U.S. patent application number 17/082087 was filed with the patent office on 2021-03-11 for oct system with bonded mems tunable mirror vcsel swept source.
The applicant listed for this patent is Axsun Technologies, Inc.. Invention is credited to Walid A. Atia, Dale C. Flanders, Bartley C. Johnson, Mark E. Kuznetsov.
Application Number | 20210075190 17/082087 |
Document ID | / |
Family ID | 1000005226113 |
Filed Date | 2021-03-11 |
United States Patent
Application |
20210075190 |
Kind Code |
A1 |
Flanders; Dale C. ; et
al. |
March 11, 2021 |
OCT System with Bonded MEMS Tunable Mirror VCSEL Swept Source
Abstract
A microelectromechanical systems (MEMS)-tunable vertical-cavity
surface-emitting laser (VCSEL) in which the MEMS mirror is bonded
to the active region. This allows for a separate electrostatic
cavity that is outside the laser's optical resonant cavity.
Moreover, the use of this cavity configuration allows the MEMS
mirror to be tuned by pulling the mirror away from the active
region. This reduces the risk of snap down. Moreover, since the
MEMS mirror is now bonded to the active region, much wider latitude
is available in the technologies that are used to fabricate the
MEMS mirror. This is preferably deployed as a swept source in an
optical coherence tomography (OCT) system.
Inventors: |
Flanders; Dale C.;
(Lexington, MA) ; Kuznetsov; Mark E.; (Lexington,
MA) ; Atia; Walid A.; (Jamaica Plain, MA) ;
Johnson; Bartley C.; (North Andover, MA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Axsun Technologies, Inc. |
Billerica |
MA |
US |
|
|
Family ID: |
1000005226113 |
Appl. No.: |
17/082087 |
Filed: |
October 28, 2020 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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16129856 |
Sep 13, 2018 |
10855053 |
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17082087 |
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15217124 |
Jul 22, 2016 |
10109979 |
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16129856 |
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13723829 |
Dec 21, 2012 |
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15217124 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01S 5/18305 20130101;
H01L 33/005 20130101; G01B 9/02004 20130101; H01S 5/02415 20130101;
H01S 5/06 20130101; H01S 5/183 20130101; H01S 5/02251 20210101;
G01B 9/02091 20130101; H01S 5/02216 20130101; H01S 5/068 20130101;
H01S 5/02235 20210101; H01S 5/18358 20130101; H01S 5/02224
20130101; H01S 5/041 20130101; H01S 5/0222 20130101; H01S 5/18366
20130101; H01S 5/02253 20210101; H01S 5/18375 20130101; H01S
5/18341 20130101 |
International
Class: |
H01S 5/068 20060101
H01S005/068; G01B 9/02 20060101 G01B009/02; H01L 33/00 20060101
H01L033/00; H01S 5/022 20060101 H01S005/022; H01S 5/024 20060101
H01S005/024; H01S 5/04 20060101 H01S005/04; H01S 5/06 20060101
H01S005/06; H01S 5/183 20060101 H01S005/183 |
Claims
1. A method for fabricating a microelectromechanical systems
(MEMS)-tunable vertical-cavity surface-emitting laser (VCSEL), the
method comprising: providing an active region substrate having
active layers that amplify light; and bonding an optical membrane
device to the active region substrate.
2. A method as claimed in claim 1, wherein bonding the optical
membrane device to the active region substrate comprises metal
bonding the optical membrane device to the active region
substrate.
3. A method as claimed in claim 1, wherein bonding the optical
membrane device to the active region substrate comprises solder
bonding the optical membrane device to the active region
substrate.
4. A method as claimed in claim 1, wherein bonding the optical
membrane device to the active region substrate comprises
thermocompression bonding the optical membrane device to the active
region substrate.
5. A method as claimed in claim 1, wherein a distance between the
active region substrate and the membrane device is less than a
micrometer.
6. A method as claimed in claim 1, further comprising fabricating
the optical membrane device by releasing a membrane structure by
partially removing a release layer.
7. A method as claimed in claim 6, wherein the membrane structure
is deflected between 1 and 3 .mu.m to tune the VCSEL.
8. A method as claimed in claim 1, further comprising using a
spacer device between the active region substrate and the optical
membrane device.
9. A method as claimed in claim 1, further comprising controlling a
polarization by applying asymmetric stress.
10. A method as claimed in claim 1, wherein the active region
substrate includes a multiple quantum well structure.
11. A method as claimed in claim 1, further comprising installing
the VCSEL in a package with a thermoelectric cooler.
12. A method as claimed in claim 1, further comprising coupling
pump light into the VCSEL with a lens that collimates laser light
exiting from the VCSEL.
13. A method as claimed in claim 1, further comprising installing
the VCSEL on an optical bench and emitting a swept optical signal
that propagates parallel to a top surface of the optical bench.
14. A method as claimed in claim 13, further comprising installing
a laser pump on the optical bench for generating pump light for
optically pumping.
