U.S. patent application number 16/745699 was filed with the patent office on 2020-05-14 for optical delay lines for electrical skew compensation.
The applicant listed for this patent is Elenion Technologies, LLC. Invention is credited to Ran Ding, Michael Hochberg, Yang Liu, Ari Novack, Alex Rylyakov, Matthew Akio Streshinsky.
Application Number | 20200153512 16/745699 |
Document ID | / |
Family ID | 56689497 |
Filed Date | 2020-05-14 |
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United States Patent
Application |
20200153512 |
Kind Code |
A1 |
Streshinsky; Matthew Akio ;
et al. |
May 14, 2020 |
OPTICAL DELAY LINES FOR ELECTRICAL SKEW COMPENSATION
Abstract
A skew compensation apparatus and method. In an optical system
that uses optical signals, skew may be generated as the optical
signals are processed from an input optical signal to at least two
electrical signals representative of the phase-differentiated
optical signals. A compensation of the skew is provided by
including an optical delay line in the path of the optical signal
that does not suffer the skew (e.g., that serves as the time base
for the skew measurement). The optical delay line introduces a
delay T.sub.skew equal to the delay suffered by the optical signal
that is not taken as the time base. The two signals are thereby
corrected for skew.
Inventors: |
Streshinsky; Matthew Akio;
(New York, NY) ; Ding; Ran; (New York, NY)
; Liu; Yang; (Elmhurst, NY) ; Novack; Ari;
(New York, NY) ; Hochberg; Michael; (New York,
NY) ; Rylyakov; Alex; (Staten Island, NY) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Elenion Technologies, LLC |
New York |
NY |
US |
|
|
Family ID: |
56689497 |
Appl. No.: |
16/745699 |
Filed: |
January 17, 2020 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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16135950 |
Sep 19, 2018 |
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16745699 |
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14931796 |
Nov 3, 2015 |
10110318 |
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16135950 |
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62118420 |
Feb 19, 2015 |
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62132742 |
Mar 13, 2015 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G02B 6/4266 20130101;
H04B 10/614 20130101; G02B 6/4292 20130101; H04B 10/616 20130101;
H04B 10/2507 20130101; H04B 10/615 20130101; G02B 6/4284 20130101;
G02B 6/4213 20130101; G02B 6/4274 20130101; G02F 1/313 20130101;
H04B 10/50 20130101; H04B 10/40 20130101; G02B 6/2861 20130101 |
International
Class: |
H04B 10/61 20060101
H04B010/61; H04B 10/40 20060101 H04B010/40; G02F 1/313 20060101
G02F001/313; G02B 6/42 20060101 G02B006/42; H04B 10/50 20060101
H04B010/50; H04B 10/2507 20060101 H04B010/2507 |
Claims
1. An optical receiver system, comprising: a receiver configured to
receive an input optical signal comprising a first optical
component signal and a second optical component signal from a
transmitter via an optical medium, and to convert the first optical
component signal and the second optical component signal into a
first electrical component signal and a second electrical component
signal, respectively; the second electrical component signal is
subject to a first timing delay relative to the first electrical
component signal caused by at least one of the transmitter, the
optical medium, and the receiver; a first skew compensation element
comprising a first delay line comprising a first single fixed
length of waveguide configured to apply a first fixed predetermined
compensation timing delay to the second optical component signal,
to achieve a first net skew with the first timing delay in the
second electrical component signal.
2. The system according to claim 1, wherein the input optical
signal also comprises a third optical component signal and a fourth
optical component signal; wherein the receiver is also configured
to convert the third optical component signal and the fourth
optical component signals into a third electrical component signal
and a fourth electrical component signal; wherein the third
electrical component signal and the fourth electrical component
signal are subject to a second timing delay and a third timing
delay, respectively, relative to the first electrical component
signal caused by at least one of: the transmitter, the optical
medium, and the receiver; and further comprising: a second skew
compensation element comprising a second delay line comprising a
second fixed length of waveguide configured to apply a second fixed
predetermined compensation timing delay to the third optical
component signal, to achieve a second net skew with the second
timing delay in the third electrical component signal; and a third
skew compensation element comprising a third delay line comprising
a third fixed length of waveguide configured to apply a third fixed
predetermined compensation timing delay to the fourth optical
component signal, to achieve a third net skew with the third timing
delay in the fourth electrical component signal.
