U.S. patent application number 11/027907 was filed with the patent office on 2005-10-20 for active optical alignment and attachment thereto of an optical component with an optical element formed on a planar lightwave circuit.
Invention is credited to Boudreau, Robert A., Shmulovich, Joseph.
Application Number | 20050232547 11/027907 |
Document ID | / |
Family ID | 35096350 |
Filed Date | 2005-10-20 |
United States Patent
Application |
20050232547 |
Kind Code |
A1 |
Boudreau, Robert A. ; et
al. |
October 20, 2005 |
Active optical alignment and attachment thereto of an optical
component with an optical element formed on a planar lightwave
circuit
Abstract
A method and apparatus is provided for attaching a external
optical component processing an optical beam to a PLC and optically
aligning the external optical component with an optical element
formed on the PLC. The method begins by securing the external
optical component to a first side of a submount. A first side of a
flexure element is secured to the first side of the submount. A
second side of the flexure element is secured to a first side of
the PLC on which the optical element is formed such that the
external optical component and the optical element are in optical
alignment to within a first level of tolerance. Subsequent to the
step of securing the second side of the flexure element, a force is
exerted on at least a second side of the submount to thereby flex
the flexure element. The force causes sufficient flexure of the
flexure element to optically align the external optical component
and optical element to within a second level of tolerance that is
more stringent than the first level of tolerance.
Inventors: |
Boudreau, Robert A.;
(Corning, NY) ; Shmulovich, Joseph; (New
Providence, NJ) |
Correspondence
Address: |
MAYER, FORTKORT & WILLIAMS, PC
251 NORTH AVENUE WEST
2ND FLOOR
WESTFIELD
NJ
07090
US
|
Family ID: |
35096350 |
Appl. No.: |
11/027907 |
Filed: |
December 30, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11027907 |
Dec 30, 2004 |
|
|
|
10826145 |
Apr 15, 2004 |
|
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Current U.S.
Class: |
385/52 |
Current CPC
Class: |
B81C 3/002 20130101;
G02B 6/4226 20130101; H01L 2924/01322 20130101; H01L 2924/12041
20130101; G02B 6/4238 20130101; G02B 6/422 20130101; H01L
2224/81801 20130101 |
Class at
Publication: |
385/052 |
International
Class: |
G02B 006/26 |
Claims
1. A method of attaching an external optical component processing
an optical beam to a PLC and optically aligning the external
optical component with an optical element formed on the PLC, said
method comprising the steps of: a. securing the external optical
component to a first side of a submount; b. securing a first side
of a flexure assembly to the first side of the submount; c.
securing a second side of the flexure assembly to a first side of
the PLC on which the optical element is located, such that the
external optical component and the optical element are in optical
alignment to within a first level of tolerance and; d. subsequent
to step (c), exerting a force on at least a second side of the
submount to thereby flex the flexure assembly, said force causing
sufficient flexure of the flexure assembly to optically align the
external optical component and optical element to within a second
level of tolerance that is more stringent than the first level of
tolerance.
2. The method of claim 1 wherein the flexure assembly has a
prescribed shape and is formed as a single unitary element.
3. The method of claim 1 wherein the flexure assembly has a
prescribed shape and is formed from a plurality of different
elements.
4. The method of claim 3 further comprising the step of forming the
flexure assembly by connecting each of the plurality of different
elements by a process selected from the group consisting of
soldering, brazing, thermo-compression bonding and welding.
5. The method of claim 1 wherein the flexure assembly comprises a
first flexure element affixed to a second flexure element such that
that the first and second flexure elements are situated one above
the other and laterally offset from one another.
6. The method of claim 5 wherein the first and second flexure
elements are substantially planar flexure elements.
7. The method of claim 1 further comprising the step of monitoring
an optical coupling efficiency of an optical beam propagating
between the external optical component and optical element on
PLC.
8. The method of claim 7 wherein the step of exerting a force is
performed on the submount, such that the coupling efficiency is
maximized.