15. A method as claimed in claim 1, further comprising deflecting
an optical membrane of the optical membrane device in a direction
away from the active region substrate to tune the VCSEL.
16. A method as claimed in claim 1, further comprising forming a
curved mirror structure on the optical membrane device.
Description
RELATED APPLICATIONS
[0001] This application is a Divisional of U.S. application Ser.
No. 16/129,856, filed on Sep. 13, 2018, which is a Divisional of
U.S. application Ser. No. 15/217,124, filed on Jul. 22, 2016, which
is a Divisional of U.S. application Ser. No. 13/723,829, filed on
Dec. 21, 2012, all of which are incorporated herein by reference in
their entirety.
BACKGROUND OF THE INVENTION
[0002] Optical coherence analysis relies on the use of the
interference phenomena between a reference wave and an experimental
wave or between two parts of an experimental wave to measure
distances and thicknesses, and calculate indices of refraction of a
sample. Optical Coherence Tomography (OCT) is one example
technology that is used to perform high-resolution cross sectional
imaging. It is often applied to imaging biological tissue
structures, for example, on microscopic scales in real time.
Optical waves are reflected from an object or sample and a computer
produces images of cross sections or three-dimensional volume
renderings of the sample by using information on how the waves are
changed upon reflection.
[0003] There are a number of different classes of OCT, but Fourier
domain OCT currently offers the best performance for many
applications. Moreover, of the Fourier domain approaches,
swept-source OCT has distinct advantages over techniques such as
spectrum-encoded OCT because it has the capability of balanced and
polarization diversity detection. It also has advantages for
imaging in wavelength regions where inexpensive and fast detector
arrays, which are typically required for spectrum-encoded OCT, are
not available.
[0004] In swept source OCT, the spectral components are not encoded
by spatial separation, but they are encoded in time. The spectrum
is either filtered or generated in successive optical frequency
sampling intervals and reconstructed before Fourier-transformation.
Using the frequency scanning swept source, the optical
configuration becomes less complex but the critical performance
characteristics now reside in the source and especially its
frequency sweep rate and tuning accuracy.
[0005] High speed frequency tuning, or high sweep rates, for OCT
swept sources is especially relevant to in-vivo imaging where fast
imaging reduces motion-induced artifacts and reduces the length of
the patient procedure. It can also be used to improve
resolution.
[0006] Historically, microelectromechanical systems (MEMS)-tunable
vertical-cavity surface-emitting lasers (VCSELs) have been used in
telecommunications applications. Their tunability enabled a single
laser to cover multiple channels of the ITU wavelength division
multiplexing grid.
[0007] More recently, these MEMS tunable VCSELs have been proposed
as the swept sources in swept source OCT systems. Here, they have a
number of advantages. Their short optical cavity lengths combined
with the low mass of their deflectable MEMS membrane mirrors enable
high sweep speeds. Moreover, they are capable of single
longitudinal mode operation and are not necessarily subject to mode
hopping noise. These characteristics also contribute to long
coherence lengths for deep imaging.
[0008] In one example, a MEMS tunable VCSEL uses an indium
phosphide (InP)-based quantum-well active region with a bonded
gallium arsenide (GaAs)-based oxidized mirror. An electrostatically
actuated dielectric mirror is suspended over the active region and
separated by an air gap that forms part of the electrostatic cavity
for the dielectric mirror. The mirror is monolithically fabricated
on top of the active region. The device is optically pumped by a
980 nanometer (nm) laser.
SUMMARY OF THE INVENTION
[0009] Monolithically forming the MEMS electrostatically actuated
dielectric mirror over the active region creates a number of
disadvantages, however. First, any processes required to form MEMS
mirror must be compatible with the chemistry of the active region.
Moreover, there is an overlap between the optical cavity that
extends between the active region and the MEMS mirror and the
electrostatic cavity of that MEMS mirror. This requires trade-offs
between the optimal electrostatic cavity, which is preferably small
to minimize drive voltage, and the air portion of the optical
cavity, which is preferably large to maximize tunability.
[0010] Another problem is that since the optical cavity and the
electrostatic cavity overlap at least to some degree, the MEMS
mirror is tuned by pulling the mirror toward the active region. If
too much voltage is applied, this mirror will then snap down and
possibly adhere to the active region destroying or damaging the
MEMS tunable VCSEL.
[0011] The present invention is similarly directed to a MEMS
tunable VCSEL. The difference is that the MEMS mirror is a bonded
to the active region. This allows for a separate electrostatic
cavity, that is outside the laser's optical resonant cavity.
Moreover, the use of this cavity configuration allows the MEMS
mirror to be tuned by pulling the mirror away from the active
region. This reduces the risk of snap down. Moreover, since the
MEMS mirror is now bonded to the active region, much wider latitude
is available in the technologies that are used to fabricate the
MEMS mirror.
[0012] In general, according to one aspect, the invention features
a MEMS tunable VCSEL, comprising an active region substrate having
active layers that amplify light and an optical membrane device
that is attached to the active region substrate.
