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Claims  |
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What is claimed is:
1. An optical interconnect comprising:
a multiple optical fiber connector comprising:
a holder having a first planar surface,
a plurality of optical fibers attached to the holder, each fiber having a
first end abutting the first surface so as to expose the first end for
receiving or transmitting optical radiation, the first ends of the fibers
forming a fiber array having a first pattern, and
guiding means disposed in said holder at predetermined positions with
respect to the fiber array; and
an optoelectronic board comprising:
an optoelectronic device array monolithically formed on a semiconductor
chip, said optoelectronic device array having substantially the same
pattern as the first pattern of said fiber array, and
aligning means formed on said chip, said aligning means being disposed at
substantially the same predetermined positions with respect to said array
of optoelectronic devices as the positions of the guiding means relative
to said fiber array, and
said aligning means receiving said guiding means so as to mechanically
align said optoelectronic device array with said optical fiber array,
whereby each optoelectronic device is aligned to an optical fiber, said
optoelectronic device emitting optical radiation into said fiber array or
receiving optical radiation from said fiber array.
2. The interconnect of claim 1 wherein said guiding means comprises two
guiding pins mounted in the holder, said pins extending through said first
planar surface of the holder, and said aligning means comprises two
aligning holes, the fiber array being aligned to the optoelectronic device
array by aligning and engaging said guiding pins with said aligning holes.
3. The interconnect of claim 2 wherein said fiber array is disposed between
the two guiding pins.
4. The interconnect of claim 1 wherein said guiding means comprises two
guiding pins mounted in the holder, said pins extending through said first
surface of the holder in a direction perpendicular to said first surface,
and said aligning means including two aligning holes, said fiber array
being aligned to said optoelectronic device array by aligning and engaging
said guiding pins with said aligning holes.
5. The interconnect of claim 4 wherein said fiber array are disposed
between said two guiding pins.
6. The interconnect of claim 1 wherein the fibers form a one-dimensional
array.
7. The interconnect of claim 1 wherein the fibers form a two-dimensional
array.
8. The interconnect of claim 6 wherein the optoelectronic device array
comprises a one-dimensional array of vertical cavity surface emitting
lasers, each laser being axially aligned to a fiber and the fibers
receiving optical radiation emitted by the lasers.
9. The interconnect of claim 8 further comprising a first electronic
circuit means interconnected to said array of lasers for driving and
modulating said lasers.
10. The interconnect of claim 8 wherein each of the optical fibers is a
plastic optical fiber.
11. The interconnect of claim 8 wherein each of the optical fibers is a
single-mode optical fiber.
12. The interconnect of claim 8 wherein each of the optical fibers is a
multi-mode optical fiber.
13. The interconnect of claim 12 wherein each multi-mode optical fiber has
a diameter of approximately 62.5 .mu.m.
14. The interconnect of claim 13 wherein each of the vertical cavity
surface emitting lasers has a diameter of approximately 20 .mu.m.
15. The interconnect of claim 6 wherein the array of optoelectronic devices
comprises an one-dimensional array of photo-detectors, each photo-detector
being axially aligned to an end of a fiber, and the detectors receiving
optical radiation emitted by the lasers.
16. The interconnect of claim 15 wherein the photo-detectors are
Schottky-barrier photo-detectors.
17. The interconnect of claim 15 further comprising a second electronic
means for receiving an electronic signal from the photo-detectors.
18. The interconnect of claim 16 wherein each of the optical fibers is a
multi-mode optical fiber.
19. The interconnect of claim 18 wherein each multi-mode optical fiber has
a diameter of approximately 62.5 .mu.m.
20. The interconnect of claim 19 wherein each photo-detector has a diameter
of approximately 100 .mu.m.