9. The method of claim 1 wherein the optical element is a facet of
a planar waveguide formed on the PLC.
10. The method of claim 1 wherein the optical element is a grating
formed on the PLC for facilitating optical coupling from the
external optical component into another structure located on the
PLC.
11. The method of claim 1 wherein the external optical component is
selected from the group consisting of a semiconductor laser, a
semiconductor optical amplifier, a LED, a beam splitter, a thin
film filter, an optical filter, a lens, a passive optical
component, a mirror, a birefringent material, a polarizer, and a
diffractive element.
12. The method of claim 1 wherein the external optical component is
a semiconductor laser.
13. The method of claim 1 wherein the external optical component is
a LED.
14. The method of claim 1 wherein the external optical component
has an active, light emitting surface that faces the PLC.
15. The method of claim 1 wherein the submount is formed from
aluminum nitride.
16. The method of claim 1 wherein the flexure assembly is
fabricated from a low yield material that is given to deformation
without a restoring reaction.
17. The method of claim 1 wherein the flexure assembly is formed
from gold or a gold alloy.
18. The method of claim 1 wherein the flexure assembly is formed
from lead.
19. The method of claim 1 wherein the flexure assembly is formed
from nickel or a nickel alloy.
20. The method of claim 1 wherein the flexure assembly element is
formed from TM Kovar.TM..
21. The method of claim 1 wherein the flexure assembly is formed
from a thermally conductive material sufficient to serve as a heat
sink for the external optical component.
22. The method of claim 1 wherein the second side of the submount
on which the force is exerted is a back surface of the submount
opposing the first side of the submount.
23. The method of claim 1 wherein the second side of the submount
on which the force is exerted is an edge of the submount.
24. The method of claim 1 further comprising the step of etching a
pocket in the PLC at a location under the external optical
component so that sufficient clearance is available for attaching
the external optical component below a surface of the PLC inside
the pocket.
25. The method of claim 1 further comprising the step of enclosing
the external optical component and the submount with a cover that
mates with the first side of the PLC.
26. The method of claim 25 wherein said cover has an etched pocket
allowing clearance for the external optical component and the
submount.
27. The method of claim 26 wherein the cover forms a hermetic seal
with the first side of the PLC.
28. The method of claim 27 wherein the cover is formed from a
material having a thermal expansion comparable to silicon so that
the cover it will not break during attachment or device
operation.
29. The method of claim 27 wherein the cover is formed from
Kovar.TM..
30. The method of claim 27 wherein the cover is formed from
silicon.
31. The method of claim 27 wherein the cover is formed from
Pyrex.TM..
32. The method of claim 27 wherein the hermetic seal is established
by a solder seal ring.
33. An optical apparatus constructed in accordance with the method
of claim 1.
34. A method of attaching an external optical component to a planar
substrate and optically aligning said device with an optical
element located on the planar substrate, said method comprising the
steps of: a. securing the external optical component to a first
side of a submount; b. securing a first side of a flexure assembly
to the first side of the submount; c. securing a second side of a
flexure assembly to a first side of the planar substrate on which
the optical element is secured such that the external optical
component and the optical element are in optical alignment to
within a first level of tolerance and; d. subsequent to step (c),
exerting a force on at least a second side of the submount to
thereby flex the flexure assembly, said force causing sufficient
flexure of the flexure assembly to optically align the external
optical component and the optical element to within a second level
of tolerance that is more stringent than the first level of
tolerance, whereby the external optical component and the planar
substrate are not in direct contact with one another.
35. The method of claim 34 wherein the flexure assembly has a
prescribed shape and is formed from a plurality of different
elements.
36. The method of claim 34 wherein the flexure assembly has a
prescribed shape and is formed as a single unitary element.
37. The method of claim 35 further comprising the step of forming
the flexure assembly by connecting each of the plurality of
different elements by a process selected from the group consisting
of soldering, brazing, thermo-compression bonding and welding.
