U.S. patent application number 09/757061 was filed with the patent office on 2003-05-29 for method and apparatus for coupling a surface-emitting laser to an external device.
Invention is credited to Anselm, Klaus Alexander, Baillargeon, James N., Hwang, Wen-Yen, Murry, Stefan J..
Application Number | 20030099273 09/757061 |
Document ID | / |
Family ID | 25046199 |
Filed Date | 2003-05-29 |
United States Patent
Application |
20030099273 |
Kind Code |
A1 |
Murry, Stefan J. ; et
al. |
May 29, 2003 |
Method and apparatus for coupling a surface-emitting laser to an
external device
Abstract
A surface emitting laser is coupled to an external modulator.
The laser and the modulator aligned by photolithographically
defined features. In a preferred embodiment, the electromagnetic
output of the laser is reflected at a right angle from a mirror
mounted on a substrate. The reflected output enters a modulator
mounted on the same substrate as the mirror. A circuit coupled to
the modulator controls the modulation undergone by the
electromagnetic output. The modulated output is coupled to an
optical fiber for transmission.
Inventors: |
Murry, Stefan J.; (Houston,
TX) ; Baillargeon, James N.; (Sugar Land, TX)
; Anselm, Klaus Alexander; (Sugar Land, TX) ;
Hwang, Wen-Yen; (Sugar Land, TX) |
Correspondence
Address: |
APPLIED OPTOELECTRONICS, INC.
13111 JESS PIRTLE BLVD.
SUGAR LAND
TX
77478
US
|
Family ID: |
25046199 |
Appl. No.: |
09/757061 |
Filed: |
January 9, 2001 |
Current U.S.
Class: |
372/108 |
Current CPC
Class: |
G02B 6/4244 20130101;
G02B 6/4266 20130101; H01S 5/423 20130101; G02B 6/4257 20130101;
H01S 5/02326 20210101; G02B 6/4214 20130101; G02B 6/4249 20130101;
G02B 6/4245 20130101 |
Class at
Publication: |
372/108 |
International
Class: |
H01S 005/00; H01S
003/08 |
Claims
What is claimed is:
1. A system for coupling one or more surface-emitting lasers to one
or more corresponding external optical devices, the system
comprising: (a) the one or more surface-emitting lasers, each laser
adapted to produce an electromagnetic output; and (b) an optical
bench substrate having photolithographically defined therein, for
each said surface-emitting laser, a coupling mirror positioned to
receive the output of a corresponding surface-emitting laser and a
corresponding optical device positioned to receive the output of
said surface-emitting laser from the coupling mirror.
2. The system of claim 1, wherein the one or more surface-emitting
lasers are vertical-cavity surface-emitting lasers (VCSELs).
3. The system of claim 1, wherein the one or more surface-emitting
lasers are fabricated in a laser substrate, the system further
comprising said laser substrate, the laser substrate and the
optical bench substrate comprising photolithographically defined
alignment features for aligning said laser and optical bench
substrates together so as to optically couple each of the
surface-emitting lasers to corresponding optical devices mounted on
said optical bench substrate via a corresponding coupling
mirror.
4. The system of claim 1, wherein said optical devices are
electroabsorption optical modulators.
5. The system of claim 4, wherein the optical bench substrate
further comprises, for each said coupling mirror and corresponding
optical modulator, a driver circuit coupled to the modulator.
6. The system of claim 5, wherein the electromagnetic output is
modulated at more than 1 Gb/s by the optical modulator.
7. The system of claim 4, wherein the optical bench substrate
further comprises, for each said coupling mirror and corresponding
optical modulator, a semiconductor optical amplifier positioned in
the path of the electromagnetic output from the corresponding
optical modulator.
8. The system of claim 4, wherein the optical modulators are
edge-absorbing, edge-emitting optical modulators.
9. The system of claim 1, wherein said optical devices are
semiconductor optical amplifiers (SOAs).
10. The system of claim 1, wherein said optical devices are optical
fibers.
11. The system of claim 1, wherein the optical bench substrate
further comprises, for each said coupling mirror and corresponding
optical device, an optical fiber coupled to the corresponding
optical device.
12. The system of claim 11, wherein the optical bench substrate has
photolithographically defined therein, for each said coupling
mirror and corresponding optical device, a V-shaped slot for
positioning the optical fiber to receive the output of the
corresponding optical device.
13. The system of claim 1, wherein: said optical devices are
electroabsorption optical modulators; the optical bench substrate
further comprises, for each said coupling mirror and corresponding
optical modulator, an optical fiber coupled to the corresponding
optical modulator; the optical bench substrate has
photolithographically defined therein, for each said coupling
mirror and corresponding optical modulator, a V-shaped slot for
positioning the optical fiber to receive the output of the
corresponding optical device; and the system is for use in one of
data communications or telecommunications.
14. The system of claim 1, wherein each coupling mirror is a planar
coupling mirror.
15. The system of claim 14, wherein each planar coupling mirror is
inclined at an angle of 45.degree. relative to the path of the
electromagnetic output from said corresponding laser.
16. The system of claim 14, wherein the optical bench substrate
comprises, for each said coupling mirror and corresponding optical
device, a lens positioned between said coupling mirror and
corresponding optical device for shaping and/or focusing the laser
output between the coupling mirror and corresponding optical
device.
17. The system of claim 1, wherein each coupling mirror is a
concave coupling mirror for reflecting and focusing the output
light of a corresponding surface-emitting laser onto the
corresponding optical device positioned to receive the output of
said surface-emitting laser from the coupling mirror.
18. The system of claim 1, wherein the optical bench substrate is a
silicon optical bench.
19. The system of claim 1, wherein the one or more surface-emitting
lasers comprise a linearly-arranged one-dimensional array of lasers
and the optical bench substrate comprises a corresponding
linearly-arranged one-dimensional array of corresponding coupling
mirrors and optical devices.
20. The system of claim 1, wherein the one or more surface-emitting
lasers comprise a two-dimensional array of lasers and the optical
bench substrate comprises a corresponding two-dimensional array of
corresponding coupling mirrors and optical devices.
