U.S. patent application number 15/678535 was filed with the patent office on 2018-03-29 for planar lightwave circuit active connector.
The applicant listed for this patent is Kaiam Corp.. Invention is credited to John Heanue, Bardia Pezeshki, Lucas Soldano.
Application Number | 20180091250 15/678535 |
Document ID | / |
Family ID | 56011281 |
Filed Date | 2018-03-29 |
United States Patent
Application |
20180091250 |
Kind Code |
A1 |
Soldano; Lucas ; et
al. |
March 29, 2018 |
PLANAR LIGHTWAVE CIRCUIT ACTIVE CONNECTOR
Abstract
An assembly of waveguide wavelength multiplexers and
demultiplexers, together with continuous wave (CW) laser
transmitters that interface to grating couplers on a silicon
photonics chip, providing CW sources, multiplexed output and
optionally multiplexed input, all using a single photonic lightwave
circuit (PLC).
Inventors: |
Soldano; Lucas; (Milan,
IT) ; Pezeshki; Bardia; (Menlo Park, CA) ;
Heanue; John; (Boston, MA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Kaiam Corp. |
Newark |
CA |
US |
|
|
Family ID: |
56011281 |
Appl. No.: |
15/678535 |
Filed: |
August 16, 2017 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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14621273 |
Feb 12, 2015 |
9768901 |
|
|
15678535 |
|
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62082529 |
Nov 20, 2014 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G02B 2006/12038
20130101; G02B 6/30 20130101; G02B 6/12021 20130101; G02B
2006/12142 20130101; H04J 14/0212 20130101; H04Q 11/0005 20130101;
H04Q 2011/0032 20130101; G02B 6/2938 20130101; H04Q 2011/0016
20130101; G02B 6/1223 20130101; G02B 6/34 20130101; G02B 6/4215
20130101; G02B 2006/12061 20130101; G02B 6/124 20130101 |
International
Class: |
H04J 14/02 20060101
H04J014/02; G02B 6/42 20060101 G02B006/42; G02B 6/30 20060101
G02B006/30; G02B 6/34 20060101 G02B006/34; H04Q 11/00 20060101
H04Q011/00 |
Claims
1. A planar lightwave circuit (PLC) chip, comprising: a
demultiplexer structure having an input and a plurality of outputs,
the demultiplexer structure configured to provide light on the
input to the plurality of outputs on a wavelength selective basis;
a multiplexer structure having a plurality of inputs and an output,
the multiplexer structure configured to provide light on the
plurality of inputs to the output on a wavelength selective basis;
and a plurality of waveguides, each having waveguide inputs and
waveguide outputs, the waveguide outputs optimized for transmission
of light into a silicon photonics chip.
2. The planar lightwave circuit chip of claim 1, wherein the
plurality of waveguides includes at least a number of waveguides
equal to a number of inputs of the plurality of inputs of the
multiplexer structure.
3. The planar lightwave circuit chip of claim 1, wherein the
demultiplexer structure comprises an arrayed waveguide grating
(AWG).
4. The planar lightwave circuit chip of claim 1, wherein the
multiplexer structure comprises an arrayed waveguide grating
(AWG).
5. The planar lightwave circuit chip of claim 1, wherein the
demultiplexer structure comprises a first arrayed waveguide grating
(AWG) and the multiplexer structure comprises a second arrayed
waveguide grating.
6. The planar lightwave circuit chip of claim 1, wherein at least
one of the demultiplexer structure and the multiplexer structure
comprises an Eschelle grating.
7. The planar lightwave circuit chip of claim 1, wherein the input
waveguide, the plurality of output waveguides, the output
waveguide, the plurality of input waveguides, and the plurality of
waveguides comprise glass waveguides.
8. The planar lightwave circuit chip of claim 7, wherein the
waveguides are formed of layers of glass.
9. The planar lightwave circuit chip of claim 8, wherein the layers
of glass are on a silicon substrate.
10. The planar lightwave circuit chip of claim 8, wherein the
layers of glass are on a quartz substrate.
11. A planar lightwave circuit chip, comprising: a substrate; a
plurality of structures on the substrate, the structures including:
a first plurality of waveguides, each waveguide of the first
plurality of waveguides coupling a corresponding one of a first
plurality of inputs and a corresponding one of a first plurality of
outputs, the first plurality of outputs being on a first side of
the chip; a demultiplexer including a demultiplexer input waveguide
and a plurality of demultiplexer output waveguides; and a
multiplexer including a plurality of multiplexer input waveguides
and a multiplexer output waveguide, the inputs of the plurality of
multiplexer input waveguides being on the first side of the
chip.
12. The planar lightwave circuit chip of claim 11, wherein the
demultiplexer is a wavelength selective demultiplexer.
13. The planar lightwave circuit chip of claim 11, wherein the
plurality of demultiplexer output waveguides are on the first side
of the chip.
14. The planar lightwave circuit chip of claim 11, wherein the
demultiplexer and the multiplexer each comprise an arrayed
waveguide grating (AWG).
15. A planar lightwave circuit chip, comprising: a first plurality
of waveguides to couple light from each of a first plurality of
discrete inputs to corresponding first discrete outputs; a
multiplexer structure to selectively couple light at predefined
wavelengths from each of a second plurality of discrete inputs to a
first single discrete output; and a demultiplexer structure to
couple light from a first single discrete input to a second
plurality of discrete outputs in a wavelength selective manner;
means for directing light to or from the first discrete outputs and
the second plurality of discrete inputs in substantially a first
direction.
16. The planar lightwave circuit chip of claim 15, wherein at least
one of the multiplexer structure and the demultiplexer structure
comprise an arrayed wavelength grating (AWG).
17. The planar lightwave circuit chip of claim 15, wherein at least
one of the multiplexer structure and the demultiplexer structure
comprise an Eschelle grating.
18. The planar lightwave circuit of claim 15, wherein the means for
directing light to or from the first discrete outputs and the
second plurality of discrete inputs in substantially a first
direction comprises an edge with an angled polish.
19. A device for use in a data communication system, comprising: a
plurality of lasers, each laser configured to emit light about a
different wavelength than other lasers of the plurality of lasers;
a silicon chip including a plurality of modulators to provide
modulated light signals through impression of data signals on the
light emitted from the lasers; a planar lightwave circuit (PLC)
chip including a first plurality of waveguides to couple light from
the lasers and the silicon chip, and a wavelength selective light
multiplexer to couple light modulated by the plurality of
modulators of the silicon chip into a single output.