38. The method of claim 34 wherein the flexure assembly comprises a
first flexure element affixed to a second flexure element such that
that the first and second flexure elements are situated one above
the other and laterally offset from one another.
39. The method of claim 38 wherein the first and second flexure
elements are substantially planar flexure elements.
40. The method of claim 34 further comprising the step of
monitoring an optical coupling efficiency of an optical beam
propagating between the external optical component and the optical
element.
41. The method of claim 40 wherein the step of exerting a force is
performed such that the coupling efficiency is maximized.
42. The method of claim 34 wherein the optical element is an
optical fiber.
43. The method of claim 34 wherein the optical element is a
grating.
44. The method of claim 34 wherein the external optical component
is selected from the group consisting of a semiconductor laser, a
semiconductor optical amplifier, or a Light Emitting Diode (LED) or
a combination thereof.
45. The method of claim 34 wherein the external optical
component
46. The method of claim 34 wherein the external optical component
is a LED.
47. The method of claim 1 wherein the external optical component
has an active, light emitting surface that faces the PLC.
48. The method of claim 34 wherein the submount is formed from
aluminum nitride.
49. The method of claim 34 wherein the flexure is fabricated from
low yield material that is given to deformation without restoring
reaction.
50. The method of claim 34 wherein the flexure assembly is formed
from gold or a gold alloy.
51. The method of claim 34 wherein the flexure assembly is formed
from lead.
52. The method of claim 34 wherein the flexure assembly is formed
from nickel or a nickel alloy.
53. The method of claim 34 wherein the flexure assembly element is
formed from Kovar.TM..
54. The method of claim 34 wherein the flexure assembly is formed
from a thermally conductive material sufficient to serve as a heat
sink for the light generating device.
55. The method of claim 34 wherein the second side of the submount
on which the force is exerted is a back surface of the submount
opposing the first side of the submount.
56. The method of claim 34 wherein the second side of the submount
on which the force is exerted is an edge of the submount.
57. The method of claim 34 further comprises forming of etching a
pocket in the planar substrate under the light generating device,
so that sufficient clearance is available for attaching the light
generating device.
58. The method of claim 34 comprising the step of enclosing the
light generating device and the submount with a cover that mates
with the first side of the planar substrate.
59. The method of claim 58 wherein said cover has an etched pocket
allowing clearance for the light generating device on the
submount.
60. The method of claim 58 wherein the cover forms a hermetic seal
with the first side of the planar substrate.
61. The method of claim 58 wherein the cover is formed from
Kovar.TM..
62. The method of claim 58 wherein the cover is formed from
silicon.
63. The method of claim 58 wherein the cover is formed from
Pyrex.TM..
64. The method of claim 60 wherein the hermetic seal is established
by a solder seal ring.
65. An optical apparatus constructed in accordance with the method
of claim 34.
66. The method of claim 1 wherein the external optical component
and the PLC are not in direct contact with one another after
performing steps (a)-(d).
67. The method of claim 34 wherein the external optical component
is a light generating device.
Description
STATEMENT OF RELATED APPLICATION
[0001] This application is a continuation-in-part of co-pending
U.S. patent application Ser. No. 10/826,145, filed Apr. 15, 2004,
entitled Active Optical Alignment And Attachment Thereto Of A
Semiconductor Optical Component With An Optical Element Formed On A
Planar Lightwave Circuit, which is incorporated herein by reference
in its entirety.
STATEMENT OF RELATED APPLICATION
[0002] 1. Field of the Invention
[0003] The invention relates generally to fiber optics, and in
particular to an arrangement for providing optical alignment
between an external optical component, such as a laser or LED, and
an optical element formed or secured on a planar substrate.