21. The system of claim 1, wherein the one or more surface-emitting
lasers comprise a plurality of surface-emitting lasers, each laser
adapted to produce an electromagnetic output having a unique
frequency different than the frequencies of said other lasers.
22. The system of claim 1, wherein the system is further for
measuring radiation absorption by a measurement species, the system
further comprising: one or more sources of single mode laser
radiation comprising the one or more surface-emitting lasers,
respectively; and a detector for detecting the single mode laser
radiation after passage thereof through a quantity of said
measurement species.
23. The system of claim 22, wherein said laser radiation is
infrared laser radiation.
24. The system of claim 23, wherein said measurement species is a
gas disposed in a measurement cell.
25. The system of claim 23, wherein said measurement species is an
unconfined gas.
26. The system of claim 23, wherein said measurement species is one
or more of human blood, a bacterial species, and a viral
species.
27. The system of claim 1, wherein said optical devices are optical
fiber amplifiers.
28. The system of claim 1, wherein each said optical device is a
top mirror for its respective laser which completes a laser cavity
for said laser.
29. A system for coupling one or more surface-emitting lasers to
one or more corresponding external optical devices, each laser
adapted to produce an electromagnetic output, the system comprising
an optical bench substrate having photolithographically defined
therein, for each said surface-emitting laser, a coupling mirror
positioned to receive the output of a corresponding
surface-emitting laser and a corresponding optical device
positioned to receive the output of said surface-emitting laser
from the coupling mirror.
30. The system of claim 29, wherein the one or more
surface-emitting lasers are fabricated in a laser substrate, the
laser substrate and the optical bench substrate comprising
photolithographically defined alignment features for aligning said
laser and optical bench substrates together so as to optically
couple each of the surface-emitting lasers to corresponding optical
devices mounted on said optical bench substrate via a corresponding
coupling mirror.
31. A method for coupling a surface-emitting laser to an external
optical device, the method comprising the steps of: (a) emitting an
electromagnetic output from the laser; and (b) reflecting the
electromagnetic output from a photolithographically defined mirror
in an optical bench substrate to the external optical device so as
to couple the electromagnetic output into the external optical
device.
32. The method of claim 31, wherein the optical device is an
optical modulator, the method comprising the further step of
modulating the reflected electromagnetic output with the optical
modulator.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] This invention relates generally to devices that emit
electromagnetic radiation and, in particular, to methods and
systems for coupling one or more surface-emitting lasers (SELs) to
external optical devices such as modulators.
[0003] 2. Description of the Related Art
[0004] The following descriptions and examples are not admitted to
be prior art by virtue of their inclusion within this section.
[0005] Lasers have a wide range of industrial and scientific uses.
There are several types of lasers, including gas lasers,
solid-state lasers, liquid (dye) lasers, and free electron lasers.
Semiconductor lasers are also in use. The possibility of
amplification of electromagnetic waves in a semiconductor
superlattice structure, i.e., the possibility of semiconductor
diode lasers, was predicted in a seminal paper by R. F. Kazarinov,
et al., "Possibility of the Amplification of Electromagnetic Waves
in a Semiconductor with a Superlattice," Soviet Physics
Semiconductors, vol. 5, No. 4, pp. 707-709 (October 1971).
Semiconductor laser technology has continued to develop since this
discovery.
[0006] There are a variety of types of semiconductor lasers.
Semiconductor lasers may be diode lasers (bipolar) or non-diode
lasers such as quantum cascade (QC) lasers (unipolar).
Semiconductor lasers of various types may be electrically pumped
(by a DC or AC current), or pumped in other ways, such as by
optically pumping (OP) or electron beam pumping. Semiconductor
lasers are used for a variety of applications and can be built with
different structures and semiconductor materials, such as gallium
arsenide.
[0007] Additionally, semiconductor lasers may be edge-emitting
lasers or surface-emitting lasers (SELs). Edge-emitting
semiconductor lasers output their radiation parallel to the wafer
surface, while in SELs, the radiation is output perpendicular to
the wafer surface.
[0008] Semiconductor lasers are typically powered by applying an
electrical potential difference across the active region, which
causes a current to flow therein. Electrons in the active region
attain high energy states as a result of the potential applied.
When the electrons spontaneously drop in energy state, photons are
produced. Some of those photons travel in a direction perpendicular
to the reflective planes of the laser. As a result of the ensuing
reflections, the photons can travel through the active region
multiple times. When those photons interact with other high energy
state electrons, stimulated emission can occur so that two photons
with identical characteristics are present. If most electrons
encountered by the photons are in the high energy state, the number
of photons traveling between the reflective planes tends to
increase. A typical laser includes a small difference in
reflectivity between its mirrors. The primary laser output is
emitted through the reflective plane having lower reflectivity.
[0009] The use of semiconductor diode lasers (both edge-emitting
and surface-emitting) for forming a source of optical energy is
attractive for a number of reasons. For example, diode lasers have
a relatively small volume and consume a small amount of power as
compared to conventional laser devices. Further, the diode laser is
a monolithic device, and does not require a combination of a
resonant cavity with external mirrors and other structures to
generate a coherent output laser beam.
[0010] At present, conventional edge-emitting semiconductor lasers
play a significant role in optical communication due to their high
operating efficiency and modulation capabilities. However,
edge-emitting semiconductor diode lasers have several shortcomings
which make them difficult to use in many applications. For example,
a conventional edge-emitting semiconductor laser typically has a
large divergence angle and an elliptical, as opposed to circular,
laser beam cross-section. This can require correction and
collimating, which can be expensive or otherwise impracticable or
undesirable.
[0011] SELs typically have a circular laser beam and a smaller
divergence angle, and are therefore more attractive than
edge-emitting lasers in some applications. One type of SEL is the
vertical cavity surface emitting laser (VCSEL). The VCSEL structure
usually consists of an active (gain) region sandwiched between two
distributed Bragg reflector (DBR) mirrors. The DBR mirrors of a
typical VCSEL can be constructed from dielectric or semiconductor
layers (or a combination of both, including metal mirror sections).