21. A method for interconnecting an optical connector to an optoelectronic
board comprising the steps of:
providing a multiple optical fiber connector, said connector comprising:
a holder having at least one planar surface,
a plurality of optical fibers attached to the holder, each fiber having a
first end abutting the first surface so as to expose the first end for
receiving or transmitting optical radiation, the first ends of the fibers
forming a fiber array having a first pattern, and
guiding means disposed in said holder at predetermined positions with
respect to the fiber array;
providing an optoelectronic board, said board comprising:
an array of optoelectronic devices monolithically formed on a semiconductor
chip, said optoelectronic devices having substantially the same pattern as
the first pattern of said fiber array, and
aligning means formed on the chip, said aligning means being disposed at
substantially the same predetermined positions with respect to said array
of optoelectronic devices as the position of the guiding means relative to
said fiber array; and
aligning and engaging said guiding means of said connector with said
aligning means of said board so as to mechanically align said array of
optoelectronic devices with said array of the optical fiber, whereby each
optoelectronic device is aligned to an end of the fiber, said array of
optoelectronic devices emitting optical radiation into said fiber array or
receiving optical radiation from the fiber array.
22. An optical interconnect comprising:
an optical fiber having a coupling end, said optical fiber comprising a
core and a cladding layer surrounding said core, said core at the coupling
end being recessed so as to form a structure having a rim, the rim
comprising substantially the cladding layer; and
a vertical cavity surface emitting laser comprising:
a semiconductor substrate,
a first mirror formed on said substrate, and a second mirror disposed above
and parallel to the first mirror and forming with said first mirror an
optical cavity that is perpendicular to the substrate, the first and
second mirrors being distributed Bragg reflectors comprising a plurality
of layers formed one on top of the other, and
an active region surrounded by first and second spacers disposed between
said mirror,
the uppermost layers of said second mirror forming a mesa having a diameter
less than an inner diameter of the rim,
said mesa engaging said rimmed structure so as to place a substantial
portion of said mesa inside the rim, and
said laser emitting optical radiation that is substantially coupled into
said optical fiber.
23. The interconnect of claim 22 further comprising holding means for
maintaining the relative positions between the coupling end of said
optical fiber and said laser.
24. The interconnect of claim 23 wherein said holding means comprises epoxy
applied to the exterior of the coupling end and the laser.
25. The interconnect of claim 22 wherein said laser further comprises a
contact surrounding the mesa and formed on the layers of said second
mirror that is close to the active region.
26. The interconnect of claim 22 wherein said fiber is a single mode fiber.
27. The interconnect of claim 22 wherein said fiber is a multi-mode fiber.
28. The interconnect of claim 22 wherein said mesa has a thickness of 1 to
3 .mu.m.
29. The interconnect of claim 22 wherein said laser further comprises an
annular implanted current confinement region surrounding the active
region.
30. A parallel optical interconnect comprising:
an optical connector comprising:
a holder having a first planar surface,
a plurality of optical fibers attached to said holder, each fiber having a
coupling end abutting the first surface so as to expose the coupling end
for receiving or transmitting optical radiation,
each said optical fiber comprising a core and a cladding layer surrounding
said core, said core at the coupling end being recessed so as to form a
structure having a rim, the rim comprising substantially the cladding
layer,
said coupling end of the fibers forming a fiber array having a first
pattern; and
an array of vertical cavity surface emitting lasers monolithically formed
on a semiconductor chip, said array having substantially the same pattern
as said fiber array, and each laser comprising:
a first mirror formed on said chip, and a second mirror disposed above and
parallel to the first mirror and forming with said first mirror an optical
cavity that is perpendicular to the chip, the first and second mirrors
being distributed Bragg reflectors comprising a plurality of layers formed
one on top of the other, and
an active region surrounded by first and second spacers disposed between
said mirror,
the uppermost layers of said second mirror forming a mesa having a diameter
less than an inner diameter of the rim,
each mesa being aligned to and engaging an optical fiber so as to place a
substantial portion of said mesa inside the rim of a fiber, and
each laser emitting optical radiation that is substantially coupled into an
optical fiber.
31. The interconnect of claim 30 further comprising holding means for
maintaining the relative positions between the coupling ends of said fiber
array and said lasers.
32. The interconnect of claim 31 wherein said holding means comprises epoxy
applied to the exterior of the coupling ends and the lasers.
33. The interconnect of claim 31 wherein each laser further comprises a
contact surrounding the mesa and formed on the layers of said second
mirror that is close to the active region.