[0004] 2. Background of the Invention
[0005] In fiber optic technology there are many instances where it
is necessary to optically align and optically couple light from an
external optical component such as a semiconductor device and/or a
micro electromechanical system (MEMS) to an optical component
located or formed on a planar substrate. In Planar Lightwave
Circuits (PLC) technology optical waveguides and other optical
elements, such as mirrors, gratings, beam-splitters, are formed
monolithically on silicon or glass wafers, using processing
techniques similar to those used in the silicon microelectronics
industry. Doped-silica waveguides are usually preferred because
they have a number of attractive properties including low cost, low
loss, fabrication process maturity and stability. Changes in
dopants (such as P or Ge) concentration change the refractive index
of the waveguide core and therefore the numerical aperture(NA) of
the waveguide. This flexibility allows for optimized coupling to
devices of different NA, for example, such as laser diodes,
detectors, high NA and standard fibers. To maximize the coupling
efficiency the component has to be accurately aligned within a
narrow range around the optimum position. The cost of achieving
proper alignment between the external optical component and an
element on PLC is often high because it involves the use of
expensive lenses and high precision actuators to accomplish the
alignment. To achieve high efficiency coupling can be particularly
hard in case of the single mode optics because the tolerance to
misalignment is so small.
[0006] The external optical components discussed includes lasers,
Light Emitting Diodes(LED), semiconductor optical amplifiers,
detectors, MEMS, filters, isolators, fibers, lenses, etc. To
integrate the above-mentioned external optical components on PLC,
there must be an alignment mechanism to optically couple the
external optical components with the optical devices located on the
PLC. Such devices can be either monolithically integrated on the
PLC or be hybrid attached to the PLC platform.
[0007] Some of the more difficult systems to align involve
semiconductor lasers, because they have a highly diverging beam. A
semiconductor laser that is to be optically aligned to a
single-mode waveguide, which is the type commonly used in optical
telecommunication systems, has a typical positional misalignment
tolerance on the order of tenths of a micron.
[0008] In a conventional alignment process in which a semiconductor
device, such as a laser, is to be attached and optically aligned
with an optical component, such as an optical fiber, the
semiconductor device is first bonded to a submount. A weldable
fixture is also attached to a submount. The optical fiber is
secured to the weldable fixture. After securing the fiber to the
fixture, the fixture is physically manipulated to achieve the
desired coupling efficiency. The semiconductor device is not moved
during the alignment process because it is electrically connected
and thermally contacted for heat sinking to maintain stability. As
a result the laser cannot be moved on the bonded submount. This
technique requires specialized fixtures and flexures to enable
bending in order to move the optical fiber back into position. The
flexures are often complex structures enabling bending in various
directions, and can be relatively expensive. If an active alignment
technique is used, an optical signal is transmitted through the
components and detected. Manipulating the optical fiber so that the
transmission is at the highest possible level for the system, which
indicates that the coupling efficiency is maximized, performs the
alignment.
[0009] This conventional approach becomes difficult, when the laser
or LED have to be aligned to an element such as a facet of a
waveguide formed on a PLC or to an optical element that is already
securely attached to some other planar substrate. The PLC chip can
be large and heavy and its manipulation and support can be
difficult.
SUMMARY OF THE INVENTION
[0010] In accordance with the present invention, a method and
apparatus is provided for attaching a external optical component
processing an optical beam to a optical element located on the PLC
chip and optically aligning the external optical component with the
latter optical element. The process starts by securing the external
optical component to a first side of a submount. A first side of a
flexure element is secured to the first side of the submount. A
second side of the flexure element is secured to a first side of
the PLC on which the optical element is located such that the
external optical component and the optical element are in optical
alignment to within a first level of tolerance. Subsequent to the
step of securing the second side of the flexure element, a force is
exerted on at least a second side of the submount to thereby flex
the flexure element. The force causes sufficient flexure of the
flexure element to optically align the external optical component
and optical element to within a second level of tolerance that is
more stringent than the first level of tolerance.
[0011] In accordance with one aspect of the invention, an optical
coupling efficiency of an optical beam propagating between the
external optical component and optical element is monitored.
[0012] In accordance with another aspect of the invention, the step
of exerting a force is performed such that the coupling efficiency
is maximized.