[0013] In embodiments, a spacer device is used that separates the
active region substrate from the optical membrane device. Further,
the active region substrate comprises a rear mirror, which can be a
layer within the active region substrate or deposited in an optical
port formed into the active region substrate. In one case, the rear
mirror is a dichroic mirror that is reflective to the wavelengths
of light amplified by the active region substrate and transmissive
to wavelengths of light generated by a pump laser.
[0014] In a current example, the optical membrane device comprises
a substrate layer, a device layer, in which a membrane is
patterned, and intervening insulating layer. This insulating layer
defines an electrostatic cavity. As a result, an optical membrane
of the optical membrane device is deflected in a direction away
from the active region substrate.
[0015] In general, according to another aspect, the invention
features a method for fabricating a MEMS tunable VCSEL, comprising
providing an active region substrate having active layers that
amplify light and bonding an optical membrane device to the active
region substrate.
[0016] In some embodiments, bonding the optical membrane device to
the active region substrate comprises thermocompression bonding the
optical membrane device to the active region substrate. In other
examples, it comprises solder bonding the optical membrane device
to the active region substrate.
[0017] In general, according to another aspect, the invention
features an integrated VCSEL swept source system. This system
comprises an optical bench and a MEMS tunable VCSEL installed on
the optical bench that emits a swept optical signal that propagates
parallel to a top surface of the optical bench.
[0018] In embodiments, a focusing lens is secured to the optical
bench for coupling the swept optical signal into an optical fiber.
A hermetic package contains the optical bench with a thermoelectric
cooler preferably being installed between the optical bench and in
the hermetic package to control a temperature of the optical
bench.
[0019] In one example, a laser pump is installed on the optical
bench for generating pump light for optically pumping an active
layer within the MEMS tunable VCSEL. Preferably, an isolator is
used between the laser pump and the MEMS tunable VCSEL for
preventing back reflections into the laser pump. In different
examples, the swept optical signal is taken from one side of the
MEMS tunable VCSEL and the pump light is coupled into the other
side of the MEMS tunable VCSEL or the swept optical signal is taken
from the same side of the MEMS tunable VCSEL as the pump light is
coupled into the MEMS tunable VCSEL.
[0020] In some embodiments, a semiconductor optical amplifier is
installed on the optical bench that amplifies the swept optical
signal. Typically, two isolators are located on either side of the
semiconductor optical amplifier.
[0021] In an embodiment, the amplified swept optical signal from
the semiconductor optical amplifier is returned to propagate
through the MEMS tunable VCSEL. This can be accomplished with a
polarization beam splitter.
[0022] In still other examples, the MEMS tunable VCSEL is
electrically pumped.
[0023] In general, according to another aspect, the invention
features an optical coherence analysis system, comprising: an
interferometer that divides a swept optical signal between a
reference arm and a sample arm and combines optical signals
returning from the reference arm and the sample arm to generate an
interference signal, a MEMS tunable VCSEL that generates the swept
optical signal, the MEMS tunable VCSEL including an active region
substrate having active layers that amplify light, and an optical
membrane device that is attached to the active region substrate,
and a detection system that detects the interference signal.
[0024] 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
[0025] 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:
[0026] FIG. 1 is an exploded perspective view of a MEMS tunable
VCSEL according to the present invention;
[0027] FIG. 2 is a cross sectional schematic view of the MEMS
tunable VCSEL according to a first embodiment;
[0028] FIG. 3 is a cross sectional schematic view of the MEMS
tunable VCSEL according to a second embodiment;
[0029] FIG. 4 is a cross sectional schematic view of the MEMS
tunable VCSEL according to a third embodiment;
[0030] FIG. 5 is a cross sectional schematic view showing a laser
cavity configuration using a K-mirror;
[0031] FIG. 6 is a top plan schematic view of a swept source using
the MEMS tunable VCSEL that is pumped through the active region
substrate and emits light through the membrane device;
[0032] FIG. 7 is a top plan schematic view of a swept source using
the MEMS tunable VCSEL that is pumped through the membrane device
and emits light through the active region substrate;
[0033] FIG. 8 is a top plan schematic view of a swept source using
the MEMS tunable VCSEL that is pumped through the membrane device
and emits light through the membrane device;
[0034] FIG. 9 is a top plan schematic view of a swept source using
the MEMS tunable VCSEL with an integrated amplification stage;
[0035] FIG. 10 is a top plan schematic view of a swept source using
the MEMS tunable VCSEL with an integrated amplification stage and
that uses the VCSEL in a self tracking configuration; and
[0036] FIG. 11 is a schematic view of an OCT system incorporating
the MEMS tunable VCSEL according to the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0037] The invention now will be described more fully hereinafter
with reference to the accompanying drawings, in which illustrative
embodiments of and detailed approaches to implement 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.
[0038] As used herein, the term "and/or" includes any and all
combinations of one or more of the associated listed items.