Other types of VCSELs sandwich the active region between metal
mirrors. The area between the reflective planes is often referred
to as the resonator. Further background discussion of VCSELs and
related matters are found in: U.S. Pat. No. 5,468,656 (1994), Shieh
et al., "Method of making a VCSEL"; U.S. Pat. No. 5,985,686 (1999),
Jayaraman, "Process for manufacturing vertical cavity surface
emitting lasers using patterned wafer fusion and the device
manufactured by the process"; MacDougal et al.,
"Electrically-Pumped Vertical-Cavity Lasers with AlO-GaAs
Reflectors", IEEE Photonics Letters, vol. 8, No. 3, March 1996. A
variant on the standard VCSEL, the vertical-external-cavity
surface-emitting laser (VECSEL), is also in use. VECSELs are
described in J. Sandusky & S. Brueck, "A CW External-Cavity
Surface-emitting Laser," IEEE Photon. Techn. Lett. 8, 313-315
(1996).
[0012] SELs such as VCSELs have other advantages, which has led to
an increased interest in these devices. For example, the
conventional VCSEL has several advantages, such as emitting light
perpendicular to the surface of the die, and the possibility of
fabrication of two dimensional arrays. Because VCSELs emit light
perpendicular to the die surface, it is cheaper and easier to test
them because they can be tested before dicing the wafer, unlike
edge-emitting lasers. Also, as noted above, the output laser beam
from an SEL has a much smaller divergence angle than a conventional
edge-emitting laser. Light with a smaller divergence angle is
easier to focus into a smaller spot size or collimate into a laser
beam that can maintain a smaller spot size after the beam has
traveled a long distance.
[0013] Semiconductor lasers such as VCSELs are used in a variety of
applications. In some applications, e.g., telecommunications and
spectroscopy among others, the output laser light is modulated to
achieve the objective of the system. Modulation consists of
modifying a characteristic of the laser output, e.g., the
amplitude, frequency, or phase. The modulation can be predetermined
by response characteristics of the optical target. U.S. Pat. No.
5,981,957, for example, shows a fluorometric system that employs a
predetermined modulation of a laser output used to detect the
fluorescence response of a sample. In this case, a major concern is
the accuracy of the modulation, because that accuracy limits the
accuracy of the measurements. The use of VCSELs for applications
such as spectroscopy is described in A. Garnache et al.,
"Application of a Diode-pumped Broadband Vertical-external-cavity
Surface-emitting Semiconductor Laser to High-sensitivity
Intracavity Absorption Spectroscopy," forthcoming in Journal of
Optical Society of America B (January 2000); A. Garnache et al.,
"High sensitivity Intra-Cavity Laser Absorption Spectroscopy with
Vertical-External-Cavity Surface-Emitting semiconductor Lasers,"
Optics Lett. 24 (1999): 826-828; and in U.S. Pat. No.
6,091,504.
[0014] In the case of telecommunications, the modulations are
patterned to correspond to information. When the radiation of the
output laser beam is detected after it has traveled to another
point, the modulations indicate the information that was encoded at
the transmitter/modulator end. A typical telecommunications system
uses optical fiber to guide the radiation from the modulation (or
emission) point to the detection point.
[0015] One major concern in such systems is ensuring that the
modulated radiation reaches the detection point with sufficient
power to be detected. Another major concern is choosing a
modulation scheme that can keep up with the rate of information
being provided, while ensuring that the receiver can decode the
information. In digital systems that information is provided as
bits. Current telecommunications systems can modulate laser
radiation according to a pattern that represents a billion or more
bits per second. Such rates of information transfer are referred to
as 1+ Gb/s. The conventional equipment necessary to modulate laser
radiation can be very expensive.
[0016] VCSELs have a thin active (gain) region (measured in the
direction of emission) and require high resonator efficiency. As a
result, low currents can drive a VCSEL. VCSELs are typically
amplitude modulated by varying the potential difference, and
therefore current, applied across the active region. A drop in
current reduces the rate at which photons are emitted. An increase
in current increases the number of photons emitted. In this manner,
the VCSEL can be "direct modulated" so as to (amplitude) modulate
the output laser beam. There is a limit, however, to the speed at
which the radiation output of the VCSEL will follow the change in
driving current. That limit can impede the use of VCSELs in
applications requiring a particular minimum rate of modulation.
[0017] It is, therefore, sometimes desirable to externally modulate
a VCSEL, or VCSELs of an array of VCSELs. To do this, the output of
the VCSEL must be coupled to an external modulator. It is also
desirable to use SELs in other applications, such as coupling the
output laser beam into a fiber or semiconductor optical amplifier
(SOA). In each such application or use of a VCSEL, it is necessary
to couple the output laser beam from the SEL into some other
optical device, such as a modulator, optical fiber, or SOA. The
term "optical device" as used herein refers to any device, external
to the SEL, to which the output of the SEL is coupled so that the
optical device receives the output laser beam. The SEL is a laser
beam source, and the optical device to which the SEL is optically
coupled may be regarded as a laser beam sink.
[0018] It can be difficult to accurately align the VCSEL and couple
it with desired optical device. For example, VCSELs are currently
"butt-coupled" with fibers, which can be expensive or
difficult.
SUMMARY OF THE INVENTION
[0019] The present invention is directed to a method and apparatus
for coupling a surface-emitting laser to an external optical device
such as an external modulator, fiber, or SOA. The laser and the
external optical device to which it is coupled are aligned by
photolithographically defined features.
[0020] An advantage of the invention is that it enables the output
of a surface emitting laser to be conveniently modulated.
[0021] Another advantage of the invention is that it permits
coupling a high percentage of power from a surface emitting laser
to an external modulator or other device.
[0022] Another advantage of the present invention is allowing data
modulation rates higher than direct modulation limits.
[0023] Another advantage is aligning arrays of surface emitting
lasers with arrays of modulators.
[0024] Still another advantage is decreasing the cost of
manufacturing lasers with high modulation rate capacity.