20. The device of claim 19, wherein the light multiplexer comprises
an arrayed waveguide grating.
21. The device of claim 19, wherein the PLC includes an edge with
an angle polish for directing light between the first plurality of
waveguides and the silicon chip.
22. The device of claim 19, wherein the PLC includes an edge with
an angle polish for directing light between an input to the light
multiplexer and the silicon chip.
23. The device of claim 19, further comprising a prism positioned
to direct light between the silicon chip and the plurality of
modulators.
24. The device of claim 19, wherein the silicon chip includes a
plurality of grating couplers to receive the light emitted by the
lasers.
25. The device of claim 19, wherein each of the grating couplers is
optimized for a particular wavelength of light.
26. The device of claim 19, wherein an output waveguide of the
light multiplexer is coupled to a fiber optic line.
27. The device of claim 26, wherein the output waveguide of the
light multiplexer is coupled to the fiber optic line by a
capillary.
28. The device of claim 19, wherein a plurality of lenses, each
positionable by a micro-electro-mechanical system (MEMS), are each
positioned to couple light from a corresponding one of the
plurality of lasers into a corresponding one of the first plurality
of waveguides of the PLC.
29. The device of claim 19, wherein the silicon chip and the PLC
are butt-coupled together.
30. The device of claim 19, wherein the PLC further includes a
wavelength selective light demultiplexer.
31. The device of claim 30, wherein an input waveguide associated
with the light demultiplexer is coupled to a second fiber optic
line.
32. The device of claim 30, wherein the silicon chip includes a
plurality of photodetectors.
33. The device of claim 32, wherein each of the plurality of
photodetectors is positioned to receive light output from a
different one of a plurality of output waveguides associated with
the light demultiplexer.
34. The device of claim 33, wherein the silicon chip includes a
plurality of transimpedance amplifiers to amplify signals from the
photodetectors.
34. The device of claim 32, wherein the silicon chip includes a
plurality of grating couplers to receive light output from output
waveguides of the light demultiplexer, and the silicon chip is
configured to route light from the grating couplers to the
photodetectors.
35. The device of claim 34, wherein the silicon chip includes a
plurality of transimpedance amplifiers to amplify signals from the
photodetectors.
36. The device of claim 31, further a comprising a second silicon
chip including a plurality of photodetectors to detect light from
the plurality of output waveguides of the light demultiplexer.
37. A method of processing light useful in a communications system,
comprising: passing light from a multiwavelength light source
through at least one waveguide of a planar lightwave circuit (PLC)
and into a silicon chip; modulating the light using a plurality of
modulators of the silicon chip; passing the modulated light out of
the silicon chip and through a wavelength selective multiplexer
structure of the PLC; and providing light output from the
wavelength selective multiplexer structure to a fiber optic
line.
38. The method of claim 37, wherein the wavelength selective
multiplexer structure is an arrayed waveguide grating.
39. The method of claim 37, wherein the at least one waveguide and
the wavelength selective multiplexer structure are on a common
substrate.
40. The method of claim 37 wherein the light from the lasers is
passed into the silicon chip by way of grating couplers on a first
surface of the silicon chip.
41. The method of claim 40, wherein the modulated light is passed
out of the silicon chip through the first surface of the silicon
chip.
42. The method of claim 40, wherein the modulated light is passed
out of the silicon chip by way of grating couplers on the first
surface of the silicon chip.
43. The method of claim 37, wherein the multiwavelength light
source comprises a plurality of lasers.
44. The method of claim 43, wherein the at least one waveguide
comprises a plurality of waveguides.
45. The method of claim 44, wherein each of the plurality of
waveguides passes light from a separate one of the plurality of
lasers.
46. The method of claim 37, wherein the at least one waveguide
comprises an input waveguide of a wavelength selective
demultiplexer of the PLC.
47. The method of claim 46, further comprising multiplexing light
from the plurality of lasers into a single beam, passing the beam
through a fiber optic line, and wherein the input waveguide of the
wavelength selective demultiplexer of the PLC receives light of the
beam passed through the fiber optic line.
48. The method of claim 47, wherein the plurality of lasers are
positioned about a front plate of switch.
49. A device for use in a data communication system, comprising: a
multi-wavelength light source; a planar lightwave circuit (PLC)
including a wavelength selective demultiplexer and a wavelength
selective multiplexer; at least one fiber optic line coupling the
multi-wavelength light source and in input waveguide of the
demultiplexer of the PLC; a silicon photonics chip including a
plurality of modulators; and means for directing light from output
waveguides of the demultiplexer of the PLC into the silicon
photonics chip for modulation by the modulators and means for
directing light modulated by the modulators of the silicon
photonics chip from the silicon photonics chip into input
waveguides of the multiplexer of the PLC.
50. The device of claim 49, wherein the multi-wavelength light
source comprises a plurality of lasers.
51. The device of claim 49, wherein the multi-wavelength light
source comprises a plurality of lasers and a second PLC with a
multiplexer to combine light from the plurality of lasers.
52. A method of processing light in a communication system,
comprising: generating a plurality of beams of light, each beam at
a different wavelength; splitting each of the beams of light into
corresponding second beams of light; providing each of the
corresponding second beams of light to corresponding ones of a
second plurality of silicon photonics chips, each having a
plurality of modulators; modulating the second beams of light using
the modulators; and multiplexing beams of modulated light by a
plurality of multiplexers, each multiplexer receiving a different
beam of the beams of modulated light from each of the silicon
photonics chips.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of the filing date of
U.S. Provisional Patent Application No. 62/082,529, filed on Nov.
20, 2014, the disclosure of which is incorporated by reference
herein.
BACKGROUND OF THE INVENTION
[0002] The present application relates generally to fiber optic
communications and, more particularly, to optical communications
using silicon photonic chips having optical modulators using
silicon interfaced with passive planar lightwave circuits.
[0003] In the past few decades, the speed of electronic processing,
powered by increasing levels of integrations and smaller gate
geometries has overwhelmed the ability of these same silicon
integrated circuits to transmit and receive the information that
they process. More and more electrical power and chip real-estate
is devoted to driving the higher capacitance lines that carry
signals off the integrated circuits. Thus the bottleneck in
electronics is frequently the communication between chips, modules,
or systems.
