0003] To date, most swept source OCT applications occur in the
1310 nanometer (nm) and 1060 nm bands. And, for OCT, it is very
important to achieve the widest tuning range in optical frequency
since the depth resolution in OCT is inversely proportional to the
tuning range. See J. Fujimoto and W. Drexler, "Introduction to
Optical Coherence Tomography," in [Optical Coherence Tomography:
Technology and Applications, First Edition], Drexler, W. and
Fujimoto, J. G., eds., ch. 1, 1-40, Springer (2008).
[0004] Two of the most important factors affecting a VCSEL's tuning
range are the (1) bandwidths of the laser cavity mirrors and (2)
the laser cavity length. Much work has gone into broad-band mirror
technology, including high-index-contrast dielectric mirrors and
GaAs/oxidized AlAs mirrors. See Jayaraman, V., Jiang, J., Potsaid,
B., Robertson, M., Heim, P. J. S., Burgner, C., John, D., Cole, G.
D., Grulkowski, I., Fujimoto, J. G., Davis, A. M., and Cable, A.
E., "VCSEL swept light sources," in [Optical Coherence Tomography:
Technology and Applications, Second Edition], Drexler, W. and
Fujimoto, J. G., eds., ch. 22, 659-686, Springer (2015). The
GaAs/AlAs material system natively has fairly high index contrast
allowing epitaxially-grown mirrors supporting tuning ranges on the
order of 100 nm.
[0005] The free-spectral range (FSR) of a VCSEL cavity in optical
frequency is c/(2L), where c is the speed of light and L is the
total length of the laser cavity including the group delay of the
mirrors (see D. I. Babic and S. W. Corzine, "Analytic expressions
for the reflection delay, penetration depth, and absorptance of
quarter-wave dielectric mirrors," IEEE Journal of Quantum
Electronics, 28, 514-524, (1992)), the length of the air gap and
the length of the gain section and other layers. The laser tuning
range is limited by the FSR because another lasing mode can pop up
one FSR away as the device is tuned. The spectral gain bandwidth of
the active quantum wells also has to be wide enough to allow wide
tuning.
[0006] A MEMS tunable VCSEL has one mirror on a flexible membrane
coupled to a 1/2-VCSEL structure that combines an optical gain
medium with a second, stationary reflector. See Flanders, D. C.,
Kuznetsov, M. E., Atia, W. A., and Johnson, B. C., "OCT system with
bonded MEMS tunable mirror VCSEL swept source." US Patent
application 2016/0329682 A1 (10 Nov. 2016) and Jayaraman, V.,
Jiang, J., Potsaid, B., Robertson, M., Heim, P. J. S., Burgner, C.,
John, D., Cole, G. D., Grulkowski, I., Fujimoto, J. G., Davis, A.
M., and Cable, A. E., "VCSEL swept light sources," in [Optical
Coherence Tomography: Technology and Applications, Second Edition],
Drexler, W. and Fujimoto, J. G., eds., ch. 22, 659-686, Springer
(2015).
SUMMARY OF THE INVENTION
[0007] Present invention concerns a way of shortening the overall
cavity length by combining the gain section with a distributed
Bragg reflector (DBR). The overall cavity length can be contracted
by placing gain structures inside the DBR. This generally applies
to a number of semiconductor material systems and wavelength bands,
but this scheme is very well suited to the AlGaAs/GaAs material
system with strained InGaAs quantum wells as a gain medium, for
example.
[0008] In general, according to one aspect, the invention features
a vertical surface emitting laser, comprising a distributed Bragg
reflector and quantum wells located in the distributed Bragg
reflector.
[0009] In embodiments, a deflectable membrane carries a mirror
defining an optical cavity.
[0010] Typically, the quantum wells are located in shallow layers
of the distributed Bragg reflector.
[0011] The laser can be fabricated in AlGaAs/GaAs, although other
material systems can be used. Also, the quantum wells might be
optically pumped or electrically pumped.
[0012] Often, the quantum wells are located between high index
layers and low index layers of the distributed Bragg reflector.
[0013] In other examples, however, the quantum wells are located in
high index layers of the distributed Bragg reflector.
[0014] Typically, the quantum wells are placed at antinodes of
standing wave patterns in the laser.