[0025] Another advantage is that it allows coupling to an SOA which
amplifies the output of the modulator.
[0026] Another advantage is separately optimizing the performance
of the laser and modulator.
[0027] Other and further features and advantages will be apparent
from the following description of presently preferred embodiments
of the invention, given for the purpose of disclosure and taken in
conjunction with the accompanying drawings. Not all embodiments of
the invention will include all the specified advantages. For
example, one embodiment may only modulate the output of a surface
emitting laser, while another only aligns an array of surface
emitting lasers with an array of modulators.
BRIEF DESCRIPTION OF THE DRAWINGS
[0028] Other features and advantages of the invention will become
apparent upon study of the following description, taken in
conjunction with the drawings in which:
[0029] FIG. 1 is a cross-sectional view of a one-dimensional array
of vertical cavity surface emitting lasers (VCSELs);
[0030] FIG. 2 is an isometric view of the one dimensional array of
VCSELs of FIG. 1;
[0031] FIG. 3 is a cross-sectional view of one embodiment of a
system for aligning and coupling the array of lasers of FIG. 2 to a
corresponding array of external modulators;
[0032] FIG. 4 is a front view of the system of FIG. 3;
[0033] FIG. 5 is a cross-sectional view of an alternative
embodiment of a system for aligning and coupling an array of lasers
to an array of external modulators;
[0034] FIG. 6 is an isometric view of a two-dimensional array of
VCSELs;
[0035] FIG. 7 is a cross-sectional view of a system for aligning
and coupling the two-dimensional array of lasers of FIG. 6 to a
corresponding array of external modulators; and
[0036] FIG. 8 is a block diagram of a spectroscopic measurement
system employing one or more VCSELs of the present invention.
DETAILED DESCRIPTION
[0037] Referring now to the drawings, the details of preferred
embodiments of the invention are schematically illustrated. Like
elements in the drawings will be represented by like numbers, and
similar elements will be represented by like numbers with a
different lower case letter suffix.
[0038] Referring now to FIG. 1, a one-dimensional array 10 of
VCSELs is shown in cross-section, the cross-sectional view
illustrating three VCSELs of VCSEL array 10. Array 10 is a
one-dimensional array having a single row of N VCSELs. In
alternative embodiments, a two-dimensional array having a plurality
of rows and columns, or other two-dimensional arrangements of
VCSELs, such as staggered rows of 2.times.N or 4.times.N, may be
employed. As will be appreciated, a VCSEL is a semiconductor laser
that emits its output perpendicular to its p-n junction. Each VCSEL
is built on a substrate 12 of semiconductor material. Various
semiconductor materials known to those in the art can be
employed.
[0039] Each VCSEL shares a first reflective plane 14. In one
embodiment, the first reflective plane 14 comprises a DBR. A DBR
consists of alternating layers of different semiconductors or
different dielectrics. In one embodiment, forty alternating layers
yield a reflectivity of 99.99%. In an alternative embodiment, the
first reflective mirror or plane 14 can comprise a metal mirror
rather than or in addition to a DBR.
[0040] Each VCSEL also shares the common active region 16 of VCSEL
array 10. The active region 16 is a p-n junction and the width of
the active region 16 controls the wavelength of emitted light. The
widths of the first reflective plane 14, active region 16, and
second reflective plane 18 are very small compared to the width of
the substrate 12. (The widths shown for the various layers in FIG.
1 are not to scale.)
[0041] In the embodiment shown, the second reflective plane 18 is
separate for each VCSEL. In another embodiment, the VCSELs can
share a common second reflective plane 18. Like the first
reflective plane 14, the second reflective plane 18 can comprise a
DBR or a metal mirror. The second reflective plane 18 is highly
reflective, but less reflective than the first reflective plane 14.
In one embodiment, twenty-five alternating layers of a DBR yield a
reflectivity of 99.9%.
[0042] A cladding 20 protects the VCSEL structure. Adjacent the
second reflective plane 18, the cladding 20 defines a gap 22 that
allows emission of radiation from the active region 16. The profile
of the emitted radiation can be controlled by modifying the
geometry of the emission area. For example, changes in the shape
and size of the gap 22 affect the spatial profile of the emitted
radiation. The distance between the active region 16 and the gap 22
also affects the spatial profile of the emitted radiation. The use
of photolithographic techniques in defining the features of the
VCSEL array allow highly accurate placing of the VCSELs and highly
accurate definition of VCSEL output profiles.
[0043] Referring now to FIG. 2, an isometric view of a linear array
10 of VCSELs is depicted. Most of the structural aspects of the
VCSELs are not visible from an outside view. The gaps 22 are
visible and allow radiation to be emitted. As with FIG. 1, the
dimensions have been rendered disproportional in order to make
visible the various features. In various embodiments, the VCSEL
separation and height are much reduced relative to the thickness of
the substrate. FIG. 2 shows a linear array 10 of VCSELs
manufactured such that radiation from each VCSEL is emitted along
substantially parallel paths. While the array shown has three
VCSELs, other embodiments include arrays having a large number of
linearly arranged VCSELs.
[0044] As noted above, there is a need to couple the output(s) of
one or more SELs of an array of SELs to respective laser beam
"sinks," i.e. devices that receive the output of a given VCSEL. For
example, it may be desired to couple the output of each VCSEL of
VCSEL array 10 to a respective modulator. Referring now to FIG. 3,
there is shown a cross-sectional view of an embodiment of a system
300 in which the VCSELs of VCSEL array 10 are aligned with and
coupled to corresponding devices, namely modulators 54, for
externally modulating the VCSELs. In an embodiment, modulators 54
are edge-absorbing and edge-emitting optical modulators of the type
typically utilized for modulation of output from edge-emitting
lasers.