[0004] At the very longest length scales, telecommunication
companies use multi-wavelength communication down a single optical
fiber to pack more than a hundred channels, each modulated using
various techniques to transport information for thousands of
kilometers. The optical line cards and transport systems are
complex, large, and expensive, justified by the need for bandwidth
efficiency in the very long links that they serve. Currently at the
shorter distance scales of a few hundred meters to a few
kilometers, the same multi-wavelength approach is used, albeit with
a smaller number of channels and simple on-off (NRZ) modulation
with more compact transceivers and at lower costs. In both types of
multi-wavelength communications, laser sources, usually in Indium
Phosphide materials systems generate the light, and the data is
then imposed on the signal. In the simplest case, the drive current
to the laser is changed to vary the optical output intensity, while
in more complex systems a separate modulator receives a continuous
optical signal from the laser and acts to vary the intensity or the
phase of the light that passes through it. The latter is of course
more expensive and complicated, but can be more precise, as a
separate modulator can more controllably vary the properties of the
light.
[0005] Recently there has been a great deal of excitement in the
prospect of using silicon as the material for the modulator. The
idea is that the industrial infra-structure that allows the
fabrication of complex electronic integrated circuits can be
leveraged to fabricate the modulators. Such technology can be
useful at all length scales, from complex modulators on the silicon
that can create intensity and phase modulation for efficient
packing of wavelength channels in very long links (for example
DQPSK modulation--Differential Quad Phase Shift Keying, used in
long haul links) to simple on-off modulation to code ones and zeros
(NRZ-non return to zero) in shorter links.
[0006] Perhaps the most significant issue with silicon photonics is
that silicon as a material, unlike Indium Phosphide, does not
possess a direct bandgap. By that we mean that electrons and holes
of the lowest energy have different momentum states, and therefore
cannot combine directly to generate light. In a forward biased
silicon pn junction, the carriers recombine non-radiatively and
thus one cannot make LEDs or lasers in silicon. Generally there
have been three workarounds for this problem. The first is
obviously to have the light off the chip, so a separate indium
phosphide laser generates the light and the light is then coupled
to the silicon chip where it is modulated and then sent out. The
challenge here is of course the complexity of getting the light on
and off the silicon chip, especially if multiple wavelengths or
multiple sources of light are needed. The second more ambitious way
is to try to incorporate the direct gap indium phosphide material
on the silicon. The different lattice constant, chemistry, and
processing requirements of the indium phosphide make it difficult
to fabricate efficient lasers this way. Furthermore, it is
impossible to test or burn-in the laser prior to assembly and the
relatively poor yield of the lasers increases the cost of the
entire assembly. Perhaps the ultimate solution is to try to make
the silicon direct gap by adding impurities or changing the crystal
through physical deformation. Needless to say, this is very
challenging.
[0007] A second related issue with silicon photonics is the
challenge of coupling light in and out of the chip. Even if the
light-source can be integrated into the silicon, one still requires
the light to exit the chip and enter an optical fiber. Silicon
modulators typically use extremely small and high contrast
waveguides. The core is usually made of silicon that is a few
hundred nanometers in scale, and the cladding is typically silicon
dioxide with a very low refractive index compared to the silicon
core (1.46 vs 3.6). Thus the light is highly concentrated in a very
tight waveguide. The high contrast has the advantage of being able
to make tight waveguide turns, the light paths almost having the
geometries of electrical wires, but also has the disadvantage of
being completely mismatched to a mode in a glass optical fiber,
where the contrast is typically much less than 1% between the core
and the cladding. Grating couplers are frequently used to help with
the alignment, but grating couplers generally work only at one
wavelength and therefore limit the coupling to a single channel per
port.
[0008] In current architectures where fiber optics is used to
connect electronic switches, the optics is separate and usually in
the form of a transceiver that is plugged in to the faceplate of
the unit. Typical switches used in datacenters can have tens or
even hundreds of optical transceivers that populate the front plate
of the unit. One advantage of this is that the customer can easily
replace faulty transceiver units at the front panel. The switch
itself generally does not need to be removed or sent back to the
supplier for repair in the event of faulty transceivers. However,
there are many penalties with this approach. First it is difficult
to cool the transceivers in the front panel. It would be much
easier if the modules were mounted on a board of the switch. A
second issue is that high speed signals have to travel from a
switch chip, somewhere on the board, all the way to the front
panel. There is frequently equalization that has to occur both on
the board and also in the transceiver to compensate for distortion
and electrical signal loss as the high speed data patterns travel
the distance from the source into the transceiver and to the
optical module.
BRIEF SUMMARY OF THE INVENTION
[0009] Aspects of the invention provide a simple and efficient
method of coupling light in and out of silicon photonics chips
using a PLC as an intermediate material. By fabricating waveguide
multiplexers and demultiplexers in glass PLCs, multi-wavelength
fibers are broken down to individual wavelengths before entering or
exiting the silicon and so each grating coupler in the silicon can
be used only at a single wavelength, compensating for the limited
optical bandwidth of the silicon grating couplers. Furthermore,
using MEMS to couple lasers into a PLC can be used in the same PLC
to provide the different wavelength laser sources for the silicon
modulators. Some embodiments include structure discussed combining
all three functions (Multiwavelength in for the receiver,
multisources in as inputs for the modulators, and multiwavelengths
out) on a single PLC and provides for a simple method of attaching
the assembly to a silicon photonics chip.
[0010] Some aspects in accordance with the invention provide a
silicon photonics chip having a plurality of grating couplers and a
photonics lightwave circuit (PLC) positioned to couple light with
the grating couplers. In some embodiments the PLC includes a
plurality of waveguides and a structure for deflecting light from
the waveguides towards a surface of the silicon photonic chip. In
some embodiments the PLC includes an arrayed waveguide grating
(AWG), such that each of the wavelengths of light coming in or out
of the assembly is separated out into a plurality of individual
waveguides or combined from a plurality of individual waveguides
into a single waveguide. In some embodiments the grating couplers
of the silicon photonics chip are configured for passing of light
of the same wavelengths as the corresponding ones of the plurality
of waveguides. In some embodiments the silicon photonics chip
includes a first set of grating couplers for coupling light from a
first set of waveguides of the PLC into the silicon photonics chip
and a second set of grating couplers for coupling light from the
silicon photonics chip into a second set of waveguides of the PLC.
In some embodiments the PLC includes a third set of waveguides for
coupling light from lasers into a third set of grating couplers of
the silicon photonics chip. In some embodiments the light from the
silicon photonics chip is light from the lasers. In some
embodiments the silicon photonics chip impresses data onto light
from the lasers. In some embodiments the silicon photonics chip
impresses the light by modulation of the light. In some embodiments
the modulation of the light is performed by modulators of the
silicon photonics chip.