34. The interconnect of claim 31 wherein each fiber is a single mode fiber.
35. The interconnect of claim 31 wherein each fiber is a multi-mode fiber.
36. The interconnect of claim 31 wherein each mesa has a thickness of 1 to
3 .mu.m.
37. The interconnect of claim 31 wherein each laser further comprises an
annular implanted current confinement region surrounding the active
region.
38. The interconnect of claim 31 further comprising guiding means disposed
in said holder, and aligning means formed on said chip, said aligning
means receiving said guiding means so as to mechanically align said array
of lasers to said fiber array.
39. The interconnect of claim 38 wherein said guiding means comprises two
guiding pins mounted in the holder, said pins extending through said first
surface of the holder, and said aligning means including two aligning
holes in said chip, the fiber array being aligned to the array of lasers
by aligning and engaging said guiding pins with said aligning holes.
40. The interconnect of claim 31 wherein said fiber array and said array of
lasers are two-dimensional arrays having substantially the same pattern.
41. The interconnect of claim 31 wherein said fiber array and said array of
lasers are one-dimensional arrays having substantially the same pattern.
42. The interconnect of claim 6 wherein the optoelectronic device array
comprises a one-dimensional array of super luminescence diodes, each diode
being axially aligned to a fiber and the fibers receiving optical
radiation emitted by the diodes.
43. The interconnect of claim 33 wherein each laser further comprises a
substrate contact disposed below the substrate. |
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Claims |
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Description |
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FIELD OF THE INVENTION
This invention relates to optical interconnects and, more particularly, to
optical interconnects that couple multiple optical fibers to arrays of
integrally formed optoelectronic devices.
BACKGROUND OF THE INVENTION
Optical fiber technology has been widely utilized in today's
telecommunication network. It is also the foundation of a future
generation of telecommunication technology which is predicted to
revolutionize the way people exchange or obtain information.
One important aspect of optical fiber technology is the interconnection of
optical fibers to optoelectronic devices such as semiconductor lasers,
photo-detectors, etc, wherein the optoelectronic devices either receive
optical radiation from the optical fibers or the optoelectronic devices
emit optical radiation into the fibers. A good optical interconnect
between an optical fiber and an optoelectronic device requires high
coupling efficiency (i.e. low loss of light from the coupling), ease of
making the coupling, and low cost for making such an interconnect.
The conventional method for coupling an optical fiber to an optoelectronic
device is by active alignment. For example, to couple a semiconductor
laser to an optical fiber by active alignment, the laser is first turned
on to emit optical radiation. A coupling end of the optical fiber is then
placed near a light emitting surface of the laser to receive optical
radiation, and a photodetector is placed at the other end of the fiber to
detect the amount of optical radiation that is coupled into the fiber. The
position of the coupling end of the fiber is then manipulated manually
around the light-emitting surface of laser until the photodetector at the
other end of the fiber detects maximum optical radiation. Optical epoxy is
then applied to both the laser and the coupling end of the fiber so as to
permanently maintain the optimized coupling.
A photo-detector can be similarly coupled to an optical fiber by shining
laser light into one end of the fiber and manually adjusting the position
of the other end of the fiber that is to be coupled to the detector until
the detector's electrical response to the optical radiation reaches a
maximum. Optical epoxy is then applied to attach the fiber to the
detector.
Because the dimensions of the light-emitting surface of a semiconductor
laser and the cross-section of an optical fiber are very small, e.g. on
the order of 10 .mu.m for single mode fiber, coupling a semiconductor
laser to an optical fiber is a task that is usually time-consuming and
requires expertise and experience. As for coupling an optical fiber to a
photo-detector, even though one may increase the size of the detector to
make such coupling easier, increasing the detector size undesirably
increases the parasitics of the detector and thus compromises the
detector's operating speed and frequency response.
One problem of the above-described optical interconnect is that the
alignment between the fiber and the optoelectronic device may suffer
misalignment under thermal strain. Such thermal strain occurs when the
temperature of the interconnect increases due to the heat generated by the
optoelectronic device, the circuits for driving the device, or by various
other factors such as the nearby electronic componenets.