3. The system according to claim 2, wherein the receiver comprises:
an input port for inputting the input optical signal; a
polarization beam splitter (PBS) for splitting the input optical
signal into a first polarized component and a second polarized
component; a local oscillator and a beam splitter for generating a
first oscillator component and a second oscillator component; a
first hybrid mixer for generating the first optical component
signal and the second optical component signals, which are phase
differentiated, from the first polarized component and the first
oscillator component; and a second hybrid mixer for generating the
third optical component signal and the fourth optical component
signal, which are phase differentiated, from the second polarized
component and the second oscillator component.
4. The system according to claim 3, wherein the first delay line is
disposed between the PBS and the first hybrid mixer or the second
hybrid mixer; and wherein the second delay line is disposed between
the local oscillator and the first hybrid mixer or the second
hybrid mixer.
5. The system according to claim 3, further comprising respective
photodiodes and electrical amplifiers for converting each of the
first optical component signal, the second optical component
signal, the third optical component signal and the fourth optical
component signal into the first electrical component signal, the
second electrical component signal, the third electrical component
signal, and the fourth electrical component signal, respectively,
and thereby contributing to generation of the first timing delay,
the second timing delay and the third timing delay.
6. The system according to claim 5, wherein the first delay line is
disposed between the first hybrid mixer and a first of the
respective photodiodes; and wherein the second delay line is
disposed between the second hybrid mixer and a second of the
respective photodiodes.
7. The system according to claim 2, wherein the first delay line,
the second delay line and the third delay line each consists of a
single mode waveguide on a substrate.
8. The system according to claim 7, wherein each of the first delay
line, the second delay line and the third delay line consists of a
silicon waveguide.
9. The system according to claim 8, wherein at least one of the
first delay line, the second delay line, and the third delay line
is about 75 .mu.m long providing about 1 ps of delay.
10. The system according to claim 8, wherein at least one of the
first delay line, the second delay line and the third delay line is
about 225 .mu.m long providing about 3 ps of delay.
11. A method of compensating skew in an optical receiver,
comprising the steps of: receiving an input optical signal
comprising a first optical component signal and a second optical
component signal from a transmitter via an optical medium in the
optical receiver; converting the first optical component signal and
the second optical component signal into a first electrical
component signal and a second electrical component signal,
respectively, wherein the second electrical component signal is
subject to a first timing delay relative to the first electrical
component signal caused by at least one of the transmitter, the
optical medium, and the receiver; passing the second optical
component signal through a first skew compensation element
comprising a first delay line, comprising a first fixed length of
waveguide configured to apply a first fixed predetermined
compensation timing delay to the second optical component signal,
to achieve a first net skew with the first timing delay.
12. The method according to claim 11, further comprising:
generating a third optical component signal and a fourth optical
component signal for the input optical signal; receiving the third
optical component signal and the fourth optical component signal in
the optical receiver; converting the third optical component signal
and the fourth optical component signal into a third electrical
component signal and a fourth electrical component signal, wherein
the third electrical component signal and the fourth electrical
component signal are subject to a second timing delay and a third
timing delay, respectively, relative to the first electrical
component signal caused by at least one of the transmitter, the
optical medium and the receiver; passing the third optical
component signal through a second skew compensation element
comprising a second delay line, comprising a second fixed length of
waveguide configured to apply a second fixed predetermined
compensation timing delay to the third optical component signal, to
achieve a second net skew with the second timing delay in the third
electrical component signal; and passing the fourth optical
component signal through a third skew compensation element
comprising a third delay line, comprising a third fixed length of
waveguide configured to apply a third fixed predetermined
compensation timing delay to the fourth optical component signal,
to achieve a third net skew with the third timing delay in the
fourth electrical component signal.
13. The method according to claim 12, further comprising: splitting
the input optical signal into a first polarized component and a
second polarized component in a polarization beam splitter (PBS);
generating a first oscillator component and a second oscillator
components with a local oscillator and a beam splitter; generating
the first optical component signal and the second optical component
signal, which are phase differentiated, from the first polarized
component and the first oscillator component in a first hybrid
mixer; and generating the third optical component signal and the
fourth optical component signal, which are phase differentiated,
from the second polarized component and the second oscillator
component in a second hybrid mixer.
14. The method according to claim 13, wherein the first delay line
is disposed between the PBS and the first hybrid mixer or the
second hybrid mixer; and wherein the second delay line is disposed
between the local oscillator and the first hybrid mixer or the
second hybrid mixer.