[0011] Some embodiments provide a combination of a plurality of
lasers of different wavelengths that operate CW (continuously), and
are coupled to silicon photonics chip that contains a plurality of
silicon modulators that impress a signal on these CW wavelengths of
light and a low index passive Planar Lightwave circuit that is used
to multiplex the different modulated wavelengths of light emanating
from the modulators, where the PLC is made is made separately using
a different material system and is attached to the silicon
photonics chip. In some such embodiments the individual lasers are
also coupled to the PLC and from the PLC enter the silicon
waveguide, where the PLC is an intermediate waveguide material. In
some such embodiments the PLC also contains a demultiplexing
structure that is used for a receiver circuit. In some such
embodiments a single PLC is used that contains different devices,
one for multiplexing the different wavelengths coming out of the
modulator into a single output, one for demultiplexing the
receiver, and one as an intermediate waveguide structure between
the lasers and the silicon chip. In some such embodiments the light
from the PLC enters and/or exits the silicon waveguide by being
deflected down onto grating couplers our out of grating couplers
made in the silicon wafer. In some such embodiments means of
coupling light between the PLC and the silicon wafer comprises an
angle polish on the edge of the PLC. In some such embodiments MEMS
coupling is used to couple the light from the individual lasers
ultimately to individual waveguides in the silicon that lead to the
individual modulators. In some such embodiments a polarity of
wavelengths is generated by a single multiwavelength source. In
some such embodiments there is an addition fiber or fibers between
the laser source and the silicon photonics chip, thereby enabling
the replacement of the laser source in case of failure of the
source. In some such embodiments the coupling from the Planar
Lightwave Circuit and the silicon modulator is realized by
attaching the two chips directly where one waveguide facet mates
directly to the other waveguide facet (end-butt coupling).
[0012] Some aspects of the invention provide as an embodiment a
planar lightwave circuit (PLC) chip, comprising a demultiplexer
structure having an input and a plurality of outputs, the
demultiplexer structure configured to provide light on the input to
the plurality of outputs on a wavelength selective basis; a
multiplexer structure having a plurality of inputs and an output,
the multiplexer structure configured to provide light on the
plurality of inputs to the output on a wavelength selective basis;
and a plurality of waveguides, each having waveguide inputs and
waveguide outputs, the waveguide outputs optimized for transmission
of light into a silicon photonics chip.
[0013] Some aspects of the invention provide as an embodiment a
planar lightwave circuit chip, comprising a substrate; a plurality
of structures on the substrate, the structures including a first
plurality of waveguides, each waveguide of the first plurality of
waveguides coupling a corresponding one of a first plurality of
inputs and a corresponding one of a first plurality of outputs, the
first plurality of outputs being on a first side of the chip; a
demultiplexer including a demultiplexer input waveguide and a
plurality of demultiplexer output waveguides; and a multiplexer
including a plurality of multiplexer input waveguides and a
multiplexer output waveguide, the inputs of the plurality of
multiplexer input waveguides being on the first side of the
chip.
[0014] Some aspects of the invention provide as an embodiment a
planar lightwave circuit chip, comprising a first plurality of
waveguides to couple light from each of a first plurality of
discrete inputs to corresponding first discrete outputs; a
multiplexer structure to selectively couple light at predefined
wavelengths from each of a second plurality of discrete inputs to a
first single discrete output; and a demultiplexer structure to
couple light from a first single discrete input to a second
plurality of discrete outputs in a wavelength selective manner;
means for directing light to or from the first discrete outputs and
the second plurality of discrete inputs in substantially a first
direction.
[0015] Some aspects of the invention provide as an embodiment a
device for use in a data communication system, comprising a
plurality of lasers, each laser configured to emit light about a
different wavelength than other lasers of the plurality of lasers;
a silicon chip including a plurality of modulators to provide
modulated light signals through impression of data signals on the
light emitted from the lasers; a planar lightwave circuit (PLC)
chip including a first plurality of waveguides to couple light from
the lasers and the silicon chip, and a wavelength selective light
multiplexer to couple light modulated by the plurality of
modulators of the silicon chip into a single output.
[0016] Some aspects of the invention provide as an embodiment a
method of processing light useful in a communications system,
comprising passing light from a multiwavelength light source
through at least one waveguide of a planar lightwave circuit (PLC)
and into a silicon chip; modulating the light using a plurality of
modulators of the silicon chip; passing the modulated light out of
the silicon chip and through a wavelength selective multiplexer
structure of the PLC; and providing light output from the
wavelength selective multiplexer structure to a fiber optic
line.
[0017] Some aspects of the invention provide as an embodiment a
device for use in a data communication system, comprising a
multi-wavelength light source; a planar lightwave circuit (PLC)
including a wavelength selective demultiplexer and a wavelength
selective multiplexer; at least one fiber optic line coupling the
multi-wavelength light source and in input waveguide of the
demultiplexer of the PLC; a silicon photonics chip including a
plurality of modulators; and means for directing light from output
waveguides of the demultiplexer of the PLC into the silicon
photonics chip for modulation by the modulators and means for
directing light modulated by the modulators of the silicon
photonics chip from the silicon photonics chip into input
waveguides of the multiplexer of the PLC.
[0018] Some aspects of the invention provide as an embodiment a
method of processing light in a communication system, comprising
generating a plurality of beams of light, each beam at a different
wavelength; splitting each of the beams of light into corresponding
second beams of light; providing each of the corresponding second
beams of light to corresponding ones of a second plurality of
silicon photonics chips, each having a plurality of modulators;
modulating the second beams of light using the modulators; and
multiplexing beams of modulated light by a plurality of
multiplexers, each multiplexer receiving a different beam of the
beams of modulated light from each of the silicon photonics
chips.
[0019] These and other aspects of the invention are more fully
comprehended upon review of this disclosure.
BRIEF DESCRIPTION OF THE FIGURES
[0020] Aspects of the disclosure are illustrated by way of
examples.
[0021] FIGS. 1A and 1B (prior art) show the design of a 4 lane
silicon photonics transmitter that needs a ribbon fiber, with one
modulated channel per fiber and all four fibers operating at the
same wavelength.
[0022] FIG. 2 (prior art) shows a MEMS-based approach of coupling
individual laser chips to a glass PLC using moveable
microlenses.
[0023] FIG. 3 shows an embodiment in accordance with aspects of the
invention.
[0024] FIG. 4 shows a schematic of a glass PLC with two arrayed
waveguide gratings, one is the demux for the receive, while the
other is the mux for the transmit, with connecting waveguides from
one side to the other bring in the CW laser light.