[0015] Further, there might be eight or more quantum wells,
especially when high power is desired. Also, the high index layers
of the distributed Bragg reflector can be thinner than the low
index layers.
[0016] In general, according to another aspect, the invention
features a vertical surface emitting laser system, comprising a
vertical surface emitting laser, including a distributed Bragg
reflector and quantum wells located in the distributed Bragg
reflector and a pump laser for optically pumping the quantum
wells.
[0017] The above and other features of the invention including
various novel details of construction and combinations of parts,
and other advantages, will now be more particularly described with
reference to the accompanying drawings and pointed out in the
claims. It will be understood that the particular method and device
embodying the invention are shown by way of illustration and not as
a limitation of the invention. The principles and features of this
invention may be employed in various and numerous embodiments
without departing from the scope of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] In the accompanying drawings, reference characters refer to
the same parts throughout the different views. The drawings are not
necessarily to scale; emphasis has instead been placed upon
illustrating the principles of the invention. Of the drawings:
[0019] FIG. 1A shows a conventional arrangement for the DBR mirror
and the quantum wells in a MEMS tunable VCSEL 10;
[0020] FIG. 1B shows an arrangement for the DBR mirror and the
quantum wells QW in a gain embedded DBR VCSEL of the present
invention;
[0021] FIGS. 2A, 2B, and 2C are plots of energy of the GaAs valence
and conduction bands in electron volts (Ev), wavelength in
nanometers (nm) as a function of optical gap in micrometers and
gain per well as a function of air optical gap in micrometers for a
conventional tunable VCSEL;
[0022] FIGS. 2D, 2E, and 2F are plots of energy of the GaAs valence
and conduction bands in electron volts (Ev), wavelength in
nanometers (nm) as a function of optical gap in micrometers and
gain per well as a function of air optical gap in micrometers for a
tunable VCSEL of the present invention;
[0023] FIG. 3 shows magnitude of the electrical field inside a gain
embedded DBR VCSEL 100 along with the refractive index along the
device's optical axis OA as a function of optical distance in
micrometers;
[0024] FIG. 4 shows magnitude of the electrical field inside a gain
embedded DBR VCSEL 100 as a function of optical distance in
micrometers for the pump light at 825 nm showing the standing wave,
forward wave, and reflected wave along with the refractive index
along the device's optical axis OA;
[0025] FIG. 5A is a plot of energy in the GaAs valence and
conduction bands in electron Voltage (eV), and FIGS. 5B and 5C are
plots of wavelength nanometers and gain per well as a function of
the optical air gap;
[0026] FIGS. 6A and 6B are an exploded perspective view of a wafer-
or die-bonded tunable VCSEL and a schematic perspective view of the
half VCSEL showing the locations of the bond pads, respectively, to
which the present invention could be applied, in one example;
and
[0027] FIG. 7 is a top plan view of an optically pumped tunable
VCSEL swept source module including the gain embedded DBR
VCSEL.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0028] The invention now will be described more fully hereinafter
with reference to the accompanying drawings, in which illustrative
embodiments of the invention are shown. This invention may,
however, be embodied in many different forms and should not be
construed as limited to the embodiments set forth herein; rather,
these embodiments are provided so that this disclosure will be
thorough and complete, and will fully convey the scope of the
invention to those skilled in the art.
[0029] As used herein, the term "and/or" includes any and all
combinations of one or more of the associated listed items.
Further, the singular forms and the articles "a", "an" and "the"
are intended to include the plural forms as well, unless expressly
stated otherwise. It will be further understood that the terms:
includes, comprises, including and/or comprising, when used in this
specification, specify the presence of stated features, integers,
steps, operations, elements, and/or components, but do not preclude
the presence or addition of one or more other features, integers,
steps, operations, elements, components, and/or groups thereof.
Further, it will be understood that when an element, including
component or subsystem, is referred to and/or shown as being
connected or coupled to another element, it can be directly
connected or coupled to the other element or intervening elements
may be present.
[0030] It will be understood that although terms such as "first"
and "second" are used herein to describe various elements, these
elements should not be limited by these terms. These terms are only
used to distinguish one element from another element. Thus, an
element discussed below could be termed a second element, and
similarly, a second element may be termed a first element without
departing from the teachings of the present invention.