It is also not practical to apply the above-described method of active
alignment to couple multiple optical fibers to an array of optoelectronic
devices that are monolithically formed on a semiconductor chip because the
array normally contains a large number of devices that are closely spaced.
However, it would be very useful to couple such an array of photo-emitters
to such an array of photo-detectors via multiple optical fibers in
applications such as local area networks (LANs) which require the coupling
of signals in parallel.
It is therefore an object of this invention to provide optical
interconnects that mechanically couple multiple optical fibers to arrays
of integrally formed optoelectronic devices.
SUMMARY OF THE INVENTION
The present invention is an optical interconnect for coupling multiple
optical fibers to an optoelectronic device array.
In a first embodiment of the invention, the optical interconnect comprises
a multiple optical fiber connector coupled with an optoelectronic board.
The multiple fiber connector comprises a holder having at least one planar
surface, a plurality of optical fibers attached to the holder, each fiber
having a coupling end abutting the first surface so as to expose the
coupling end for receiving or transmitting optical radiation, the first
ends of the fibers forming a fiber array, and guiding means disposed in
the holder at predetermined positions with respect to the fiber array.
The optoelectronic board comprises an array of optoelectronic devices that
are monolithically formed on a semiconductor chip in substantially the
same pattern as the fiber array. The board further comprises aligning
means formed on the chip at substantially the same predetermined positions
with respect to the array of optoelectronic devices as the guiding means
are positioned relative to the fiber array.
The fiber connector is coupled to the optoelectronic board by aligning and
engaging the guiding means into the aligning means, whereby the
optoelectronic device array is aligned with the optical fiber array, and
whereby each optoelectronic device is coupled to an optical fiber.
Preferably, the guiding means are guiding pins and the aligning means are
aligning holes formed in the chip.
The optoelectronic devices may be photo-emitters which emit optical
radiation into the fiber array or photo-detectors which receive optical
radiation transmitted from the fiber array.
In a second embodiment, the optical interconnect comprises a multiple
optical fiber connector and an optoelectronic board. The fiber connector
is the same as that of the first embodiment.
The optoelectronic board comprises a substrate having a first surface and
first aligning means, a semiconductor chip mounted on the first surface of
the substrate, and an array of optoelectronic devices monolithically
formed on the chip. The optoelectronic devices are arranged on the chip in
substantially the same pattern as the fiber array. The optoelectronic
board further comprises a second aligning means for aligning the
optoelectronic device array to the first aligning means. The first
aligning means are disposed at substantially the same predetermined
positions with respect to the optoelectronic device array as the guiding
means are positioned relative to the fiber array.
The connector is coupled to the optoelectronic board by aligning and
engaging the guiding means into the aligning means, whereby the
optoelectronic device array is aligned with the optical fiber array, and
whereby each optoelectronic device is coupled to a fiber. Preferably, the
guiding means are guiding pins and the aligning means are aligning holes
formed in the substrate. Again, the optoelectronic devices may be
photo-emitters or photo-detectors.
This invention reduces misalignment due to thermal strain caused by thermal
expansion mismatch between the different materials that are used in the
interconnect. This is achieved by anchoring the optical fiber assembly to
the locations of the guiding pins.
In a third embodiment of the invention, an optical interconnect comprises
an optical fiber and a vertical cavity surface emitting laser (VCSEL)
coupled to a coupling end of the fiber.
The optical fiber comprises a core and a cladding layer surrounding said
core. The core at the coupling end of the fiber is recessed so as to form
a rimmed structure, wherein the rim comprises substantially the cladding
layer.
The VCSEL comprises a semiconductor substrate, a first mirror formed on
said substrate, a second mirror parallel to and disposed above the first
mirror and forming with said first mirror an optical cavity that is
perpendicular to the substrate, and an active region surrounded by first
and second spacers disposed between said mirrors.
The first and second mirrors are distributed Bragg reflectors comprising a
plurality of layers formed one on top of the other. The uppermost layers
of said second mirror form a mesa having a diameter less than the inner
diameter of the rim. The mesa engages said rimmed structure so as to place
a substantial portion of said mesa inside the rim. As a result, the laser
is self-aligned and the optical radiation emitted from the laser is
substantially coupled into said optical fiber.