[0025] FIG. 5 shows how the angle polish on the PLC directs the
light down to the silicon grating coupler.
[0026] FIG. 6 shows a semi-schematic, semi-block diagram of the
silicon photonics chip, with a receive chain going to high speed
integrated photodetectors and amplifiers, and a transmit chain
taking CW laser sources to the modulators and then back to the
output, and in which integrated taps can monitor power levels and
provide feedback for MEMS coupling, for example during a
manufacturing stage.
[0027] FIG. 7 illustrates a different embodiment, where the
multi-wavelength CW source is fiber coupled to the PLC with a
connector.
[0028] FIG. 8 illustrates an alternate embodiment where instead of
grating couplers, direct end-butt coupling is used.
[0029] FIG. 9 illustrates an embodiment using a plurality of
silicon photonics chips.
DETAILED DESCRIPTION
[0030] Silicon modulators are readily fabricated using standard
processes in foundries and therefore can be used to generate
modulated signals that can transmit information between electronic
modules. As mentioned previously, it is extremely difficult to
generate light in silicon, and therefore a separate light source,
for example a laser, is generally needed that provides light
coupled into the silicon photonics chip with the modulators.
Similarly, the modulated light is coupled out of the chip and into
fiber(s) to transmit the information. One way of coupling light in
and out of very tight silicon waveguides is with a grating coupler,
which can only operate efficiently at one wavelength. Given these
limitations of standard silicon photonics, often a single high
power external laser is used as the source for multiple modulators.
As input coupling into the waveguides is challenging, coupling a
single laser is simpler than coupling multiple lasers. This means
that for multiple lanes, multiple fibers are needed, making the
cable plant much more complicated. Even if multiple lasers were
used, making a multiplexer to combine the wavelengths together is
challenging in silicon photonics, so it would still be difficult to
combine all the wavelengths into a single low cost fiber.
[0031] FIGS. 1A and 1B show a current implementation of silicon
photonics, which exhibits many of the limitations mentioned above.
FIG. 1A is a diagram of the structure, while FIG. 1B is a schematic
of the different elements of the silicon photonics chip. The
silicon photonics chip (101) that contains the modulators acts as
the base of the optical assembly and is wirebonded to the package
such that the drive voltages of the modulators can be obtained from
external sources. On top of the silicon photonics chip is a small
packaged laser (102). In this small assembly an InP laser diode is
combined with a focusing lens, an isolator, and a turning mirror to
reflect the light onto the silicon photonics chip (101). The laser
beam is carefully aligned on top of a grating coupler (104) in the
silicon photonics chip that couples the laser light into a small
silicon waveguide. The light is then divided into 4 separate
waveguides using a splitter (105). Each independent output of the
splitter goes into a silicon modulator (106) and a separate output
grating coupler 107 that sends the light vertically out of the
silicon photonics chip. The array of four output grating couplers
interface with a fiber block (103) containing a single mode fiber
ribbon, which then exits the assembly. It would be preferable that
such a module operate at multiple wavelengths and use only a single
output fiber, As providing ribbon cabling over long lengths and
with many connectors can be expensive, but as previously mentioned,
this is difficult given the limitations of silicon photonics.
[0032] FIGS. 1A & 1B describe prior art for only a transmitter
with four lanes. Of course a larger number of lanes could carry
additional information, but would need more fibers and the laser
power would also have to be higher for the increased splitting
loss. Similarly a receiver could also be implemented on the silicon
photonics chip. In this case rather than just four output fibers,
one would need eight fibers in total, four for output and four for
input. The inputs would go to additional grating couplers that
direct the light to four photodetectors and transimpedance
amplifiers. A wavelength multiplexed system of course would only
need two fibers, one for output and one for input rather than eight
fibers.
[0033] FIG. 2 shows prior art method of coupling light from laser
chips to a PLC--a photonic lightwave circuit with waveguides made
of glass rather than silicon. By using lenses between each laser
and each input waveguide of the PLC, one can obtain very good
coupling between the two.
[0034] The PLC chip 201 has four input waveguides (not shown) and
contains a wavelength multiplexer such as an AWG (not shown) with a
single output on the other side of the chip (not shown). The
assembly contains four lasers 204 that emit light into four lenses
203, one lens per laser. The lens focuses the light and matches the
mode to the input waveguides of the PLC 201. Given that very
precise positioning is needed on these lenses, the lenses are
mounted on a movable stage built on a silicon chip 202 using
silicon MEMS (micro-electro-mechanical systems) techniques. Each
movable stage is connected to a lever 205 that magnifies the motion
of the lens. At the end of the lever is a heater 206 used to lock
down the lever in the optimal position. The assembly process starts
with bonding all the components on the MEMS chips 202. Each lens is
then separately aligned using the lever and the levers are locked
with the heaters. This process has proven itself a simple and high
yield technique for aligning lasers to PLCs.
[0035] FIG. 3 shows the overall structure in accordance with
aspects of the invention that overcomes at least some limitations
of the prior art, that can operate for example on just two single
mode fibers, one for input and one for output. Details of various
of the components are further described in subsequent figures. In
various embodiments, on the receive side, multiwavelength light,
with many modulated channels, enters from a single fiber and is
demultiplexed by a planar lightwave circuit (PLC). The
demultiplexed light is passed into a silicon photonics chip. In
some embodiments the demultiplexed light is directed into the
silicon photonics chip. In some embodiments an angle polished side
on the PLC directs the light down into grating couplers on the
silicon photonics chip. On the transmit side, light from multiple
continuous wave (CW) laser sources are coupled into the PLC using
MEMS-based coupling. The light from the lasers passes through
waveguides of the PLC to the silicon photonics chip. In some
embodiments the light from the lasers is directed from outputs of
the waveguides into the silicon photonics chip In some embodiments
the angle polished side of the PLC directs the light into the
silicon photonics chip, where each wavelength is modulated. The
light is generally guided by waveguides in the silicon photonics
chip to optical modulators of the silicon photonics chip, which
perform the modulation. The output, modulated light, goes back
through the PLC and is multiplexed, for example using a multiplexer
structure of the PLC, back into a single fiber. All three functions
are obtained through one PLC. In some embodiments, however, the PLC
includes the waveguides and multiplexer structure for passing light
from the lasers to the silicon photonics chip, and for providing
light modulated by modulators of the silicon photonics chip to an
optical fiber as a single beam of light, and may not include a
demultiplexer for receive side operations. In such embodiments
multiwavelength light on the receive side may pass through a
demultiplexer of another PLC, and be provided to either the silicon
photonics chip or another silicon photonics chip.