[0031] Unless otherwise defined, all terms (including technical and
scientific terms) used herein have the same meaning as commonly
understood by one of ordinary skill in the art to which this
invention belongs. It will be further understood that terms, such
as those defined in commonly used dictionaries, should be
interpreted as having a meaning that is consistent with their
meaning in the context of the relevant art and will not be
interpreted in an idealized or overly formal sense unless expressly
so defined herein.
[0032] FIG. 1A shows a conventional arrangement for the DBR mirror
and the quantum wells in a MEMS tunable VCSEL 10.
[0033] In more detail, the MEMS tunable VCSEL 10 includes a front
MEMS membrane structure 214 that carries a membrane mirror 250. A
1/2 or half VCSEL chip or device 112 is separated from the front
MEMS membrane structure 214 by an adjustable air optical gap 105,
the length of which along the optical axis is tuned by deflecting
the membrane structure.
[0034] The half VCSEL device 112 typically has an antireflective
coating 114 facing the air optical gap 105. This is typically a
dielectric coating that minimizes reflections at the interface with
the air optical gap 105. A gain medium GM of the half VCSEL device
112 typically has multiple quantum well layers QW1, QW2, QW3, QW4.
A DBR stack mirror follows the gain medium in the half VCSEL device
112. This mirror DBR is characterized by a stack of alternating
layers of high refractive index material H and low refractive index
material L. In general, the DBR mirror can have as few as 4 layers
to 70 layers or more, depending on the desired reflectivity and the
difference in the refractive indices of the two materials used.
Finally, the half VCSEL device 112 further includes a substrate
115, on which the various layers are deposited and which provides
mechanical support.
[0035] It is conventional that the gain medium GM is separate from
the DBR mirror in the half VCSEL device 112.
[0036] FIG. 1B shows an arrangement for the DBR mirror and the
quantum wells QW in a gain embedded DBR VCSEL 100 constructed
according to the principles of the present invention.
[0037] Here, the overall cavity length is contracted in the
direction of the optical axis by placing the gain medium GM inside
the DBR stack mirror to create a combined gain medium and DBR
mirror GM/DBR.
[0038] In general, in the illustrated example, the quantum wells
are associated with the portion of the DBR mirror that is closest
to the air gap 105 in the half VCSEL device 112. Specifically,
after antireflective coating 114, there is low index material layer
H, a first quantum well QW1, then a high index material layer L,
low index material layer H, a second quantum well QW2, then a high
index material layer L, low index material layer H, a third quantum
well QW3, then a high index material layer L, low index material
layer H, and a fourth quantum well QW4. Then the remaining layers
H,L of the DBR mirror are added for the desired reflectivity. As
shown, preferably, the quantum wells are added between the high
index and low index layers of the DBR mirror.
[0039] The quantum well layers QW1-QW4 are preferably placed high
in the stack of the DBR mirror where the electrical fields of the
reflected light are strongest. This maximizes their respective
contributions to the gain.
[0040] This combined gain medium and DBR mirror, the GM/DBR, is
generally applicable to a number of semiconductor material systems
and wavelength bands. Common material systems are based on III-V
semiconductor materials, including binary materials, such as GaN,
GaAs, InP, GaSb, InAs, as well as ternary, quaternary, and
pentenary alloys, such as InGaN, InAlGaN, InGaP, AlGaAs, InGaAs,
GaInNAs, GalnNAsSb, AlInGaAs, InGaAsP, AlGaAsSb, AlGaInAsSb, AAsSb,
InGaSb, InAsSb, and InGaAsSb. Collectively, these material systems
support operating wavelengths from about 400 nanometers (nm) to
2000 nm, including longer wavelength ranges extending into multiple
micrometer wavelengths. Semiconductor quantum well and quantum dot
gain regions are typically used to obtain especially wide gain and
spectral emission bandwidths. Quantum well layers may be purposely
strained or unstrained depending on the exact materials and the
desired wavelength coverage.
[0041] Nevertheless, the combined gain medium and DBR mirror, the
GM/DBR, is very well suited to the AlGaAs/GaAs material system with
strained InGaAs quantum wells as a gain medium. This system lases
in a band around 1050 nm. In this material system, the high index
layers H are GaAs, and the low index layers L are AlAs or similar
high Al content AlGaAs alloys.