This interconnect reduces the misalignment due to thermal strain because
the laser is embedded in the optical fiber.
BRIEF DESCRIPTION OF THE DRAWINGS
These and other objects, features and advantages of the invention will be
more apparent from the following detailed description in conjunction with
the appended drawings in which:
FIG. 1a illustrates a three-dimensional view of a first embodiment of the
optical interconnect of the present invention;
FIG. 1b depicts a view of surface 130 of FIG. 1a;
FIGS. 2a-b depicts a second embodiment of the invention;
FIG. 3 illustrates the packaged interconnect of FIG. 2;
FIGS. 4a-b illustrate an application of the optical interconnects of the
present invention;
FIGS. 5a-c depict a third embodiment of the optical interconnect of this
invention; and
FIGS. 6a-b illustrate an array of optical interconnects of FIGS. 5a-c.
DETAILED DESCRIPTION
In accordance with the invention, a first embodiment of the optical
interconnect of the present invention comprises a multiple optical fiber
connector and an optoelectronic board. The connector comprises a holder, a
plurality of optical fibers attached to the holder, and guiding means. The
optoelectronic board comprises an optoelectronic device array that is
monolithically formed on a semiconductor chip and aligning means defined
on the chip. The optical fibers are coupled to the optoelectronic device
array by mechanically engaging the connector's guiding means to the
aligning means of the board.
FIG. 1a illustrates a preferred embodiment of the optical interconnect. The
preferred embodiment comprises an optical fiber connector 150 and an
optoelectronic board 100.
Optical fiber connector 150 comprises a holder 125 having a planar surface
130, a plurality of optical fibers 135 embedded in the holder, and two
guiding pins 140 slideably mounted in two cylindrical tunnels 141.
The fibers have coupling ends 131 that vertically abut surface 130. The
coupling ends of the fiber are exposed so as to receive or emit optical
radiation. As depicted in the end view of surface 130 in FIG. 1b, coupling
ends 131 form a one-dimensional optical fiber array 132. The distance
between any two fibers in the array is equal to that of any other two
adjacent fibers. Preferably, the distance between centers of any two
adjacent fibers is about 0.25 mm.
Tunnels 141 and pins 140 are positioned symmetrically with respect to the
fiber array. The positions of the tunnels and pins with respect to the
fiber array are predetermined.
Optoelectronic board 100 comprises a semiconductor chip 105, a
one-dimensional optoelectronic device array 110 monolithically formed on
the semiconductor chip, and a plurality of conducting lines 115 for
contacting the optoelectronic devices. The chip contains two aligning
holes 120 having approximately the same diameter as tunnel 141 for
receiving the guiding pins. Advantageously, array 110 and aligning holes
120 of the board have substantially the same pattern as that formed by
optical fiber array 132 and tunnels 141 of the fiber connector, i.e., the
distance between the centers of any two adjacent optoelectronic devices
are the same as the distance between the centers of any two adjacent
fibers. Thus, by simply engaging the guiding pins 140 in the tunnels of
the fiber connector to the aligning holes 120 on the optoelectronic board,
the optical fiber array is axially aligned to the optoelectronic device
array. Each optical fiber is also aligned to a corresponding
optoelectronic device.
Optical connectors similar to the one shown in FIG. 1a have been disclosed
by Hayashi, et al in European Patent Application publication No. 0,226,274
which is incorporated herein by reference. The same type of optical
connectors are commercially available with a tolerance on the order of 10
.mu.m between the centers of two adjacent fibers.
The optoelectronic device array and the aligning holes on the semiconductor
chip are defined by well developed semiconductor processing technologies
such as photolithography and chemical or reactive ion etching.
Consequently, the tolerance of the device features or the aligning holes
can be controlled to within the order of 10 .mu.m. Combining this
tolerance with the tight tolerance of the available fiber connector, this
interconnect provides good coupling efficiency. It can also be easily
connected or disconnected.