Further, the terms: includes, comprises, including and/or
comprising, when used in this specification, specify the presence
of stated features, elements, and/or components, but do not
preclude the presence or addition of one or more other features,
elements, components, and/or groups thereof. Further, it will be
understood that when an element 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.
[0039] FIG. 1 shows a MEMS tunable VCSEL 100 comprising an optical
membrane device 110 that is bonded to an active region substrate
112, which has been constructed according to the principles of the
present invention.
[0040] Generally, in the MEMS tunable VCSEL 100, a spacer device
114 separates the active region substrate 112 from the membrane
device 110 to thereby define its laser cavity. As a general rule,
the thickness of the spacer device is about a micrometer thick. In
some examples, the spacer is thicker than a micrometer to provide a
longer air portion of the optical cavity. In other examples, it is
less than a micrometer. Typically, the spacer device 114, active
region substrate 112, and the membrane device 110 are bonded
together using a metal bonding technique such as solder bonding or
thermocompression bonding.
[0041] The optical membrane device 110 comprises handle material
210 that functions as a support. Preferably, the handle material is
wafer material such as from a silicon handle wafer, which has been
subsequently singulated into the illustrated device.
[0042] An optical membrane or device layer 212 is added to the
handle wafer material 210. The membrane structure 214 is formed in
this optical membrane layer 212. In the current implementation, the
membrane layer 212 is silicon. An insulating layer 216 separates
the optical membrane layer 212 from the handle wafer material
210.
[0043] During manufacture, 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.
Currently, the membrane layer is manufactured from a silicon wafer
that has been bonded to the insulating layer under elevated heat
and pressure. During operation, the insulating layer provides
electrical isolation between the device layer 212 and the handle
material 210.
[0044] In the current embodiment, the membrane structure 214
comprises a body portion 218. The optical axis 10 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.
[0045] 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.
[0046] An optically curved surface 250 is disposed on the membrane
structure 214. This optically curved surface 250 forms an optically
concave optical element to thereby form a curved mirror laser
cavity in conjunction with the active region substrate 112, which
currently includes is a flat mirror structure.
[0047] An optical coating dot 230 is typically deposited on the
body portion 218 of the membrane structure 214, specifically
covering the optically curved surface 250 of the optical element.
The optical dot 230 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 10%, to the
wavelengths of laser light generated in the laser 100, whereas the
optical dot is transmissive to wavelengths of light that are used
to optically pump the active layers in the active region substrate
112. In other examples, the optical dot is a reflective metal layer
such as aluminum or gold.
[0048] In the illustrated embodiment, artifacts of the manufacture
of the membrane structure 214 are etchant holes 232. These holes
allow an etchant to pass through the body portion 218 of the
membrane structure 214 to assist in the removal of the insulating
layer 216 during the release process.
[0049] In the illustrated embodiment, metal pads 234 are deposited
on the proximal side of the membrane device 210. These are used to
solder or thermocompression bond, for example, the spacing
structure 114 onto the proximal face of the membrane device
210.
[0050] This discrete spacing device 114 is avoided in other
embodiments where the spacing structure 114 is formed to be
integral with the membrane device 110 or active region substrate
112.
[0051] Bond pads 234 are also useful when installing the filter 100
on a micro-optical bench, for example.
[0052] Also provided is a membrane layer wire bond pad 334 that is
used as an electrode for electrical connections to the membrane
layer 212. A handle wafer wire bond pad 336 is used as the
electrode for electrical connections to the handle wafer material
210. The membrane layer bond pad 334 is a wire bonding location for
electrical control of the membrane layer 212. The handle wafer bond
pad 336 is a wire bond pad for electrical access to the handle
wafer material 210.
[0053] According to the invention, the active region substrate 112
comprises an active layer 118. This is preferably a single or
multiple quantum well structure.
[0054] The material system of the active region substrate 112 is
selected based on the desired spectral operating range. 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, GaInNAsSb, AlInGaAs,
InGaAsP, AlGaAsSb, AlGaInAsSb, AlAsSb, 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.
[0055] In the preferred embodiment, the polarization of the MEMS
tunable VCSEL 100 is preferably controlled and at least stabilized.
In general, this class of devices has a cylindrical resonator that
emits linearly polarized light. Typically, the light is polarized
along the crystal directions with one of those directions typically
being stronger than the other. Moreover, the direction of
polarization can change with laser current or pumping levels. The
behaviors often exhibit hysteresis. In one embodiment, polarization
selective mirrors are used. In another example, non-cylindrical
resonators are used. In still a further embodiment, asymmetrical
current injection is used when electrical pumping is used. In still
other examples, the active region substrate includes trenches or
materials layers that result in an asymmetric stress, strain, heat
flux or optical energy distribution that are used in order to
stabilize the polarization along a specified stable polarization
axis.
[0056] Defining the other end of the laser cavity is the rear
mirror 116 that is formed in the active region substrate 112. In
one example, this is a layer within the active region substrate
that creates the refractive index discontinuity that provides for a
portion of the light to be reflected back into the cavity, such as
between one and 10%. In other examples, the rear mirror 116 is a
high reflecting layer that reflects over 90% of the light back into
the laser cavity.