[0042] The combined gain medium and DBR mirror, the GM/DBR, is
optically pumped in a current embodiment. The laser emission light
and pump light needs to come through the MEMS membrane structure if
the substrate is absorptive at the pump wavelength, as it is for a
GaAs substrate.
[0043] That is not necessarily true for other material systems,
however. The VCSEL emission may be configured to come out of the
MEMS side, the substrate side, or both. The MEMS mirror 250 is
often curved for spatial mode control; but, it does not necessarily
have to be so, for example in cases where thermal lensing in the
half VCSEL device 112 is strong.
[0044] For optically pumped lasers, gain quantum wells have to be
placed inside the pump-absorbing low-bandgap Bragg reflector
layers. The concept of placing gain section inside the Bragg
reflector mirror can also be applied to the electrically pumped
semiconductor lasers.
[0045] FIGS. 2A, 2B, and 2C are plots of energy of the GaAs valence
and conduction bands in electron volts (eV), wavelength in
nanometers (nm) as a function of optical gap in micrometers and
gain per well as a function of optical gap in micrometers for a
conventional tunable VCSEL. The graphs show about 90 nm of tuning
range for the VCSEL by changing the optical gap by about 0.5 .mu.m
for a six well VCSEL of conventional design.
[0046] On the other hand, FIGS. 2D, 2E, and 2F are plots of energy
of the GaAs valence and conduction bands in electron volts (eV),
wavelength in nanometers (nm) as a function of optical gap in
micrometers and gain per well as a function of optical gap 105 in
micrometers for a tunable VCSEL employing the combined gain medium
and DBR mirror GM/DBR. The graphs show about 110 nm of tuning range
for the VCSEL by changing the optical gap 105 by less than 0.5
.mu.m. This improved performance is due to the much shorter cavity
length afforded by the combined gain medium and DBR mirror
GM/DBR.
[0047] The two designs have similar threshold gain requirements as
seen in FIGS. 2C and 2F. These curves were calculated using an
optical multilayer method, see Orfanidis, S. J., "Multilayer
structures," n [Electromagnetic Waves and Antennas], ch. 6,
186-240, Self published (2016).
https://www.ece.rutgers.edu/.about.orfanidi/ewa/ch06.pdf, using a
complex index for the quantum wells to simulate the gain. This
allows the threshold gains to be calculated and results in the
correct lasing field standing wave locations. The FIGS. 2A and 2D
show the conduction and valence band edges for the example where
the materials are
GaAs/A.sub.xGa.sub.1-xAs/In.sub.yGa.sub.1-yAs.
[0048] FIG. 3 shows standing wave patterns (magnitude of the
electric field plotted) inside the gain embedded DBR VCSEL 100 with
six quantum wells QW1-QW6. The electric field is shown at the band
center 1040 nm, short band edge 994 nm, and long band edge 1090 nm.
The refractive index profile of the semiconductor and dielectric
materials show the structure of the DBR and placement of the
quantum wells. The wells QW1-QW6 are placed, and roughly aligned
with the antinodes in the DBR of the standing wave patterns for all
wavelengths.
[0049] Another advantage of the combined gain medium and DBR mirror
GM/DBR is that if more gain is required, more quantum wells can be
added without appreciably changing the tuning range. In the
AlGaAs/GaAs system, pump absorption is weak enough that wells
further away from the pump laser are still pumped appreciably.
[0050] In fact, the present approach can additionally be used to
fabricate high-power tunable VCSELs.
[0051] Since there is no tunability penalty for adding quantum
wells and absorption material, many more wells could be added
beyond the four and six wells shown, such as greater eight wells
and even more than ten wells. More wells yield more power. The
absorption of the pump wavelength in the GaAs is low enough that
power will get to the deeper wells.
[0052] FIG. 4 shows standing wave patterns (magnitude of the
electric field plotted) inside the gain embedded DBR VCSEL 100 for
the pump light at 825 nm. As shown, there is enough pump light to
power deeper wells.