[0013] In accordance with another aspect of the invention, the
optical element is a facet of a planar waveguide formed on the
PLC.
[0014] In accordance with another aspect of the invention, the
external optical component is a semiconductor laser.
[0015] In accordance with another aspect of the invention, the
external optical component is selected from the group consisting of
a semiconductor laser, an LED, a semiconductor optical amplifier, a
beam splitter, a thin film, a filter, a mirror, a birefringent
material, a polarizer, and a diffractive element.
[0016] In accordance with another aspect of the invention, the
flexure element is formed from gold or a gold alloy, or other metal
with low mechanical yield.
[0017] In accordance with another aspect of the invention, the
flexure has a preferred shape, and can be constructed as a
monolithic element or from multiplicity of individual elements.
[0018] In accordance with another aspect of the invention, the
flexure element and submount are one element.
[0019] In accordance with another aspect of the invention, the
flexure element is formed from a thermally conductive material
sufficient to serve as a heat sink for the external optical
component.
[0020] In accordance with another aspect of the invention, the
second side of the submount on which the force is exerted is a back
surface of the submount opposing the first side of the
submount.
[0021] In accordance with another aspect of the invention, the
second side of the submount on which the force is exerted is an
edge of the submount.
[0022] In accordance with another aspect of the invention, the
submount is mounted on the edge of a pocket etched into the first
side of the PLC, whereas such pocket allows clearance for the
external optical component.
[0023] In accordance with another aspect of the invention, the
submount is enclosed with a cover that mates with the first side of
the PLC. In accordance with another aspect of the invention, the
cover forms a hermetic seal with the first side of the PLC.
[0024] In accordance with another aspect of the invention, the
cover is formed from Kovar.TM., silicon, or Pyrex.
[0025] In accordance with another aspect of the invention, the
hermetic seal is established by a solder seal ring.
[0026] In accordance with another aspect of the invention, a
retaining disk is secured between the first side of the flexure
element and the first side of the submount.
[0027] In accordance with another aspect of the invention, the
retaining disk has a diameter greater than a diameter of the
flexure element.
[0028] In accordance with another aspect of the invention, the
flexure element and the retaining disk are formed from a common
material.
BRIEF DESCRIPTION OF THE DRAWINGS
[0029] FIG. 1 shows a semiconductor laser mounted on a submount
before attachment to PLC platform.
[0030] FIG. 2 shows a first embodiment of the optical apparatus
constructed in accordance with the present invention in which an
external optical component is attached to and optically aligned
with an optical element formed on a PLC. The external optical
element is selected to be a laser.
[0031] FIGS. 3A-3C show an embodiment of the optical apparatus
constructed in accordance with the present invention in which a
hermetic seal is enclosing the external optical component.
[0032] FIG. 4 shows a second embodiment of the optical apparatus
constructed in accordance with the present invention
[0033] FIGS. 5A and 5B show a third embodiment of the optical
apparatus constructed in accordance with the present invention in
which an external optical component is attached to and optically
aligned with an optical element formed on a PLC.
[0034] FIGS. 6A and 6B show a fourth embodiment of the optical
apparatus constructed in accordance with the present invention in
which an external optical component is attached to and optically
aligned with an optical element formed on a PLC.
DETAILED DESCRIPTION OF INVENTION
[0035] The present invention describes a method and an apparatus
for alignment of an external optical component such as a
semiconductor laser to an optical element formed on a PLC. While
the external optical component will be described below for
illustrative purposes only as a semiconductor laser, the external
optical component alternatively may comprise a variety of different
active and/or passive elements that process an optical beam. For
example, active devices include semiconductor lasers and
amplifiers, LEDs, as well as more complex devices offering higher
levels of functionality. Passive devices include, for example, beam
splitters, thin films, filters, mirrors, birefringent material,
polarizers, and diffractive elements. In the same spirit, the
optical element formed on PLC will be for illustrative purposes a
facet of a waveguide, but it could alternatively be a grating, a
mirror, or other passive or active component formed on a PLC, or
secured on PLC or other planar substrate.