[0045] FIG. 3 shows in cross-section VCSEL 22 of VCSEL array 10. In
alternative embodiments, SELs other than VCSELs may be employed,
such as a grating-coupled surface emitting laser (GCSEL). GCSELs
are described in U.S. Pat. No. 5,867,521 (Macomber); and R. J. Noll
& S. H. Macomber, "Analysis of Grating Surface Emitting
Lasers," IEEE J. Quantum Electronics, vol. 26, no. 3, March 1990,
pp. 456-466, the entireties of which are incorporated herein by
reference. In alternative embodiments, the VCSELs may be
operatively (optically) coupled to devices other than modulators
54.
[0046] As shown in FIG. 3, contacts 60, 62 are electrically coupled
to different sides of the active region of VCSEL 22. Those of skill
in the art are aware of several means by which contacts can be
mounted on a surface-emitting laser. The laser can then be powered
by current provided through the contacts 60, 62. The laser emits a
coherent electromagnetic output, i.e. a laser beam, which is to be
coupled to a device such as modulator 54. When the output laser
beam of a given VCSEL is coupled to the input of a given device
receiving the laser output, the VCSEL may be said to be coupled (or
operationally coupled or optically coupled) to the device.
[0047] In an embodiment, laser array 10 is mounted on a substrate
50. The substrate 50 includes a planar mirror 52 that is inclined
at some desired angle relative to the surface of the substrate 50.
In alternative embodiments, the mirror 52 is positioned at other
angles. In a preferred embodiment, mirror 52 is inclined at
45.degree. relative to the surface of substrate 50, because devices
such as modulator 54, SOA 56, and/or fiber 58 are preferably
mounted on substrate 50 in the plane of the surface of substrate 50
and the laser beam emitted by VCSEL 22, for example, is
perpendicular to the surface of substrate 50, so that a 45.degree.
angle mirror will optimally couple laser output from VCSEL 22 to
modulator 54.
[0048] As will be appreciated, various fabrication techniques may
be employed to form inclined mirror 52 having a desired angle, such
as 45.degree.. For example, KOH may be used as an etchant to form a
45.degree. angle inclined mirror 52, when using a silicon optical
support bench substrate 50. KOH etches into silicon at
approximately 54.7.degree. relative to the (001) crystal plane
along the (111) plane, however. Therefore, in one embodiment, the
silicon substrate is cut at an offset from the (001) crystal plane
towards, e.g., the (110) plane, so that during etching, the mirror
52 is etched at the desired angle (e.g., 45.degree.). In the case
of a linear array of VCSELs, the silicon substrate may be cut such
that the laser radiation is reflected into the input side of a
device, such as modulator 54, mounted in a V-groove also etched
into the substrate 50. However, for other types of VCSEL arrays,
such as a 2.times.N array, the silicon may be cut at a rotating
offset from the <011> crystal plane. This can cause the
reflected laser radiation to be in the plane of the surface of
substrate 50, but offset at an azimuthal angle from V-grooves. In
this case, alignment structures other than V-grooves (e.g.,
mechanical stops and guides) may be employed to properly align
coupling devices such as modulator 54, SOA 56, and/or fiber 58 with
the reflected laser radiation.
[0049] Alternatively, silicon substrate 50 may not be cut at an
offset from the <011> crystal plane, so that KOH etching to
form the inclined planar mirror surface 52 is at an angle of
approximately 54.7.degree. relative to the surface. In this case,
if it is desired to reflect the VCSEL laser radiation into the
plane of the surface of substrate 50, VCSEL array 10 may be mounted
at an angle off of normal to the surface. For example, an end of
VCSEL array 10 may be etched at an end (e.g., the left end shown in
cross-section in FIG. 3) to have a mounting surface angled relative
to its own surface, and a mounting structure formed on substrate 50
with a top surface parallel to substrate 50's surface (this
mounting structure, e.g., may be formed to the left of mirror 52 in
the cross-sectional view shown in FIG. 3). In such an embodiment,
the angle of the emitted radiation from VCSEL 22 is such that it
reflects off a 54.7.degree. inclined mirror 52 into the plane of
the surface of substrate 50 and thus is coupled into a device such
as modulator 54. When other materials are used for substrate 50,
other etchants may be employed, (e.g., H.sub.2SO.sub.4,
H.sub.2O.sub.2, H.sub.2O for a GaAs substrate).
[0050] Whether a Si, GaAs, or other substrate is used, a metal
coating or dielectric coating may be employed to minimize mirror 52
losses. For example, metals such as Al, Au, or Ag may be employed
to form a reflective surface for mirror 52. Mirror 52 may also be
fabricated with a suitable DBR dielectric stack, e.g. Si/SiO.sub.2
or Al.sub.2O.sub.3/Si, or in combination with Al, Au, or Ag metals.
[also CAN BE USED FOR S]
[0051] In an embodiment, mirror 52 and V-shaped slot (V-groove) 72
(illustrated in FIG. 4) can be fabricated using lithographic
alignment of a mask with the substrate 50 that has had a desired
pattern imposed on the substrate by lithography. By aligning the
substrate 50 in a preferred orientation prior to lithography,
subsequent etching can take place preferentially along various
crystal axes of the substrate, including the <111> axis or
another axis that is oriented at a desired angle to the <100>
axis. These and other alignment techniques are described in Hauffe,
et al. "Methods of Passive Fiber Chip Coupling of Integrated
Optical Devices," Proc. 2000 Electronic Components and Technology
Conference (May 2000), pp.238-243, and in Optoelectronic Packaging,
eds. Mickelson et al. (New York: John Wiley & Sons, 1997), esp.
ch. 9, "Array Device Packaging," by Nagesh R. Basavanhally &
Ronald A. Nordin ("Basavanhally & Nordin"). Chapter 9
(Basavanhally & Nordin) of the Optoelectronics Packaging text
is incorporated herein by reference.
[0052] In an embodiment, to precisely align the VCSELs of array 10
with corresponding optical devices, for effective optical coupling,
array 10 is mounted on substrate 50, which forms a silicon (or
other material) optical bench, using various photolithographically
formed or etched alignment features, such as mechanical alignment
stops, notches, pedestals, standoffs, and the like. These and other
"silicon optical bench" (SiOB) alignment techniques are described
in Basavanhally & Nordin, esp. pp. 143-148. By employing such
alignment techniques, the VCSELs of a VCSEL array may be easily
aligned with and thus coupled to corresponding optical devices
mounted in precisely aligned positions on a support bench.