[0057] In still other examples, the rear mirror 116 is a dichroic
mirror-filter that provides a defined reflectivity, such as between
1 and 10%, to the wavelengths of laser light generated in the laser
100, whereas the rear mirror 116 is transmissive to wavelengths of
light that are used to optically pump the active layers in the
active region substrate 112, thus allowing the active region
substrate 112 to function as an input port of pump light.
[0058] FIG. 2 schematically shows the MEMS tunable VCSEL 100 in
cross-section.
[0059] An optical port 240 is provided, extending from a distal
side of the handle wafer material 210 to the membrane structure 214
in cases where the reflector 230 is used as an output reflector or
to provide for monitoring. If the reflector 230 is used as a back
reflector, then the port 240 is not necessary in some cases.
[0060] Further, whether or not this optical port 240 is required
also depends upon the transmissivity of the handle wafer material
210 at the optical wavelengths over which the MEMS tunable VCSEL
100 must operate. Typically, with no port, the handle wafer
material 210 along the optical axis must be anti-reflection coated
(AR) coated if transmission through the backside is required for
functionality.
[0061] The optically concave surface 250 is formed either as a
surface with a continuous curvature, a binary element, or a stepped
curvature of a Fresnel structure.
[0062] The optical port 240 has generally inward sloping sidewalls
244 that end in the port opening 246. As a result, looking through
the distal side of the handle wafer material, the body portion 218
of the membrane structure is observed. The port is preferably
concentric with the optical coating 230 and the optical surface
250. Further, the backside of the body portion 218 is coated with
an AR coating 119 in some examples. This AR coating is used to
facilitate the coupling of pump light into the laser cavity and/or
the coupling of laser light out of the cavity. In still other
examples, it is reflective to pump light to return pump light back
into the laser cavity.
[0063] The thickness of insulating layer 216 defines the
electrostatic cavity length. Presently, the insulating layer is 216
is between 3.0 and 6.0 .mu.m thick. It is a general rule of thumb,
that electrostatic elements can be tuned over no greater than one
third the distance of the electrostatic cavity. As result, the body
portion 218, and thus the mirror optical coating 230 can be
deflected between 1 and 3 .mu.m, in one embodiment.
[0064] FIG. 3 schematically shows the MEMS tunable VCSEL 100 in
cross-section according to another embodiment.
[0065] In this example, the rear mirror 116 that is formed in the
active region substrate 112 is deposited at the bottom of rear
optical port 122. This optical port is formed into the back side of
the active region substrate 112. It is preferably formed with a
flat bottom. This is achieved by etching through to an etch stop
layer in the active region substrate 112.
[0066] The advantage of this embodiment is that the rear mirror 116
need not be formed within the active region substrate 112 as an
integral material layer. Instead, the rear mirror 116 is deposited
using standard thin-film deposition tools, in one implementation,
or as a metal layer. The use of the optical port 122 has the
advantage of locating the rear mirror 116 close to the active layer
118 and also close to the optical coating dot 230 that is typically
deposited on the body portion 218 of the membrane structure 214.
This has the effect of reducing the length of the laser optical
cavity that extends between the rear mirror 116 and the optical
coating dot 230. Such a short optical cavity increases the
potential tuning speed of the laser 100 while also reducing or
eliminating mode hopping noise.
[0067] FIG. 4 schematically shows the MEMS tunable VCSEL 100 in
cross-section according to another embodiment.
[0068] In this example, the rear mirror 116 again is formed in the
active region substrate 112 and deposited at the bottom of rear
optical port 122.
[0069] A current source 124 is used to electrically pump the active
layer 118 in the active region substrate 112. Specifically, the
current source 124 establishes a voltage across the active region
118. This embodiment avoids the need to optically pump the active
region 118 and the concomitant increase in complexity associated
with coupling the pump light into the laser cavity.
[0070] FIG. 5 shows a laser cavity configuration using a spatially
limited or K-mirror optically curved surface 250.
[0071] In more detail, the optically curved surface 250, which is
disposed on the body portion 218 of the membrane structure 214, is
spatially limited to preferentially support the resonance of only
the lower order optical spatial modes within the laser cavity.
Specifically, in the illustrated embodiment, only the lowest order
mode 126 fits within the extent of the curvature of the curved
surface 250.
[0072] The basic operation of such resonators is generally
disclosed in U.S. Pat. No. 7,327,772, which is incorporated herein
by this reference in its entirety. Such resonators typically rely
on the use of curved mirrors in which the spatial extent of the
mirrors is limited to preferentially reflect only the desired
modes, which is typically the only the lowest order spatial mode.
Such mirrors are sometimes referred to as K-mirrors.
[0073] FIG. 6 shows a swept source laser system 400 that
incorporates the MEMS tunable VCSEL 100.
[0074] In general, the laser system 400 is contained within a
butterfly package 410. The package 410 provides a hermetically
sealed environment for the components of the laser system 400.