The optical fibers may be single mode fibers or multi-mode fibers, e.g.
plastic fibers. Preferably, the optical fibers are multi-mode fibers
having a core diameter ranging from 50 to 100 .mu.m. Multiple mode optical
fibers normally have greater core diameters than single mode optical
fibers; and as a result they offer higher alignment tolerance than single
mode fibers.
The optoelectronic devices may be various types of devices such as
edge-emitting lasers, Super Luminescence Diodes (SLEDs), Vertical Cavity
Surface Emitting Lasers (VCSELs), other surface emitting devices or
photo-detectors. They may also be integrated devices combining one or more
devices such as the combination of VCSELs and transistors, or
photo-detectors and transistors.
When the optoelectronic devices are VCSELs, the optical interconnect
functions as a transmitter sending optical signals into the optical
fibers. Generally, the tolerance of alignment in this interconnect is in
the range of 2-20 .mu.m. For example, each VCSEL has a diameter of
approximately 20 .mu.m and each fiber is a multi-mode fiber having a core
diameter of about 62.5 .mu.m, the alignment tolerance is about 10 .mu.m.
The optical interconnect may further comprise electronic circuit means
interconnected to the VCSEL array for driving and modulating the VCSELs.
Such electronic circuit means may be monolithically formed on the
semiconductor chip, or integrated with the VCSEL array as a hybrid
integrated circuit.
If the optoelectronic devices are photo-detectors formed on the
semiconductor chip, the optical interconnect functions as a receiver for
receiving optical signals from the optical fibers. For example, if each
photo-detector is a Schottky-barrier photo-detector having a diameter of
approximately 100 .mu.m, and each fiber is a multi-mode fiber having a
core diameter of about 62.5 .mu.m, the alignment tolerance between the
fibers and the detectors is about 20 .mu.m.
In the above-described preferred embodiment, one-dimensional arrays are
used as an example. However, two-dimensional arrays can also be similarly
utilized to form a two-dimensional optical interconnects.
In the second embodiment of the present invention, the optical interconnect
also comprises an optical connector and an optoelectronic board. The
connector comprises a holder, a plurality of optical fibers attached to
the holder, and guiding means. The optoelectronic board comprises a
substrate, a semiconductor chip disposed on the substrate, and an
optoelectronic device array monolithically formed on the semiconductor
chip. The optoelectronic board further comprises first aligning means
formed in the substrate, and second aligning means disposed between the
chip and the substrate so as to properly position the optoelectronic
device array with respect to the first aligning means. By mechanically
aligning and engaging the guiding means of the connector to the first
aligning means of the board, the optical fibers are coupled to the
optoelectronic device array.
FIG. 2a depicts the preferred embodiment of the optical interconnect which
comprises an optical connector 150 and an optoelectronic board 101. For
convenience, like elements in FIGS. 2a-b are designated with the same
numbers as in FIG. 1. Optical fiber connector 150 in this embodiment is
the same as the optical connector in the first embodiment, and is
therefore not discussed in detail here.
Optoelectronic board 101 comprises a dielectric substrate 90, a
semiconductor chip 105 disposed on the substrate, a linear optoelectronic
device array 110 monolithically formed on the semiconductor chip,
electronic circuit means 117 interconnected to the optoelectronic device
array, and contact pads 108 defined at the periphery of the substrate for
making contact to the circuit means. The optoelectronic board further
includes two aligning holes 119 formed in the substrate as first aligning
means for receiving the guiding pins. As depicted in FIG. 2b, the board
further comprises second aligning means 91 formed between a second surface
95 of the semiconductor chip and a first surface 96 of the substrate. The
second alignment means laterally aligns the optoelectronic device array to
first aligning means 119 that is formed in the substrate.
The principle of forming this optical interconnect is as follows:
The optoelectronic devices are first formed on the semiconductor chip. They
are arranged to form an array having the same pattern as the optical fiber
array. Utilizing the well developed semiconductor technology, the
positions of the optoelectronic devices can be precisely defined such that
the distance between centers of any two adjacent devices are the same as
the distance between centers of any two adjacent fibers.