[0053] For example, VCSELs of VCSEL array 10 are lithographically
formed in precise positions in a common substrate 12 (FIG. 1).
Similarly, various coupling structures (e.g., mirror 52) and
external optical device alignment features (e.g., V-groove 72) are
lithographically defined into optical support bench substrate 50.
The latter alignment features permit devices such as modulator 54,
SOA 56, and fiber 58 to be mounted on bench 50 in precise alignment
with mirrors 52. The VCSEL substrate 12 and bench substrate 50 also
include alignment features that permit the precise mounting of the
VCSEL array 10 with corresponding external optical devices mounted
in external optical device alignment positions of bench substrate
50. In this manner, an entire array 10 of VCSELs can be aligned
together, with their corresponding optical devices (54), instead of
having to align each VCSEL independently.
[0054] In addition to alignment, to actually "flip-chip" mount
laser array 10 on substrate 50, any suitable mounting technique may
be utilized. For example, solder bump technology, bonding metal, or
epoxy bonding may be employed.
[0055] The laser output will diverge in profile as it travels. In
another embodiment, therefore, a lens is used to shape the output
between the laser and the mirror 52. In such embodiments, the lens
can be placed in an etched groove between the mirror 52 and the
modulator 54, or between the SOA 56 and the fiber 58.
Alternatively, two lenses may be employed, with one being
positioned in either of the aforementioned positions.
[0056] An alternative embodiment of the invention involves the
etching of a non-planar mirror in place of the plane mirror 52. For
example, in the embodiment illustrated in FIG. 3, such a non-planar
mirror would preferably be concave so as to focus the light output
of the laser 22 onto the modulator 54 or other optical component
(e.g., SOA, fiber, etc.). In this way, the lens envisioned in the
previously described embodiment may be eliminated. Lenses may also
be placed between other components in various other alternative
embodiments of the present invention.
[0057] The laser output reflects off the mirror 52 and a modulator
54 is positioned on the substrate 50 in the path of the output. In
this manner, the laser output from VCSEL 22 is coupled to modulator
54, i.e. VCSEL 22 is optically coupled with modulator 54.
[0058] In one embodiment, the optical modulator 54 is an
electroabsorption modulator (EAM). Contacts 64, 68 are provided to
control the modulation. The contacts 64, 68 can be provided by
various methods known to persons of skill in the art. For example,
the contacts 64, 68 can be flip-chip bonded. In another embodiment,
the contacts 64, 68 are wirebonded from the modulator to an
adjacent contact pad mounted on the substrate 50. When modulation
is desired, a driver circuit can be used to provided electrical
signals to one or both of the contacts 64, 68, as will be
appreciated by those skilled in the art. The laser output is then
modulated in accordance with the electrical signals.
[0059] In an embodiment, the output of the modulator 54 is
optically coupled to SOA 56, which increases the power of the
modulated laser output. The SOA 56 includes contacts 66, 70 that
can be mounted and placed as discussed with respect to the
modulator 54. The contacts 66, 70 provide power to the SOA 56. For
some applications, the SOA 56 is not necessary. In alternative
embodiments, amplifiers or optical devices other than the SOA 56
may be used. For example, an optical fiber can be doped to produce
an optical amplifier and used in place of the SOA 56. In one of
many possible embodiments, the SOA 56 is mounted on the substrate
50. In alternative embodiments, the SOA 56 is placed between the
mirror 52 and the modulator 56 in the path of the laser output.
[0060] An optical fiber 58 is preferably provided and optically
coupled to the SOA 56. The optical fiber 58 guides the modulated
and amplified laser output to its destination. Persons of ordinary
skill in the art are aware of techniques for coupling the optical
fiber 58 to the SOA 56 (or in an alternative embodiment to the
modulator 54), such as butt-coupling. The alignment of the VCSEL
and modulator 54 may typically allow, in various embodiments, 3 dB
of the power from the laser to enter the modulator 54 (i.e., 50%
coupling efficiency). The use of an external modulator such as
modulator 54 allows data to be encoded in the laser output at rates
above 1 Gb/s.
[0061] Referring now to FIG. 4, there is shown a front view of
system 300. As illustrated in FIG. 4, the substrate 50 can continue
to the left and right sides of FIG. 4 with additional
laser-modulator combinations being mounted thereon. From the view
of FIG. 4, some of the contacts 60, 62, and 66 are visible as well
as a portion of the mirror 52. Additionally, the SOA 56 can be seen
behind the fiber 58 to which it is coupled. A V-shaped slot or
channel 72 in the substrate 50 is visible in FIG. 4. The V-shaped
slot 72 precisely aligns the fiber 58 to receive the modulated and
amplified laser output with minimum loss of power. In alternative
embodiments, the fiber 58 may be positioned without a slot, or with
a differently configured slot. As will be appreciated, the
alignment of various components of embodiments of the present
invention may employ SiOB techniques, as described above. In such
SiOB techniques, a single crystal semiconductor material such as
silicon is typically utilized as the support structure (optical
bench) for various optical devices. This typically includes etching
channels (e.g., the V-groove 72) on the surface of a silicon
substrate in order to provide for mounting of optical components or
to increase coupling efficiency between the components.
Accordingly, the embodiment of the present invention shown in FIGS.
3-4 provides a method and system for accurately and efficiently
aligning and coupling a SEL to an optical device such as a
modulator.
[0062] Referring now to FIG. 5, there is shown a cross-sectional
view of an alternative embodiment of a system 500 for aligning and
coupling an array of lasers to an array of external modulators. The
view of FIG. 5 shows a single VCSEL 501 of the array, in
cross-section. The embodiment of FIG. 5 differs from that of FIGS.