Typically, a moisture getter is included in the package 410.
Further, in some embodiments, the atmosphere in the hermetic
package 410 is largely an inert gas such as helium or nitrogen. In
other examples, a reactive component is included within the
atmosphere that is sealed within the package 410. In one example,
this reactive component is oxygen or ozone. The reason for this is
described in more detail in US Pat. Publ. No. US 2012/0257210 A1,
which is incorporated herein in its entirety by this reference.
Briefly, the reactive element, such as oxygen chemically reacts
with organics contained within the package to produce a gas such as
carbon dioxide to thereby avoid package induced failure (PIF) that
is otherwise caused by the deposition of these organics on hot
optical facets of the laser.
[0075] In the preferred embodiment, the temperature of the laser
system 400 is also controlled. This can be achieved through the use
of a heater that heats the inside of the package 410 to a
temperature above the ambient temperature. In the preferred
embodiment, a thermoelectric cooler 412 is secured to the bottom of
the butterfly package 410. The electric drive currents are provided
to the thermoelectric cooler 412 via electrodes of 414 and 416.
[0076] A micro-optical bench 420 is secured to the top of the
thermoelectric cooler 412. As result, the heat generated by active
components such as pump lasers that are mounted to the optical
bench 420 are removed from the package 410 via the thermoelectric
cooler 412.
[0077] The optical components are mounted to the top side of the
micro-optical bench 420. In particular, in this embodiment and the
other illustrative embodiments, the optical components are
installed on the optical bench 420 such that there optical axes are
parallel to the planar top surface of the optical bench 420.
[0078] In more detail, in the illustrated embodiment, the MEMS
tunable VCSEL 100 produces an output optical signal through the
membrane device 110. Preferably, the MEMS tunable VCSEL 100 is
directly, tombstone mounted, to the top of the bench 420.
Particularly, the membrane device 110, for example, is mounted such
that its membrane is orthogonal to the plane of the top surface of
the bench 420. In a similar vein, the plane of the active layers
118 in the active region substrate 112 are also similarly
perpendicular to the plane of the top surface of the bench 420.
[0079] As a result, the light exiting from the MEMS tunable VCSEL
100 propagates in a direction that is parallel to the top surface
of the bench 420. It is collimated by output lens 438 that is
mounted to the bench 420 and coupled into the fiber facet of the
optical fiber 440 that is secured to the bench 420. This optical
fiber passes through the side wall of the hermetic package 410 via
a fiber feed through 418.
[0080] In the illustrated example, the MEMS tunable VCSEL 100 is
optically pumped. Specifically, light at pump frequencies is
generated by a laser chip 430 that is also mounted to the top of
the bench 420. Sometimes the chip is mounted on a sub mount, which
is in turn bonded to the bench 420. In one example, the laser pump
chip 430 is operated at 980 nm. The pump light exiting from the
laser chip is collimated by a first pump lens 432. The collimated
light passes through an isolator 434. The light exiting from the
isolator 434 is then focused via a second pump lens 436 into the
active region substrate 112 of the MEMS tunable VCSEL 100. In the
illustrated embodiment, the first pump lens 432, the isolator 434,
and the second pump lens 436 are mounted to the bench 420.
[0081] FIG. 7 illustrates another embodiment of the laser system
400. This embodiment is largely similar to the embodiment
illustrated and discussed with respect to FIG. 6. The difference
is, however, in the orientation of the MEMS tunable VCSEL 100. The
MEMS tunable VCSEL 100 produces an output optical signal through
the active region substrate 112. This light from the MEMS tunable
VCSEL 100 is similarly collimated by output lens 438 and coupled
into the fiber facet of the optical fiber 440. The light exiting
from the pump chip 430 is coupled into the MEMS tunable VCSEL 100
through the membrane device 110.
[0082] FIG. 8 illustrates another embodiment of the laser system
400. In the specific embodiment shown, the swept signal that is
generated in the MEMS tunable VCSEL 100 is coupled out through the
membrane device 110 and the light from the pump laser 430 is
coupled in through the membrane device 110.
[0083] Certainly in alternative embodiment, the orientation of the
MEMS tunable VCSEL 100 could be reversed with the light being
coupled in and out through the active region substrate 112.
[0084] In either case, the light exiting from the MEMS tunable
VCSEL 100 is collimated by a collimating lens 442 and then
transmitted to the output lens 438 that couples the swept optical
signal into the optical fiber 440.
[0085] The use of the two relay lenses 442, 438 collimates the pump
light to be transmitted through a beam splitter/beam combiner
element 444. In one example, the splitter/combiner element 444 is a
polarization beam splitter. In other examples, it is a wavelength
division multiplexing filter element. In either case, the
splitter/combiner element 444 is transmissive to the light exiting
from the MEMS tunable VCSEL 100 either due to its polarization, or
its wavelength.