[0053] Pump Absorption
[0054] This gain embedded DBR structure is compatible with optical
pumping. Absorption into the first few GaAs layers of the DBR
channel electron-hole pairs into the co-located strained quantum
well layers. The reflected waves are small, and the overall
absorption constant is about half that of bulk GaAs since half of
the material in the DBR is non-absorbing AlGaAs. The propagating
optical power density is calculated from the Poynting vector, and
is proportional to n|E|.sup.2, where n is the refractive index. The
incident forward wave in air from the left is normalized to a value
of n|E|.sup.2=1. The light absorbed in the gain region is about 12%
in this example. The forward and backward waves are plotted, along
with the standing wave.
[0055] This plot, however, only considers linear absorption. In the
case where there is saturated pump absorption in the quantum wells
and quantum well barrier layers, deeper wells would be even more
advantageous. Depositing more pump power inside the VCSEL cavity
naturally provides more VCSEL emission power, assuming the absorbed
pump power is channeled into a quantum well that can increase VCSEL
emission. The other advantage is that more quantum wells would
provide more gain. This would allow the output coupling to be
increased by reducing the reflectivity of the output mirror. More
output coupling increases the VCSEL's slope efficiency and will
further yield more power, depending on the internal losses of the
cavity.
[0056] In some examples, the power would be high enough to use the
VCSEL light directly, without requiring a separate amplification
stage, e.g., semiconductor optical amplifier (SOA).
[0057] Placement of Quantum Wells in the Gain Embedded DBR
Structure
[0058] For efficient extraction of gain from a VCSEL structure, the
quantum wells QW need to be placed in a region of high optical
field, preferably at an antinode of the VCSEL's standing wave
pattern. For example, in a GaAs/A.sub.0.97Ga.sub.0.03As DBR, the
strained InGaAs quantum well is placed near the junction of the
GaAs and Al.sub.0.97Ga.sub.0.03As, which is where the antinode is,
as shown in FIG. 3. The GaAs also serves as an absorbing region
where pump light is absorbed, and the excited electrons and holes
are funneled into the potential well formed by the strained InGaAs
quantum wells. As drawn, the GaAs and Al.sub.0.97Ga.sub.0.03As
regions have equal optical lengths to maximize the reflectivity of
the DBR. However, thicker GaAs could be grown with thinner
Al.sub.0.97Ga.sub.0.03As in order to keep the DBR period the same.
This would have the effect of increasing optical absorption while
decreasing the DBR reflectivity and bandwidth. This might be a
favorable tradeoff in some cases. It may also be preferable to
offset the quantum wells slightly from the
GaAs/Al.sub.0.97Ga.sub.0.03As interfaces so that the quantum well
barrier is GaAs on both sides. The quantum well should not be
placed at the left edge or in the center of the GaAs sections of
the DBR since this is at or close to the nodes of the standing wave
patterns at many wavelengths.
[0059] Hybrid Structures
[0060] Hybrid structures can be employed that are intermediate
between a separate gain region structure and a gain embedded DBR
structure. One way this can be done is by increasing the GaAs
thickness near the quantum wells and decreasing the AlGaAs
thickness to keep the same DBR period.
[0061] FIG. 5A shows a structure of this type, showing the energy
in the valence and conduction bands in electron Voltage (eV) in the
combined gain medium and DBR mirror GM/DBR. Here, the quantum wells
QW1-QW6 are located within respective high refractive index layers
H, and the low refractive index layers have a reduced thickness.
Yet, the optical period corresponds to roughly half a
wavelength.
[0062] FIGS. 5B and 5C are plots of wavelength nanometers and gain
per well as a function of the optical air gap. Calculations for
three of these structures are shown where the GaAs/AlGaAs DBR
period optical thicknesses are 50/50, 63/37, and 75/25 in percent
thickness. Adding GaAs is advantageous since it funnels more pump
light into excitation of the quantum wells.
[0063] Another variant could place some wells inside the DBR and
some outside. A 100/0 thickness variant would be considered a well
"outside" the DBR.
Exemplary Embodiment
[0064] A specific design for a MEMS tunable VCSEL 100 is presented
in FIGS. 6A and 6B that employs the combined gain medium and DBR
mirror, the GM/DBR. While many designs are possible, this one is
formed by bonding the half VCSEL chip or device 112 to a silicon
MEMS optical membrane device 110, which has an electrostatically
movable mirror for wavelength tuning.