[0036] As detailed below, the external optical component is first
attached to its own submount in a conventional manner and the
resulting subassembly is bonded to the PLC via a deformable flexure
element of low yield strength that allows for active optical
alignment. Active alignment is then achieved by moving the external
optical component into its proper position. In the present
invention the external optical component is directly attached to
its own submount and not the PLC. For example, in some
circumstances, the external optical component is preferably aligned
in an orientation that is upside-down in comparison to the
conventional process. Among its other advantages, the invention
eliminates the need for laser soldering or laser welding equipment.
It also eliminates the need for hardware or fixtures to hold the
PLC chip in place, which is more difficult due to the usually
larger size of the latter. The invention allows attachment of the
external optical component using conventional die-bonding equipment
and allows pre-assembly of the external optical component with its
submount by conventional pre-qualified means. The flexure element
can be flexed to provide for active alignment of the external
optical component to the PLC optical component after the bonding
processes are completed, eliminating alignment losses caused by
bonding. The flexure element may be, for example, a small gold shim
made by hole punching a metal sheet and thus does not require any
expensive machining.
[0037] FIG. 1 shows a semiconductor laser 110 that is first
attached to its own submount 111, which in the case of a
semiconductor laser is often aluminum nitride (AIN). Aluminum
nitride is commonly used as a submount for semiconductor lasers
because AIN has excellent thermal conductivity and is expansion
matched to the GaAs material from which such lasers are formed,
reducing stress which could otherwise alter the lasing wavelength
of the device. In FIG. 1 the laser is bonded in a standard way,
such that the laser active ridge 116 is on top. Metal pads 113 and
a metallized through hole 117 are provided for biasing the laser.
The bottom of the laser is contacting pad 113, and the top is
connected to pad 114 using two wire bonds 115.
[0038] A flexure element 112 is also attached to the submount 111.
As detailed below, the flexure element 112 enables the laser active
area 116 to be optically aligned to the waveguide 210 on PLC 211 in
FIG. 2 after the laser 110 has been bonded to the submount 111.
This is an important feature of the invention because the bonding
process would otherwise cause misalignment and because during
active alignment the bonded laser can be operated at room
temperature since the bonding process that normally uses heat is
not needed during the alignment step. The laser 110 and waveguide
210 can be optically aligned in an active alignment process by
applying force to the laser submount 111 to thereby bend the
flexure element 112 by an appropriate amount.
[0039] The flexure element 112 is preferably made of a material
that has low yield strength, meaning that it will bend but not tend
to spring back. The flexure element 112 should also be stable and
remain in position as long as sufficient force is not exercised. A
preferred material for the flexure element 112 is gold or a gold
alloy, which have low yield strength characteristics. Other
exemplary materials with a low yield strength that may be employed
are lead, nickel, nickel alloys, copper, silver and Kovar.TM.. One
advantage of gold is that it is compatible with a gold tin eutectic
solder and can be die-bonded to bond it in position using performs
of AuSn (80/20) eutectic solder, a solder commonly used to bond
laser chips to AIN submounts. The absence of spring action and
stability could be also achieved by proper design of a more complex
multi-element flexure, which is also covered by this
disclosure.
[0040] The inventive attachment and optical alignment process
begins by die-bonding and wire-bonding the laser 110 to the AIN
submount 111 in a conventional manner, after which the resulting
laser subassembly is aged and tested, also in a conventional
manner.
[0041] After the laser 110 is bonded, burned-in and tested on the
AIN submount 111 in the aforementioned manner, the flexure element
112 is bonded to the AIN submount 111 at a location in front of the
laser facet 110a. The flexure element 112 may be bonded by the same
technique used to bond laser 110 to the AIN submount 111. That is,
the same equipment can be used to bond both the laser 110 and the
flexure element 112. For example, an AuSn (80/20) eutectic solder
may be used to establish both bonds.