[0086] In contrast, the splitter/combiner element 444 is reflective
to the light that is generated by the pump laser 430. As result,
the light that is generated by the pump laser 430, collimated by
the first pump lens 432 and collimated by the second pump lens 436,
transmitted through the isolator 434, and redirected by fold mirror
446 is reflected by the splitter/combiner element 444 to be focused
by the collimating lens 442 to be coupled into the MEMS tunable
VCSEL 100.
[0087] Thus in this embodiment, the swept optical signal is taken
from the MEMS tunable VCSEL 100 from the same side from which the
MEMS tunable VCSEL 100 is optically pumped.
[0088] FIG. 9 illustrates another embodiment of the laser system
400. This embodiment is similar to that shown in FIG. 8 in terms of
how the swept optical signal is coupled out of the MEMS tunable
VCSEL 100 and the manner in which the light from the pump laser 430
is coupled into it. This embodiment differs, however, in that it
has an integrated amplification stage.
[0089] In more detail, the swept optical signal that is transmitted
through the splitter/combiner element 444 is collimated by a first
gain stage lens 450 to a first gain stage isolator 452. The light
exiting from the first gain stage isolator 452 is then focused by a
second gain stage lens 454 to be coupled into a semiconductor
optical amplifier (SOA) 456.
[0090] This SOA 456 is installed on the top of the optical bench
420. Often, an intervening sub mount is used. The SOA 456 amplifies
the swept optical signal generated by the MEMS tunable VCSEL 100
and the amplified signal is emitted through the output facet and
collimated by a third gain stage lens 458. This collimates the
amplified swept optical signal to pass through a second gain stage
isolator 460. Finally, the output lens 438 couples the amplified
swept optical signal into the output optical fiber 440.
[0091] FIG. 10 illustrates another embodiment of the laser system
400 that similarly has an integrated amplification stage. This
embodiment differs from the embodiment described with respect to
FIG. 9 in that the MEMS tunable VCSEL 100 is implemented in a
double pass, self-tracking configuration.
[0092] In more detail, the light that is transmitted through the
splitter/combiner element 444, in addition to being coupled through
the first gain stage isolator 452, is also transmitted through a
polarization beam splitter 470. In the configuration illustrated,
the polarization of light that is emitted by the MEMS tunable VCSEL
100 is transmitted directly through the polarization beam splitter
470.
[0093] Similar to the previous embodiment, this light is coupled
into the SOA 456, and passes through the second gain stage isolator
460. The light then passes through a fourth gain stage lens 462 and
is reflected by a series of fold mirrors 464, 466, and 468 to be
coupled back to the polarization beam splitter 470.
[0094] The second gain stage isolator 460 in this embodiment
further includes a half wave plate. This has the effect of rotating
the polarization of the swept optical signal that was amplified by
the SOA 456 by 90.degree.. At this polarization, the light is
reflected by the polarization beam splitter to be transmitted back
to the MEMS tunable VCSEL 100. It is then coupled through the MEMS
tunable VCSEL 100 and exits out through the active region substrate
112 to be collimated by the output lens 438 into the optical fiber
440.
[0095] This configuration helps to remove any amplified spontaneous
emissions from the SOA 456 when the light is transmitted back
through and filtered by the MEMS tunable VCSEL 100.
[0096] In one example, the MEMS tunable VCSEL lases in only one
polarization to eliminate ASE from the second pass.
[0097] FIG. 11 shows an optical coherence analysis system 12, such
as a tomography system, in which the MEMS tunable VCSEL 100 is used
to generate the swept optical signal.
[0098] An optical swept source system 400 generates the tunable or
swept optical signal on optical fiber 440 that is transmitted to
interferometer 500. The swept optical signal scans over a scan band
with a narrowband emission.
[0099] The swept source system 400 is generally intended for high
speed tuning to generate swept optical signals that repeatedly scan
over the scan band(s) at rates of greater than 1 kiloHertz (kHz).
In current embodiments, the swept source system 400 tunes at speeds
greater than 20 or 100 kHz. In very high speed embodiments, the
multi-sweep rate swept source system 100 tunes at speeds greater
than 200 or 500 kHz.
[0100] Typically, the width of the tuning or scan band is greater
than 10 nanometers (nm). In the current embodiments, it is
preferably between 50 and 150 nm, although even wider tuning bands
are contemplated in some examples. On the other hand, the bandwidth
of the narrowband emission has a full width half maximum (FWHM)
bandwidth of less than 20 or 10 GigaHertz (GHz), and is usually 5
GHz or less. For optical coherence tomography, this high spectral
resolution implies a long coherence length and therefore enables
imaging deeper into samples, for example deeper than 5 millimeters
(mm). On the other hand, in lower performance applications, for
example OCT imaging less than 1 mm deep into samples, broader FWHM
passbands are sometimes appropriate, such as passbands of about 200
GHz or less.