[0065] In more detail, the optical membrane device 110 comprises
handle wafer material 210 that functions as a support. Currently,
the handle is made from doped silicon, with a resistivity <0.1
ohm-cm, carrier concentration >1.times.10.sup.17 cm.sup.-3, to
facilitate electrical contact.
[0066] An optical membrane or device layer 212 is added to the
handle wafer material 210. Typically, silicon on insulator (SOI)
wafers are used. An optical membrane structure 214 is formed in
this optical membrane layer 212. In the current implementation, the
membrane layer 212 is silicon that is low doped with resistivity
>1 ohm-cm, carrier concentration <5.times.10.sup.15
cm.sup.-3, to minimize free carrier absorption of the transmitted
light. For electrical contact, the membrane layer surface is
usually additionally doped by ion implantation to create a highly
doped surface layer (doped usually to >1.times.10.sup.18
cm.sup.-3, but at least 1.times.10.sup.17 cm.sup.-3 and at least
200 Angstroms (A) thick, usually 500-2000 A thick). This method
minimizes optical absorption in the membrane layer itself that
would occur if the entire layer were highly doped. An insulating
(buried silicon dioxide) layer 216 separates the optical membrane
layer 212 from the handle wafer material 210.
[0067] During manufacture of the membrane device in the SOI wafer
material, the insulating layer 216 functions as a
sacrificial/release layer, which is partially removed to release
the membrane structure 214 from the handle wafer material 210. Then
during operation, the remaining portions of the insulating layer
216 provide electrical isolation between the patterned device layer
212 and the handle material 210.
[0068] In the current embodiment, the membrane structure 214
comprises a body portion 218. The optical axis of the device 100
passes concentrically through this body portion 218 and orthogonal
to a plane defined by the membrane layer 212. A diameter of this
body portion 218 is preferably 300 to 600 micrometers; currently it
is about 500 micrometers.
[0069] Tethers 220 (four tethers in the illustrated example) are
defined and delineated by arcuate slots 225 fabricated into the
device layer 212. The tethers 220 extend radially from the body
portion 218 to an outer portion 222, which comprises the ring where
the tethers 220 terminate. In the current embodiment, a spiral
tether pattern is used.
[0070] A membrane mirror dot 250 is disposed on body portion 218 of
the membrane structure 214. In some embodiments, the membrane
mirror 250 is optically curved to form an optically concave optical
element to thereby form a curved mirror laser cavity. In other
cases, the membrane mirror 250 is a flat mirror, or even possibly
convex.
[0071] When a curved membrane mirror 250 is desired, this curvature
can be created by forming a depression in the body portion 218 and
then depositing the material layer or layers that form mirror 250
over that depression. In other examples, the membrane mirror 250
can be deposited with a high amount of compressive material stress
that will result in its curvature.
[0072] The membrane mirror dot 250 is preferably a reflecting
dielectric mirror stack. In some examples, it is a dichroic
mirror-filter that provides a defined reflectivity, such as between
1 and 99.9%, to the wavelengths of laser light generated in the
laser 100, whereas the optical dot 250 is transmissive to
wavelengths of light that are used to optically pump the active
region in the VCSEL device 112. Instill other examples, the optical
dot is a reflective metal layer such as aluminum or gold.
[0073] In the illustrated embodiment, four metal pads MP1, MP2,
MP3, and MP4 are deposited on the proximal side of the membrane
device 110. These are used to solder or thermocompression bond, for
example, the half VCSEL device 112 onto the proximal face of the
membrane device 110.
[0074] Also provided are two wire bondpads 334A, 334B. Membrane
wire bond pad 334A is used to provide an electrical connection to
the membrane layer 212 and thus the membrane structure 214. The
handle wire bond pad 334B is used to provide an electrical
connection to the handle wafer material 210.
[0075] The half VCSEL device 112 generally comprises an
antireflective coating 114, which is optional, and the combined
gain medium and DBR mirror GM/DBR. The cap layer can be used
between the antireflective coating 114, if present, and the
combined gain medium and DBR mirror GM/DBR. The cap layer protects
the active region from the surface/interface effects at the
interface to the AR coating and/or air.