3-4 in that an optical fiber section 58 is placed between the
mirror 52 and the modulator 54. Thus, in system 500, the VCSEL 501
is optically coupled to fiber 58, which is itself optically coupled
to modulator 54. In this manner, VCSEL 501 is optically coupled
(via fiber 58) to modulator 54. As will be understood, optical
fiber 58 may, in some embodiments, be able to be placed closer to
mirror 52 than modulator 54 can be, thus reducing the laser path
distance from the mirror to the input of the device. This can
reduce the divergence of the laser output. In an alternative
embodiment, a lens is used in place of the fiber 58 to focus the
laser output and reduce or counteract divergence. While VCSEL 501,
optical fiber 58, modulator 54, SOA 56, and optical fiber 58 are
shown mounted on a common substrate 50, in another embodiment the
optical fiber 58 can be employed between the mirror 52 and
modulator 54 where the modulator 54 is mounted on a separate
substrate.
[0063] Referring now to FIG. 6, an isometric view of a substrate
mounted array 80 of VCSELs is illustrated. The array 80 extends in
two dimensions having both rows and columns. While the 2-D array 80
is shown with two columns and two rows, other embodiments include
arrays with more rows and/or columns, or with other 2-D
arrangements (such as staggered columns). Arrays of VCSELs (both
one dimensional and two dimensional) can be used to provided input
sources for Wavelength Division Multiplexed (WDM) systems. In an
embodiment, each VCSEL of array 80 generates a different wavelength
laser beam. The different wavelength output beams are modulated and
then combined in a single optical fiber. At the destination, the
wavelengths are separately detected and demodulated. Combining
several modulation systems such as those shown in FIGS. 3 and 5, a
WDM system can be configured.
[0064] Referring now to FIG. 7, there is shown a cross-sectional
view of a system 700 for aligning and coupling the two-dimensional
laser array 800 of FIG. 6 to a corresponding array of external
modulators 54. The 2-D array 80 is shown having two columns. The
number of rows is not shown. In various embodiments, there could be
one to many rows. A single substrate 50 mounts various optical
devices for aligning, modulating, and amplifying the VCSELs in each
column. These devices are mounted for each column as discussed with
reference to FIG. 3. The configuration of FIG. 7 allows for
efficient manufacture of a plurality of VCSELs that can be
accurately aligned with and coupled to a corresponding plurality of
modulators for external modulation.
[0065] The coupled VCSELs and systems of the present invention may
be employed in a variety of applications, such as data
communications, telecommunications, spectroscopy, and biosensing
applications. For example, one or more VCSELs of the present
invention may be used as radiation sources to perform molecular
spectroscopy to determine the molecular composition of various
measurement species. The measurement species can be a gas or
liquid, for example (disposed in a measurement cell or an
unconfined gas or liquid). The measurement species can also be
human compounds, such as blood, in the case of biosensing
spectroscopy applications. For biosensing, the measurement species
could also be a human- or biological-related compound containing
bacteria and/or viruses to be detected, in which case the
measurement species may be regarded as a bacterial species or viral
species, respectively. Thus, the VCSELs of the present invention
may also be utilized as radiation sources for pollution monitoring
and other applications that involve absorption measurements.
[0066] These embodiments are illustrated in FIG. 8, which is a
block diagram of a spectroscopic measurement system 800 employing
one or more VCSELs of the present invention, such as VCSEL 300 of
FIG. 3. The electromagnetic infrared radiation (laser beam) 811 is
emitted from fiber 58 of a VCSEL of the VCSEL array of system 300
of FIG. 3, so that it passes through a given measurement species
812. The measurement species may be, for example, gas in cell,
unconfined gas, or blood sample. The molecules in the species
selectively absorb various wavelength radiation, and the exited
radiation then impinges on a conventional detector 815. A computer
814 may then analyze the result to determine the presence of
certain elements in the species 812.
[0067] In an embodiment for spectroscopic absorption analysis of a
gas in a cell, for example, one or more VCSELs of the present
invention may be employed in a point sensing apparatus, as
described, for example, in U.S. Pat. No. 5,901,168, the entirety of
which is incorporated herein by reference. In such an embodiment,
the (one or more) VCSEL(s) provides mid-IR radiation and is mounted
on a temperature-controlled stage for coarse wavelength tuning.
Mid-IR radiation from the laser passes through a conventional gas
cell (optionally a multi-pass cell), with exited radiation
impinging on a conventional detector. The gas cell, in this case,
contains the measurement species. The electrical output of the
detector is supplied to a lock-in amplifier (together with an
appropriate modulation signal, e.g., a 1.2 kHz sine wave from a
modulation signal generator), and the lock-in amplifier output is
supplied to a computer for data analysis and formatting. The data
is then displayed and/or stored in any suitable manner. The VCSEL
is pumped with an appropriate electrical current. For instance, a
low frequency current ramp (e.g., 250 ms period) from a ramp
current generator, short bias pulses (e.g., 5 ns pulse width, 2
.mu.s period) from a bias current generator, and a modulation
signal from a modulation current generator are supplied to a
combiner, and the resultant current ramp with superimposed current
pulses and sine wave is applied to the laser. The current ramp
serves to sweep the laser temperature over a predetermined range,
and the pulses cause the emission of short laser pulses. The pulse
wavelength is slowly swept over a range of wavelengths, and
absorption as a function of wavelength is determined. Thus, the
presence in the cell of a gas that has an absorption line in the
range of wavelengths is readily detected, and the gas can be
identified. In such an embodiment, the output of the VCSEL is
coupled in accordance with the invention to the gas in the cell,
e.g. by coupling the VCSEL to a fiber which is then used to pass
the mid-IR radiation through the gas in the cell. In alternative
embodiments, VCSEL(s) of the present invention produce radiation at
other than mid-IR wavelengths, for spectroscopic or other
applications.