[0042] In addition to its low yield strength, the flexure element
112 should have a sufficiently high thermal conductivity to serve
as a heat sink for the heat generated by laser 110. This alleviates
the need to attach any additional heat sinks or cooling elements
such as a thermo-electric cooler (TEC) to the AIN submount 111,
which could adversely impact optical alignment by flexing the
submount 111. Of course, the thermal conductivity of the flexure
element 112 can be increased, as needed by increasing its size
along the dimensions that contact the AIN submount 111 and the PLC
211. For example, if the flexure element 112 is configured in the
shape of a disk, its diameter can be increased to increase its
thermal capacity. The thickness of the flexure element 112,
however, is preferably about the same as the thickness of the laser
110 to facilitate initial alignment. Of course, the present
invention encompasses flexure elements 112 of any shape and size
and is not intended as a limitation on the invention.
[0043] The resulting laser, flexure element and submount
subassembly is next die-bonded to the PLC 211 also using, for
example, a AuSn (80/20) solder. This step may be conveniently
performed by inverting the sub-assembly so that the top of the
laser 110 (usually the p side) and the top of the flexure element
112 are facing the PLC 211. The top of the flexure element 112 is
bonded to the PLC 211, so that the laser 110 extends in front of
the facet of the waveguide 210 in rough optical alignment, as shown
in FIG. 2.
[0044] As shown in FIG. 2, a wire bond 214 extends from the back of
the AIN submount 111 to a point on the PLC 211 to complete the
electrical circuit. The wire bond 214 serves as one electrical
connection for the laser 110. The other electrical connection to
the laser 110 is established through the flexure element 112
itself. The bottom contacts for the laser 110 and the flexure
element 112 are located on the same metal pad. The two resulting
connections allow the laser 110 to be powered.
[0045] Once the bonding process is complete, optical alignment
between the laser 110 and waveguide 210 may be performed in an
active manner. That is, the laser 110 is powered and aligned to the
waveguide 210 by exerting a downward force on the back of the AIN
submount 111 until the optical signal coupled into the waveguide
210 is maximized. Since a conventional die-bonder is generally able
to initially place the laser 110 to within about 5-10 microns of
its target position, the flexure element 112 only needs to bend
sufficiently so that the laser 110 can be adjusted over these
remaining 10 microns. It should be noted that because the aperture
of waveguide 210 is significantly larger than the output aperture
of the laser 110, the coupling efficiency is not very sensitive on
the angular misalignment between the axis of the laser 110 and the
axis of the waveguide 210, at least up to an angular misalignment
of about 3 degrees in a typical application. In such an application
the angular misalignment provided by exerting a force on the
flexure element 112 will typically be less than about 0.1 degrees
to achieve maximum coupling efficiency.
[0046] Among the directions along which alignment must be achieved,
the most sensitive are vertical alignments, such as up and down
(i.e., in a direction perpendicular to the axis of waveguide 210
that also traverses the PLC chip 211) and side to side (i.e., in a
direction perpendicular to the axis of waveguide 210 that is also
parallel to the planes encompassing the submount 111 and the PLC
211). Up and down alignment is achieved by exerting a downward
force on the AIN submount 111, either in front of or behind the
flexure element 112. Exerting a force in front of the flexure
element 112 causes the laser 110 to move down while exerting the
force behind the flexure element 112 causes the laser to move
upward. Side to side movements is accomplished by exerting a force
on a side or edge of the AIN submount 111. Since the linear
displacements should be very small, the angular misalignment will
be negligible.
[0047] The present invention achieves multiple advantages with
respect to the conventional bonding and optical alignment
techniques. First, the invention enables the use of commercially
available semiconductor lasers or other external optical components
that are already mounted, burned-in, and pre-tested without the
need for modifications. The laser or other external optical
component is simply provided as a chip located on its own submount.