Next, aligning holes 119 in the substrate are defined by using techniques
such as precision laser drilling. The aligning holes have substantially
the same diameters as tunnels 141 of the optical fiber connecter and the
distance between the centers of the two aligning holes is substantially
the same as that between the tunnels. Thus, guiding pins can be aligned to
the aligning holes.
The optoelectronic device array is then laterally aligned to the aligning
holes by utilizing the flip-chip bonding technology. As illustrated in
FIG. 2b, the flip-chip bonding technology mainly comprises the steps of
forming a first set of metal pads 93 on first surface 96 of the substrate,
the patterns having predetermined positions with respect to aligning holes
119; this is followed by depositing a layer of solder on the substrate.
The solder layer is then patterned by photolithography and etching to form
a second set of solder patterns 94 on top of the first set of metal
patterns. A third set of metal pads 92 that is the mirror image of the
first set is then defined on second surface 95 of the semiconductor chip.
By utilizing infrared alignment techniques, the third set of patterns is
aligned with the optoelectronic device array in such a way that, if the
third set is vertically aligned with the first set, the optoelectronic
device array will laterally align to the aligning holes in a way such that
the optoelectronic device array will also be vertically aligned to the
optical fiber array.
Advantageously, the third set of patterns is aligned to the first set by a
self-alignment process. In the self-alignment process, the third set is
first coarsely aligned with the second set. Then, heat is applied to melt
the solder. The melted solder is confined between the third and first
metal patterns and does not flow laterally beyond the patterns. Due to the
surface tension of the solder, the third set of patterns is automatically
aligned to the first set of patterns. When the heat is removed, the solder
cools down and the self-alignment process is completed. This flip-chip
technology is explained in detail by Wale et al, "Flip-Chip Bonding
Optimize Opto-Ics", IEEE Circuit and Devices, pp.25-31, Nov. 1992, which
is incorporated herein by reference.
Consequently, at the completion of the solder flow self-alignment process,
the optoelectronic device array is laterally aligned to the aligning holes
in the same way as the fiber array is aligned to the guiding pins. Thus,
by vertically aligning and engaging the guiding pins with the aligning
holes, the fiber array is aligned with and coupled to the optoelectronic
device array.
As in the case of the optical interconnect of FIG. 1a, if the
optoelectronic devices are VCSELs and electronic circuit means 117 are
integrated circuits for driving and modulating the VCSELs, the
interconnect functions as an optical transmitter that injects optical
signals into the fibers. If the optoelectronic devices are
photo-detectors, the interconnect functions as an optical receiver that
receives optical signals transmitted through the optical fibers. The
photo-detectors send electrical signals that correspond to the optical
signals to the electronic circuit means which process the signals before
forwarding them to other circuits such as computers or processors. If the
optoelectronic devices comprise both VCSELs and photo-detectors, this
optical interconnect is effectively both a transmitter and a receiver.
Note that by using the solder flow self-aligning process, two or more
individual semiconductor chips may be aligned to one optical fiber array.
In this case, the devices on one chip are coupled to some optical fibers
of the fiber connector, whereas the devices on the other wafer(s) are
coupled to the other optical fibers. In this way, a transmitter and a
receiver may be coupled to a single optical connector.
FIG. 3 illustrates the optical interconnect of FIG. 2 as mounted in a
twenty-pin package 200. The fiber connector can be easily attached to or
detached from the optoelectronic board.
FIG. 4a illustrates the use of the optical interconnect of the present
invention for board-to-board interconnect. In this application, board 400
is connected to board 410 via connectors 405 and 415. FIG. 4b is an
enlarged illustration of connector 405 or 415. Each of 405 or 415
comprises an optical interconnect such as the one depicted in FIG. 1a or
FIG. 3. If connector 405 contains only the optical interconnect that is a
transmitter and connector 415 contains only the optical interconnect that
is a receiver, then data are transmitted only from board 400 to board 410.
However, if both connector 405 and 415 include optical interconnects that
have both a transmitter and a receiver, then data can be transmitted in
full duplex fashion between the boards via an optical fiber ribbon.