[0068] VCSELs of the present invention may also be employed in an
embodiment in which the measurement species is an unconfined gas
(e.g. for pollution monitoring). For example, one or more VCSELs of
the present invention may be utilized in a remote-sensing system,
wherein an emission source such as a factory emits a gaseous
emission cloud (the unconfined gas or measurement species). One or
more VCSELs of the present invention is positioned to emit mid-IR
radiation which propagates through the emission cloud, and is
reflected (e.g., by means of a corner reflector). The reflected
radiation is then detected by means of a detector. The VCSEL can be
pumped in any appropriate manner, e.g., as described above, and the
detector output can be utilized in any appropriate manner, e.g.,
also as described above. A mirror or other appropriate reflector
can be used instead of a corner reflector. The reflector can be on
an aircraft or any elevated feature, including the smoke stack that
is being monitored. The detector could also be on an aircraft, or
be on an elevated feature. In general, any arrangement that results
in a line-of-sight disposition of laser and detector is
contemplated. In such an embodiment, the output of the VCSEL(s) is
coupled in accordance with the invention to the unconfined gas. For
example, multiple VCSELs of an array may each be coupled to a
respective fiber, which are then combined into a single fiber,
which directs the combined mid-IR radiation from all the VCSELs
through the unconfined gas.
[0069] In such applications and embodiments, the VCSEL of the
present invention will generally be mounted in an appropriate
housing for protection and control. The package will typically
comprise cooling means (e.g., water cooling, thermoelectric
cooling), temperature sensor means (e.g., a thermocouple) for use
in a feedback loop for temperature control, and means for applying
the pump current to the laser. The VCSEL is attached in
conventional fashion to the cooling means. Optionally the housing
may also contain detector means for controlling laser output power.
The housing will typically have a window that is transparent for
the laser radiation, and will typically be evacuated or filled with
inert gas.
[0070] In the case of biosensing applications, for example, the
output radiation at wavelengths appropriate for absorption by the
element or compound to be detected, is coupled in accordance with
the present invention to a fiber, which directs the radiation into
the measurement species (e.g., the human blood beneath the skin of
a finger).
[0071] As will be appreciated, multiple VCSELs may be employed in
such applications. For example a 2.times.2 array of 4 VCSELs, each
having a different wavelength, may be used to detect multiple
gases, or to better detect one gas. In such an application, for
example, each of the VCSELs may be coupled to a fiber, using the
coupling technique and system of the present invention. These
fibers may be used to direct the radiation of all four VCSELs
through the gas species to be measured. Other variations may be
employed, such as coupling the output of multiple VCSELs to a
single fiber to emit the combined (superimposed) radiation through
the gas species.
[0072] Therefore, in such applications, one or more VCSELs are
coupled in accordance with the present invention to a measurement
species such as a gas disposed in a measurement cell or an
unconfined gas. The VCSELs may be coupled to the gas via fibers to
which they are respectively coupled, in accordance with the present
invention, for example. In this application, the coupled VCSELs of
the present invention are part of a system for measuring infrared
radiation absorption by a measurement species, where the system has
one or more sources of single mode infrared laser radiation
(comprising the coupled VCSEL(s)), and a detector for detecting the
single mode infrared laser radiation after passage thereof through
a quantity of said measurement species.
[0073] Coupled VCSELs of the present invention may also be utilized
for communications applications, such as datacom and telecom. For
example, an array of VCSELs may be fabricated, each of which has a
narrow and closely-spaced wavelength in an appropriate range (e.g.,
around 850 or 1310 nm for datacom, or 1550 or 1310 nm for telecom),
and each coupled in accordance with the invention to an external
modulator. The modulated signals may be combined in a single fiber,
and transmitted to subsequent destinations (e.g. shorter distances
for datacom applications, larger distances for telecom
applications). Alternatively, all of the VCSEL outputs may be
combined together and modulated together for distribution on
different wavelengths to various laser beam sinks.
[0074] In embodiments of the present invention described above, the
output of a VCSEL (of VCSEL array 10, for example) is coupled to
some optical device utilizing various coupling structures (e.g.,
mirror 52) and external optical device alignment features (e.g.,
V-groove 72) which are lithographically defined into optical
support bench substrate 50. In an alternative embodiment, one or
more of the optical devices in substrate 50 form part of the
working VCSEL. For example, optical support bench substrate 50 may
include the top mirror on the other side of mirror 52 to complete
the laser cavity. This top mirror may be formed, for example, in a
fiber amplifier or fiber portion. Thus, for example, referring once
more to FIG. 5, in an embodiment, fiber 58 has a modulated
refractive index to form both a gain and mirror (DBR) portion.
Thus, in this embodiment, VCSEL 501 is only a "partial" VCSEL,
having some gain, where the overall VCSEL has gain portions both in
VCSEL 501 and in fiber portion 58, having its top mirror portion in
fiber 58, and having the laser cavity along the laser path from the
bottom edge of VCSEL 501 and the right-most edge of fiber 58 (in
accordance with the orientations shown in FIG. 5). Thus, in this
embodiment, the VCSEL portion 501 comprising part of the gain and
the "bottom" mirror is coupled, via mirror 52 and substrate 50, to
gain/top mirror portion of fiber 58. Alternatively, fiber 58 may
have only a DBR on the fiber end (right-most) to form the top
mirror, with the gain region being located exclusively within VCSEL
portion 501. As used herein, in such configurations, the VCSEL
portion (e.g. 501) fabricated in its own substrate (e.g., substrate
12) may be referred to as a "surface-emitting laser" which is
coupled to an external optical device, namely the "top" mirror (and
possibly a gain portion) which is mounted in support bench
substrate 50, where the coupling is by way of mirror 52 and the
coupling completes the surface-emitting laser operation by
providing the laser cavity and the top mirror.
[0075] The present invention, therefore, is well adapted to carry
out the objects and attain the ends and advantages mentioned, as
well as others inherent therein. While the invention has been
depicted and described and is defined by reference to particular
preferred embodiments of the invention, such references do not
imply a limitation on the invention, and no such limitation is to
be inferred. The invention is capable of considerable modification,
alteration and equivalents in form and function, as will occur to
those ordinarily skilled in the pertinent arts. The depicted and
described preferred embodiments of the invention are exemplary only
and are not exhaustive of the scope of the invention. Consequently,
the invention is intended to be limited only by the spirit and
scope of the appended claims, giving full cognizance to equivalents
in all respects.
* * * * *