[0101] The tuning speed can also be expressed in wavelength per
unit time. In one example, for an approximately 110 nm tuning band
or scanband and 100 kHz scan rate, assuming 60% duty cycle for
substantially linear up-tuning, the peak sweep speed would be 110
nm*100 kHz/0.60=18,300 nm/msec=18.3 nm/.mu.sec or faster. In
another example, for an approximately 90 nm tuning range and 50 kHz
scan rate, assuming a 50% duty cycle for substantially linear
up-tuning, the peak sweep speed is 90 nm*50 kHz/0.50=9,000
nm/msec=9.0 nm/.mu.sec or faster. In a smaller tuning band example
having an approximately 30 nm tuning range and 2 kHz scan rate,
assuming a 80% duty cycle for substantially linear tuning, the peak
sweep speed would be 30 nm*2 kHz/0.80=75 nm/msec=0.075 nm/.mu.sec,
or faster.
[0102] Thus, in terms of scan rates, in the preferred embodiments
described herein, the sweep speeds are greater than 0.05 nm/.mu.sec
and preferably greater than 5 nm/psec. In still higher speed
applications, the scan rates are higher than 10 nm/psec.
[0103] A controller 590 generates a filter, or tunable element,
drive waveform or waveform that is supplied to a digital to analog
converter (DAC) 572. This generates a tunable element drive signal
508 that is amplified by amplifier 574 and applied to the optical
swept source system 400 as the electrostatic drive signal that is
applied across the electrostatic cavity of the membrane substrate
110 via the membrane layer bond pad 334 and the handle wafer bond
pad 336. In one example, the controller 590 stores the filter drive
waveform that linearizes the frequency sweep of the swept source
system 400.
[0104] A clock system 592 is used to generate k-clock signals at
equally spaced optical frequency sampling intervals as the swept
optical signal is tuned or swept over the scan or tuning band. A
swept source signal splitter 506 is used to provide a portion of
the swept source signal to the clock system 592.
[0105] In the illustrated example, a Mach-Zehnder-type
interferometer 500 is used to analyze the optical signals from the
sample 5. The swept optical signal from the optical swept source
system 400 is transmitted on fiber 440 to a 90/10 optical fiber
coupler 510 or other beam splitter, to give specific examples. The
swept optical signal is divided between a reference arm 520 and a
sample arm 512 of the system 12.
[0106] The optical fiber of the reference arm 520 terminates at the
fiber endface 524. The light 502R exiting from the reference arm
fiber endface 524 is collimated by a lens 526 and then reflected by
a reference mirror 528 to return back, in some exemplary
implementations.
[0107] The reference mirror 528 has an adjustable fiber to mirror
distance, in one example. This distance determines the depth range
being imaged, i.e. the position in the sample 5 of the zero path
length difference between the reference arm 520 and the sample arm
512. The distance is adjusted for different sampling probes and/or
imaged samples. Light returning from the reference mirror 528 is
returned to a reference arm circulator 522 and directed to an
interference signal combiner 540, such as a 50/50 fiber coupler. In
other examples, such as those using free space optical
configurations, the combiner 540 is a partially reflecting
mirror/beam splitter.
[0108] The fiber on the sample arm 512 terminates at the sample arm
probe 516. The exiting swept optical signal 502S is focused by the
probe 516 onto the sample 5. Light returning from the sample 5 is
returned to a sample arm circulator 514 and directed to the
interference signal combiner 540.
[0109] The reference arm signal and the sample arm signal are
combined or mixed in the interference signal combiner 540 to
generate an interference signal.
[0110] The interference signal is detected by a detection system
550. Specifically, a balanced receiver, comprising two detectors
552, is located at each of the outputs of the fiber coupler 540 in
the illustrated embodiment. The electronic interference signal from
the balanced receiver 552 is amplified by amplifier 554, such as a
transimpedance amplifier.
[0111] A data acquisition and processing system 555 of the
detection system 550 is used to sample the interference signal
output from the amplifier 554. The k-clock signals derived from the
clock system 592 are used by the data acquisition and processing
system 555 to synchronize system data acquisition with the
frequency tuning of the optical swept source system 400.
Specifically, the data acquisition and processing system 555
samples the interference signals in response to the k-clock signals
to generate evenly spaced samples of the interference signal in the
optical frequency domain.
[0112] A complete data set is collected of the sample 5 by
spatially raster scanning the focused probe beam point over the
sample 5 in a Cartesian geometry x-y fashion or a cylindrical
geometry theta-z fashion. The spectral response at each one of
these points is generated from the frequency tuning of the optical
swept source system 400. Then, the data acquisition and processing
system 555 performs a Fourier transform on the data in order to
reconstruct the image and perform a 2D or 3D tomographic
reconstruction of the sample 5. This transformed data is displayed
by the display system 580.
[0113] In one application, the probe 516 is inserted into blood
vessels and used to scan the inner walls of arteries and veins. In
other examples, other analysis modalities are included in the probe
such as intravascular ultrasound (IVUS), forward looking IVUS
(FLIVUS), high-intensity focused ultrasound (HIFU), pressure
sensing wires, and image guided therapeutic devices. In still other
applications, the probe is used to scan different portions of an
eye or tooth or other structure of a patient or animal.
[0114] 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.
* * * * *