[0076] In still other examples, the combined gain medium and DBR
mirror GM/DBR is a dichroic mirror-filter that provides a defined
reflectivity, such as greater than 99% to the wavelengths of laser
light generated in the laser 100, whereas the GM/DBR is
transmissive to wavelengths of light that are used to optically
pump the active region in the VCSEL device 112, thus allowing the
half VCSEL device 112 to function as an input port of pump
light.
[0077] In the example of a VCSEL operating with a center wavelength
around 1050 nm, the mirror reflectivities tend toward higher
numbers for light around 1050 nm in wavelength. For example, the
reflectivity of the GM/DBR is about 99.99% c. On the other hand,
the front mirror dot 250 is usually 99% or greater. In current
embodiments, the mirror dot 250 has a reflectivity of about 99.4%
or higher.
[0078] In operation, the VCSEL device is electrically or optically
pumped. The generated light resonates between the combined gain
medium and DBR mirror GM/DBR and membrane mirror 250. The
wavelength of the generated light is tuned within the scan band of
the device by the out of plane electrostatic deflection of the body
portion 218 and thus the membrane mirror 250 by controlling the
electrostatic field between the body portion 218 and the handle
wafer material 210 and/or the half VCSEL device 112.
[0079] FIG. 6B is schematic view showing the hidden bond pads
VP1-VP4 on the half VCSEL device 112.
[0080] The thin gold pads VP1-VP4 on the 1/2 VCSEL device 112 are
thermocompression bonded to corresponding membrane pads MP1, MP2,
MP3, and MP4 of the MEMS membrane (mirror) device 110 by placing
them in contact at high pressure and temperature to form the bond.
Typically, this is done at 300-360.degree. C. and 1-10N force.
Other elevated temperature bonding methods, such as gold-tin and/or
eutectic soldering, can be used to produce a similar effect,
however.
[0081] In some examples, when the bonded structure cools, strain is
introduced because of the different coefficients of thermal
expansion (CTE) between the silicon MEMS membrane device 110 and
the substrate material used for the optical gain medium of the half
VCSEL device 112. An asymmetric bond pad arrangement is used to
produce asymmetric stress. Specifically, the bond pad arrangement
is asymmetric in that the x-axis distance (X1) between the pads is
greater than the z-axis distance (Z1) between the pads. Said
another way, a distance between the bond pads is different between
two axes of a plane of the membrane device and the half VCSEL
device. The bond pad asymmetry is designed so that the added stress
asymmetry to the % VCSEL from bonding to the membrane device 112 is
between 20 and 100 MPa.
[0082] More details of the design and other designs can be found in
US Patent Application Pub. No. US 2014/0176958 A1, filed on 21 Dec.
2012, which is incorporated herein by this reference.
[0083] FIG. 7 also shows an example optically pumped tunable VCSEL
swept source system 101 employing the gain embedded DBR VCSEL 100,
which system has been integrated into a single module.
[0084] Light from a pump chip 760 is coupled to a bench 740 via a
pump optical fiber 742. The pump light 712 from the optical fiber
742 is collimated by a first lens LensA that is affixed to the
bench 740. The pump light 712 then is transmitted through the
dichroic mirror 732 and then focused by a second lens LensB onto
the half VCSEL 112 of the VCSEL 100.
[0085] Preferably, the bench 740, in turn, is installed in a
hermetic package 744 with optical fibers passing through
fiber-feedthroughs 746, 748 of the package 744.
[0086] The dichroic mirror 732 is reflective to longer wavelength
of the VCSEL light 734, emitted by the VCSEL 100, but transmissive
to the pump light 712, 724 in the illustrated example. Specifically
in the illustrated example, the tunable signal from the VCSEL 100
is reflected by the dichroic mirror 732, which is affixed to the
bench 740, and directed to a fold mirror 750 which is also affixed
to the bench 740 and then to a third lens 752, which is affixed to
the bench 740. The third lens 752 focuses light into an entrance
aperture of an output optical fiber 754.
[0087] More details of this specific design can be found in U.S.
Pat. Appl. Pub. No. US 2019/0348813 A1, which is incorporated
herein by this reference in its entirety.
[0088] While this invention has been particularly shown and
described with references to preferred embodiments thereof, it will
be understood by those skilled in the art that various changes in
form and details may be made therein without departing from the
scope of the invention encompassed by the appended claims.
* * * * *
References