In a third embodiment of invention, an optical interconnect comprises an
optical fiber having a coupling end and a vertical cavity surface emitting
laser coupled to the coupling end of the fiber.
The optical fiber comprises a core and a cladding layer surrounding the
core. The core at the coupling end is recessed so as to form a rimmed
structure wherein the rim comprises the cladding layer.
The VCSEL has a second mirror that is a distributed Bragg reflector
comprising a plurality of layers formed one on top of the other.
Advantageously, the outermost layers of the second mirror form a mesa that
has a diameter smaller than the inner diameter of the rim. The mesa
engages the coupling end so as to place a substantial portion of the mesa
inside the rim. The VCSEL emits optical radiation that is substantially
coupled into the optical fiber.
FIGS. 5a-c illustrate a preferred embodiment of the invention.
Specifically, FIG. 5a depicts a three-dimensional view of an optical fiber
500 and a VCSEL prior to their coupling. Fiber 500 comprises a core 505
and a cladding layer 510 surrounding the core. At a coupling end 515 of
the fiber, the core is recessed so as to form a rim 516 that is mainly
made of the cladding layer. The VCSEL includes a mesa 555 that comprises
the outermost layers of a second mirror, several underlying layers
described in FIG. 5b, and a contact 565 that surrounds the mesa.
FIG. 5b illustrates a cross-section view of the optical interconnect. The
VCSEL comprises a first mirror 525 formed on substrate 520, a first spacer
530, an active region 535, a second spacer 540, a portion of a second
mirror 545, and the mesa portion of the second mirror 555. The VCSEL
further comprises contact 565 surrounding the mesa and contact 575 formed
on the substrate. Additionally, there is a passivation layer 560 that
protects most of the surface from the atmosphere. Preferably, the
passivation layer is a SiO.sub.2 layer. The laser further includes an
annular implanted current confinement layer 550 that surrounds the active
region.
Both the first and second mirrors are distributed Bragg reflectors, each
comprising a plurality of layers formed one on top of the other.
Additionally, each layer is a quarter-wavelength thick wherein the
wavelength is the wavelength in the layer. Preferably, mesa 555 comprises
dielectric layers; and layers 545 are semiconductor layers having high
doping concentration so as to form low resistance contact with contact
565.
The coupling end of the fiber is firmly fixed to the upper portion of the
VCSEL by an optical epoxy 570.
The optical fiber in this interconnect can be a single-mode fiber, a
multi-mode fiber, or a plastic fiber.
FIG. 5c depicts the three-dimensional view of such interconnect. In this
interconnect, all the optical radiation is substantially coupled into the
fiber without requiring additional alignment means.
This interconnect is fabricated as follows: First, the VCSEL as illustrated
in FIG. 5b is prepared, followed by the fabrication of the optical fiber
with a recessed core at the coupling end. The core and the cladding layer
of a glass optical fiber are normally both made of silica. However, the
core is usually doped to increase the refractive index and hence it can be
selectively etched from the cladding layer by buffered hydrofluoric acid
(BHF) or other acid. Next, the etched fiber is aligned to the VCSEL and
the mesa of the VCSEL is fitted into the rim. Optical epoxy is then
applied to the fiber and laser.
Such an interconnect can also be made in one or two dimensional arrays,
wherein an array of integrally formed VCSELs is coupled to an array of
optical fibers.
FIGS. 6a-b illustrate an example of such an optical interconnect wherein a
fiber connector 650 comprising a linear array of optical fibers is coupled
to a linear array of VCSELs. Specifically, FIG. 6a depicts the VCSEL array
and the fiber array prior to their coupling, whereas FIG. 6b illustrates
the interconnect after their coupling.
In this embodiment, each VCSEL and each optical fiber has the same
construction as those described in FIG. 5b, and thus they are not
discussed in detail here. Additionally, the fiber connector 650 can be
aligned and coupled to the VCSEL array by using a pair of tunnels and
guiding pins in the fiber connector and aligning holes adjacent to the
VCSEL array as described in the embodiments shown in FIGS. 1-4.
As will be apparent to those skilled in the art, numerous modifications may
be made within the scope of the invention, which is not intended to be
limited except in accordance | | |