Thus, the present invention advantageously makes use of a low cost,
pre-qualified components. Moreover, the critical manufacturing
process of the laser or other external optical component is
completely separated from the alignment process. This is a key
advantage, because doing otherwise might require a custom
semiconductor device that would be highly specialized and thus much
more expensive. A second key advantage is that the invention allows
alignment to take place after the bonding steps are performed. In
this way the process yield arising from alignment can be dealt with
separately from the process yield arising from attachment. Since
the alignment process is independent of the attachment process it
does not affect the optimization of the attachment process. A third
key advantage is that the alignment can be reworked, meaning that
if for some reason proper alignment is not achieved, the optical
components can always be repositioned by bending the flexure
element until the alignment is satisfactory. A fourth key advantage
is that the overall size of the assembly is relatively small, and
in some cases may not be much larger than the size of the
semiconductor laser itself. A fifth key advantage is that because
the semiconductor laser rather than the waveguide is moved to
achieve alignment, the invention allows many additional devices to
be mounted and independently aligned on a single PLC chip, enabling
larger scale integration of optical components.
[0048] The assembly shown in FIG. 2 may be finalized in the manner
shown in FIGS. 3A-3C to provide a very compact, hermetically sealed
component. In FIG. 2 a pocket 212 is shown etched in the PLC chip,
such as it has a vertical facet 213 formed in the waveguide, to
which the laser is coupled. The pocket is big enough as to
accommodate part of the laser and the wire-bonds. Around the hole a
metal ring 301 (FIG. 3B) is deposited that is sufficiently wide to
encompass the hole and the laser on the submount mounted on the
side of the hole. The metal ring is electrically isolated from the
leads connecting the laser with the wire-bonding pads. A cap 302
made of a material close in expansion to silicon is fabricated, and
can be soldered to the ring, so the laser is now hermetically
sealed in the space formed between the pocket and the cap. One
advantage of this approach is that the entire package does not have
to be hermetic, but only a small part of it. It is significant,
because this method allows using non-hermetic components
simultaneously with the laser inside, further lowering the overall
cost of the package
[0049] In an alternative embodiment, as shown in FIG. 4, a laser is
mounted on a PLC an edge of a PLC chip, such that the facet of the
waveguide is either etched or polished. Here element 303 is an end
facet of the waveguide 210, and the laser is aligned to it.
[0050] In other embodiments of the invention the AIN submount 111
to which the laser 110 or other external optical component is
attached may be eliminated by directly attaching the laser 110 or
external optical component to the flexure element 112. That is, the
flexure element 112 can serve as both the support for the external
optical component and the component that is flexed during the
active alignment process.
[0051] Various embodiments of the optical-alignment flexure
assembly are possible without deviating from the spirit of the
invention. For example, by changing the laser position on the AIN
submount, the AIN submount can be arranged at a 90 degree angle to
the buried optical waveguide, as shown in FIGS. 5A and 5B. This
variation now makes the x direction (along the optical axis) more
adjustable by displacing it further away from the attachment points
of the flexures. The x-direction adjustment would take place by
twisting the attachment points on the flexures rather than trying
to sheer them. Another variation of the basic idea is an attachment
scheme in which the buried optical waveguide is located opposite or
across from the attachment point of the submount on another edge of
the PLC. This is shown in FIGS. 6A and 6B. With this geometry there
is a well in the silicon bench that can accommodate the laser. The
laser can then be aligned to the facet of the optical waveguide in
a similar way as done before, except that the laser now points away
from the flexure attachment point.
[0052] In some embodiments of the invention connections to the
contacts may be established as follows. A wirebond connects the
p-contact of the laser to a pad on the AIN submount. This pad is
holed through 117 the AIN (see FIG. 1) to the backside of the AIN,
which is coated with conductor as well as coating within the hole,
making electrical connection to the backside. After assembly, the
backside is then wirebonded down to a pad on the Si substrate,
enabling an electrical path to the p-contact of the laser. The
n-contact of the laser is simply made by conduction through the
flexure itself to a pad on the PLC, eliminating the need for
wirebonds.
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