[0036] In the embodiment of FIG. 3, multi-wavelength light, for
example carrying, four modulated wavelengths from a distant source,
enters through fiber 306. This fiber is glued into a capillary 303
that is aligned to the PLC 302. This light is then demultiplexed
into four individual channels in the PLC 302, for example using a
conventional wavelength sensitive structure such as an AWG. In some
embodiments the PLC provides glass waveguides, which may be formed
on silicon or quartz wafers, with the AWG formed on a surface of
the waveguiding structure using for example lithographic
techniques. The four channels are then in four separate waveguides,
each waveguide carrying one wavelength of light. The waveguides
terminate, in the embodiment of FIG. 3, at the other end of the
PLC. In some embodiments the other end of the PLC includes a
structure for directing, or deflecting, this light down into the
silicon photonics chip 301. This structure could be as simple as a
high reflection coated angle polish on the end of the PLC, as shown
in the figure, or it could be a diffraction grating or a separate
mirror or prism formed into the PLC, for example. The four channels
enter the silicon photonic chip through separate grating couplers,
each of which is optimized for that particular wavelength of light.
For each channel, the light then goes into an integrated
photodetector on the silicon photonics chip and is converted into
an electrical signal and electrically amplified by a subsequent TIA
(transimpedance amplifier). The silicon photonic chip may
additionally include circuitry to further process the electrically
amplified signal prior to passing the signal to other components of
a receiver of which portions of the silicon photonic chip is a
part. The further processing may include, in some embodiments,
digitization and/or data recovery circuitry and other circuitry.
Alternately, the silicon photonic chip may simply pass the
electrically amplified signal to other components.
[0037] A MEMS coupling structure 305 couples light from lasers into
the PLC. The MEMS coupling structure 305 is similar to, and in some
embodiments the same as, the prior art described in FIG. 2, where
four individual lasers of different wavelengths are coupled into
four waveguides on the PLC 302. In some embodiments the MEMS
coupling structure is as described in U.S. Pat. No. 8,346,037,
entitled Micromechanically Aligned Optical Assembly, the disclosure
of which is incorporated by reference herein for all purposes. Of
course other techniques not using MEMS could be used to couple the
light from the lasers into the PLC. The light in the PLC goes into
four waveguides, that also terminate on an end of the PLC chip, and
are reflected down onto grating couplers on the silicon photonics
chip, using the same method as the received channels described
previously. In this case, however, the four channels go, generally
via waveguides in the silicon photonics chip, to four modulators of
the silicon photonics chip, where data are impressed upon the
signal. The modulated light signal exits the modulators and goes
back to four grating couplers which deflect the modulated light
signal upwards into the PLC 302, where they enter four separate
waveguides. In some embodiments, it is important to note, the input
grating couplers, output grating couplers and the modulators in
this transmit chain are all optimized for the particular wavelength
in each path. The PLC 302 also contains a wavelength multiplexer
that combines all four channels together into a single waveguide.
The output of this multiplexer, which again could be an AWG, is
coupled to another capillary fiber assembly 304 and 307 that send
the information out of the module. This output is aligned and
epoxied to the PLC in the same manner as the input 303 and 306.
[0038] FIG. 4 shows an example of the PLC 302 in greater depth. On
the right hand side of the figure, where the PLC interfaces to the
fibers and the lasers, there are three features. At the very top of
the figure is an input waveguide 401 for a demultiplexer structure
406. This input waveguide would be aligned and affixed to the
capillary and fiber assembly 303/306 of the previous figure. In the
center there are four input waveguides 402 that connect to the
laser assembly 305. At the bottom 403 is an output waveguide that
connects to the output capillary and fiber 304/307 of the previous
figure. There are three main structures on the chip, illustrated as
being in the middle of the chip. At the top is the demultiplexing
AWG 404, at the bottom is the multiplexing AWG 405, and in the
center are simple waveguide connections. On the left hand side of
the chip, as illustrated in FIG. 4, at the top are the four
demultiplexer output waveguides 406 that connect to the receiver.
At the bottom are the four multiplexer input waveguides 408 that
come from the four modulators and are subsequently multiplexed on
the PLC. In the center are the four waveguides 407 that send the
continuous wave (CW) signals from the laser chips into the input of
the silicon modulators. In some embodiments, outputs of the
demultiplexer output waveguides, inputs of the multiplexer input
waveguides, and outputs of the four waveguides that send the CW
signals from the laser signals direct light from the PLC towards
inputs of the silicon photonics chip, in the case of the outputs of
the demultiplexer output waveguides and the four waveguides, or, in
the case of the inputs of multiplexer input waveguides, direct
light from the silicon photonics chip into the PLC. In some
embodiments a structure may be used to directionally couple light
between the PLC and the silicon photonics chip. The structure may
be, for example, and angled polish, or structures for directing or
deflecting light such as, for example, discussed with respect to
FIG. 5. In the embodiment of FIG. 4, at the left hand side of the
chip, there is an angle polish to match the input acceptance angle
of the grating coupler on the silicon photonics chip. This angled
polish (409) shows as a vertical line in the top view. All the
waveguides terminate at this angle polish and therefore the light
is reflected down into the grating coupler of the silicon photonics
chip. In the embodiment of FIG. 4 the angled polish is configured
to direct light from or to the various waveguides in substantially
the same direction.
[0039] In the figure Arrayed Waveguide Gratings are shown as an
example for the demultiplexing geometry. Of course many different
kinds of wavelength combiners or splitters could be used. For
example, an Eschelle grating provides essentially the same
functions. Wavelength dependent directional couplers using
asymmetry between the waveguides, or diffraction gratings etched
into the waveguides or couplers could act as filters. The material
of the PLC may be glass on silicon, as previously mentioned, but in
various embodiments a variety of wave materials may instead be
used, for example such as silicon-on insulator (SOI) waveguides,
polymer waveguides, or higher contrast SiON waveguides, and the
waveguides and other structures may be on different materials such
as Silicon, quartz, or fused-silica.
[0040] FIG. 5 shows a detail of the side polish of the PLC and the
grating coupler. The PLC wafer 302 includes a silicon substrate 501
on which various layers of glass are formed. The wafer is polished
at an angle and then attached upside down to the silicon photonics
chip 301. The PLC waveguides have two outer cladding regions 502
and 504, and a higher index core region 503. When the wafer is
placed on the silicon photonics chip 301, the angle polish 409
causes the beam to reflect against the polished surface, diffract
through the roughly 10 um of top cladding 504 and imping on the
grating coupler 505. The grating coupler pitch and contrast is such
that a particular wavelength couples to the silicon core waveguide
506. FIG. 5 shows an angle polish on the PLC to direct, or deflect,
light into the grating couplers of the silicon photonics chip or to
direct light from the silicon photonics chip into inputs of
multiplexer waveguides. Of course there are other techniques of
directing, or deflecting, the light into the grating coupler. For
example, the PLC could have a perpendicular facet, but the light
would then enter a prism that would reflect the light down. It may
be easier to coat and handle a prism rather than create an angular
polish on the PLC. Similarly, one could create a grating on the
waveguide PLC which would direct or deflect the light much the same
way that the grating in the silicon couples a vertical beam into a
horizontal guided mode.
[0041] On the receiver side, a grating coupler may not be
necessary. Depending on the silicon technology, the light in the
glass PLC 302 could be deflected down by the side polish 409 and
instead of hitting a grating coupler 505 and entering a waveguide
506, could instead hit a photodetector that would be placed in lieu
of the grating coupler 505. The photodetector would then be
electrically connected to the transimpedance amplifier. This may be
simpler than the light first going via a grating coupler to a
waveguide and then to a photodetector. Furthermore, it would
resolve some polarization complexities, since the receiver
generally have to be polarization insensitive and a grating coupler
that works with both polarization usually is less efficient and is
in fact generally a combination of two gratings that go to two
different waveguides and two different detectors. In this case a
single p-i-n diode would suffice.
[0042] FIG. 6 shows a schematic of the silicon photonics chip 301.
On the right hand side of the figure are all the optical inputs and
outputs, received from the PLC chip from the angle polished face
and onto the grating couplers 505. There are 12 sets of inputs and
outputs. A top four 607 are the receiver inputs, a middle four 608
are the inputs from the lasers which go to the modulators 603, and
a bottom four in the FIG. 609 are the modulated outputs of the
transmitter. The input chain of the receiver simply goes to high
speed photodetectors 601 that are integrated with the silicon chip
and in turn goes to transimpedance amplifiers 602. The CW laser
inputs go to the modulators 603 and then exit the chip. Optionally,
one may have low speed photodetectors on the chip that tap a small
amount of the transmit or receive chain. Those tapping the receive
chain 606 can monitor the input power and adjust the laser bias to
compensate for temperature variations of laser output power or for
aging. The output of these detectors are particularly useful in the
MEMS alignment process, because the position of the microlenses 203
preferably make use of some sort of a signal to optimize position.
The taps on the output 605, for example, could be used to monitor
the health of the modulators and set off an alarm should the power
vary outside the specifications. The silicon photonics chip could
of course also contain electronics 610 for the control of signals
or to process signals. The control and driver function can also be
implemented in a separate chip that would be bonded to the main
silicon photonics chip.
[0043] Currently integrated detectors in silicon do not have the
performance of separately fabricated InP detectors. So one may
desire to implement only the modulators on the silicon chip and
have a more conventional detector path. In this case waveguides 607
would terminate on a standard InP photodetector array and either
use a separate TIA or be electrically connected to the 620, the
TIAs in the silicon chip.
[0044] As discussed in the background section, bringing the optical
signals directly to the processor has many advantages, such as
reducing the need for equalization and compensation for the loss
and distortion that electrical signals suffer going all the way to
the front panel. There is a potential penalty which is the
increased difficulty of replacing failed components. Should one of
the lasers fail, it is more difficult to replace the multi-chip
module on the board than a pluggable component on the faceplate.
FIG. 7 addresses this issue.
[0045] In this case, a multiwavelength source is separate from an
interface PLC 703, which provides light to a silicon photonics chip
301. A multi-wavelength source 700 emits radiation at multiple
wavelengths simultaneously into a single output. The
multiwavelength source could simply be a set of individual lasers,
which may be in the form of an array of lasers on a single chip or
an array of lasers on discrete chips. Alternatively, the
multiwavelength source may be another type of multiwavelength
source, for example a quantum dot laser with an external grating.
Light of the multiwavelength source may combine into the single
output, for example using a PLC having a wavelength selective
multiplexer, for example provided by an AWG. In some such
embodiments light from discrete wavelength laser sources may be
provided to the PLC using MEMS-mounted lenses as discussed with
respect to FIG. 2. The output of the multiplexer of the PLC may be
coupled to a fiber or fiber pigtail, for example as discussed with
respect to FIG. 3. Should the multi-wavelength source fail, a
technician could simply unplug or disconnect the source, which
might be conveniently placed in the front panel, and replace it
with a spare. The fiber from the PLC is coupled to a connector 701.
Another fiber is also coupled to the connector, with the other
fiber providing an optical path to a fiber assembly 707 coupled
with the interface PLC 703. The interface PLC coupled to the
silicon photonics chip 301 is slightly different than the PLC of
FIG. 3. Rather than having two pigtailed fibers 303 and 304, as in
FIG. 3, there are now three. The new pigtailed connection 702
brings in light from the multiwavelength source and the interface
PLC 703 demultiplexes this, perhaps through another AWG that splits
up the light from the multiwavelength source into multiple
waveguides.
[0046] In some embodiments it may be preferred that the connector
701 and the fiber connecting the multiwavelength source 700 to the
pigtail 702 be polarization maintaining single mode fiber. This
makes the design of the grating coupler easier in the silicon
photonics chip.
[0047] Conceivably one could replace the multiplexer in the
multiwavelength source, the single fiber connector 701 and the
additional demultiplexer in PLC 703 with a ribbon fiber and an
arrayed connector, such as a single mode MTP connector. This has
the advantage of lower loss, as it would remove the insertion loss
of the additional multiplexer and demultiplexer, but it would add
cost and complexity, since a ribbon fiber and an array connector
would be needed.
[0048] Embodiments discussed above generally have been discussed
with grating couplers in the silicon for passing light into the
silicon. For example the light in the PLC may be deflected down
onto the silicon grating coupler and is then transferred into a
waveguide in the silicon. Some silicon photonic technologies do not
possess efficient grating couplers and/or are better interfaced
using edge coupling. Though edge coupling is not generally
wavelength sensitive and it is possible to make multiplexers and
demultiplexers in the silicon waveguides, these silicon mux/demuxes
are generally more lossy and difficult to make. The higher index of
the silicon makes the wavelength of these devices extremely
dependent on the geometry of the waveguides, and slight variations
that occur in normal manufacturing can dramatically vary the
performance of these devices. So it may be preferable to
manufacture wavelength multiplexers and demultiplexers in one or
more low index silica PLCs that are edge coupled to the silicon
photonics chip. FIG. 8 shows such an implementation. A silicon
modulator chip 801 is almost identical to the chip 301 discussed
with respect to FIG. 6, except that there are no grating couplers.
Instead input and output waveguides of the silicon chip terminate
at an edge of the chip. The edge or facet of the chip may be coated
to increase the transmission of light out of the edge of the chip.
Similarly, the PLC chip 802 is almost identical to the PLC 302
discussed with respect to FIG. 4, but there is no angle polish on
the edge of the chip. Like the silicon photonics chip 801, the
waveguides of the PLC terminate at an edge of the PLC, which also
could be polished and coated. The two chips 801 and 802 are aligned
such that the waveguides interface, and the chips are attached
together in some embodiments, for example with epoxy or other
means. In addition to the coating applied to the ends of the chips
801 and 802, tapers could be used in the waveguides of both chips
to match the size of the optical modes and further reduce coupling
loss between the two chips. Lasers 305 may be coupled to the PLC,
for example using a MEMS based approach discussed with respect to
FIG. 2, for example.
[0049] The exact implementation on the speed of the lanes, the
number of lanes, and the number of lanes per silicon photonic chip
may all depend on the application and yield and cost points.
Implementations discussed above have been in terms of a single
silicon photonics chip used with a single PLC chip. However,
multiples of either, or both, chips can be implemented, for example
to increase capacity. A particular useful embodiment is shown in
FIG. 9, where four separate silicon photonics chips, each with 4
channels of modulators, are used with a single 16 channel PLC that
directs light into an array of 4 transmit fibers, with each
transmit channel carrying four wavelengths.
[0050] The light source in this case is the same as previously
described. Assembly 305 has four separate lasers, each of which
operates at a different wavelength. However, the PLC 802 directs
each wavelength of light to a different silicon photonics chip
(803-806). In various embodiments, each of the silicon photonics
chips are also slightly different in that parts of each chip, in
some embodiments all parts, are designed to work at a particular
wavelength. A 6 dB or a divide by 4 optical splitter may take the
appropriate wavelength of light from assembly 305 and splits it
into four modulators of each of four silicon photonics chips, and
then sends modulated light back to the PLC 802. The PLC 802
includes four multiplexers, each of which receives one different
channel from each of the silicon photonics chips and muxes them
together, with the multiplexers providing four outputs. These four
outputs, each of which contain four wavelengths coming from the
different silicon chips are coupled to a parallel single mode fiber
ribbon 801. Of course the number of channels, silicon photonics
chips, wavelengths and fibers in the ribbon can be optimized for
particular application.
[0051] The topology discussed in this disclosure is generally very
scalable, and may be extremely useful for many applications. For
example, in various embodiments the chips can be used in
transceivers that are now normally used in routers and switches of
datacenters. Modules can also be used for midboard or embedded
applications, for example with the modules on a circuit board and
fiber is routed to a front panel. Further, the chips in various
embodiment may be co-packaged with processor chips or integrated
with processor chips. For example a switch chip or a
microprocessor, instead of taking the high speed data signals to
drive high current transistors that can power the capacitance of
package pins and traces, instead may route the high speed signals
to on-board modulators. With the implementation described, the
signals are taken off the chip optically, and in various
embodiments with many wavelengths in the same fiber. For example a
20 laser assembly, with light from each laser routed to four
modulators, as in FIG. 8, and driving at 25 Gbaud with PAM 8
modulation would provide 1 Tb/s per fiber, or 4 Tb/s in PSM4 fiber
(parallel single mode with 4 fibers in each direction). Such an
optical interconnect may be used, in various embodiments, in the
transport of data between chips, backplanes, or switches.
[0052] There are many variations on this structure that fall within
the realm of this invention. For example, in some embodiments the
number of channels can be increased to far more than four. Thirty
six channels modulated at 25 Gbaud using PAM4 modulation results in
a total bandwidth of 1.8 Tb/s and in various embodiments only a
single input fiber is used for an input and another single output
fiber for the output. In various embodiments the wavelength
spacing, for example of light from the lasers, is placed close
together and the entire system temperature controlled to allow for
additional channels. In addition, in various embodiments arrays of
lasers are used, for example on one side of the PLC, instead of
individual laser dies, and an array of lenses could couple this
into the PLC. Since the laser array, the PLC input waveguides and
the lens array spacing are determined lithographically, the
alignment would be relatively simple. Further, whether using arrays
or discrete lasers, alternative methods of aligning to the PLC
could be used instead of MEMS. For example, the lenses could be
individually adjusted with micropositioners and glued in place.
Instead of lenses, butt-coupling could be used where the laser end
faced is placed in close proximity to the PLC input waveguides.
[0053] Another alternative embodiment uses a multi-wavelength laser
within the main PLC instead of a number of individual lasers. The
PLC incorporates another AWG to demultiplex the wavelengths of the
multi-wavelength laser and separate them into individual waveguides
that enter the silicon photonics chip(s) at different points.
[0054] In some embodiments optical isolators are incorporated with
the microlenses, used to couple light from the lasers into the PLC,
to prevent feedback into the lasers. In some embodiments optical
taps are implemented in the PLC, rather than the silicon photonic
chip. Additional inputs and outputs, such as straight-through
waveguides, are added to the PLC in some embodiments to ease
alignment and assembly of the PLC and silicon chips. Alternatively,
one can increase the number of channels of an uncooled system and
space them closer together. All the channels will drift up and down
with temperature together, and one can use a demultiplexer to track
this drift and appropriately lock on to a DWDM wavelength. This can
be done in many ways. For example, the receiver can be made tunable
by controlling the temperature of the demultiplexer. Since the
demultiplexer does not generate heat, it can be thermally insulated
from the environment and therefore only a small amount of power
from a heater would vary the temperature substantially. This would
tune the filter. In some embodiments this heater could be made
local--for example on a polymer insert into the PLC, or it could
heat the entire assembly. To track, a low frequency dither tone can
be placed on one channel of the transmitter. A receiver would
detect this dither tone, and generate a signal to adjust the
temperature of the receiver with heater power such that the dither
would be maximized at the appropriate channel.
[0055] Various aspects of the invention are useful, for example, as
they provide, in some embodiments, a single chip interface to the
silicon photonics and provides for all the inputs and outputs
simultaneously.
[0056] Although the invention has been discussed with respect to
various embodiments, it should be recognized that the invention
comprises the novel and non-obvious claims supported by this
disclosure.
* * * * *