15. The method according to claim 14, wherein the receiver further
comprises respective photodiodes and electrical amplifiers for
converting each of the first optical component signal, the second
optical component signal, the third optical component signal and
the fourth optical component signal into the first electrical
component signal, the second electrical component signal, the third
electrical component signal, and the fourth electrical component
signal, respectively, and thereby contributing to generation of the
first timing delay, the second timing delay and the third timing
delay.
16. The method according to claim 15, wherein the first delay line
is disposed between the first hybrid mixer and a first of the
respective photodiodes; and wherein the second delay line is
disposed between the second hybrid mixer and a second of the
respective photodiodes.
17. The method according to claim 12, wherein the first skew
compensation element, the second skew compensation element, and the
third skew compensation element are disposed in the
transmitter.
18. The method according to claim 12, further comprising: i)
determining the first timing delay in the second electrical
component signal, the second timing delay in the third electrical
component signal, and the third timing delay in the fourth
electrical component signal based on experience or measurement; ii)
fabricating the first delay line, the second delay line and the
third delay line based on step i).
19. A transmitter system, comprising: a transmitter configured for
generating an input optical signal comprising a first optical
component signal and a second optical component signal, and
transmitting the input optical signal via an optical medium to a
receiver, which is configured for converting the first optical
component signal and the second optical component signal into a
first electrical component signal and a second electrical component
signal, respectively; the second electrical component signal is
subject to a first timing delay relative to the first electrical
component signal caused by at least one of the transmitter, the
optical medium and the receiver; a first skew compensation element
comprising a first delay line, comprising a first fixed length of
waveguide configured to apply a first fixed predetermined
compensation timing delay to the second optical component signal,
to pre-compensate for the first timing delay in the second
electrical component signal.
20. The system according to claim 19, wherein the input optical
signal also comprises a third optical component signal and a fourth
optical component signal; wherein the receiver is also configured
to convert the third optical component signal and the fourth
optical component signals into a third electrical component signal
and a fourth electrical component signal; wherein the third
electrical component signal and the fourth electrical component
signal are subject to a second timing delay and a third timing
delay, respectively, relative to the first electrical component
signal caused by at least one of: the transmitter, the optical
medium, and the receiver; and further comprising: a second skew
compensation element comprising a second delay line comprising a
second fixed length of waveguide configured to apply a second fixed
predetermined compensation timing delay to the third optical
component signal, to pre-compensate for the second timing delay in
the third electrical component signal; and a third skew
compensation element comprising a third delay line comprising a
third single fixed length of waveguide configured to apply a third
fixed predetermined compensation timing delay to the fourth optical
component signal, to pre-compensate for the third timing delay in
the fourth electrical component signal.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority to and the benefit of
co-pending U.S. patent application Ser. No. 16/135950 filed Sep.
19, 2018, which is a continuation of U.S. patent application Ser.
No. 14/931,796 filed Nov. 3, 2015, which claims priory from and
benefit of U.S. provisional patent application No. 62/118,420 filed
Feb. 19, 2015, and co-pending U.S. provisional patent application
No. 62/132,742 filed Mar. 13, 2015, each of which applications is
incorporated herein by reference in its entirety.
FIELD OF THE INVENTION
[0002] The invention relates to systems and method for controlling
signal propagation in dual-polarization coherent communication
systems in general and particularly to skew compensation in such
systems.
BACKGROUND OF THE INVENTION
[0003] FIG. 1 is a schematic block diagram 100 of the electrical
and optical components of a prior art coherent optical transceiver.
Skew can be introduced in the traces between elements as well as
within each element. It would be advantageous for each element to
compensate for skew.
[0004] In dual-polarization coherent communication, there are at
least four signal paths from the digital signal processor (DSP) to
the output optical signal. These are the in-phase and quadrature
modulator inputs for X- and Y-input optical polarizations. X- and
Y-polarizations are orthogonal polarizations in the input optical
fiber. In such a scenario, it is important that the relative timing
skew between each of these signal paths from the DSP to the output
optical signal is kept as low as possible. There is skew between X-
and Y-polarizations, as well as between the In-phase and Quadrature
components of a signal within a certain polarization. These are
called XY and IQ timing skews, respectively. Similarly, there are
four such paths from the incoming optical signal to the DSP. There
can be both XY and IQ timing skew in the transmitter and in the
receiver.
[0005] There is a need for improved systems and methods for
correcting skew.
SUMMARY OF THE INVENTION
[0006] According to one aspect, the invention features a skew
compensation apparatus, comprising: a signal converter selected
from the group of signal converters consisting of a signal
converter that is configured to convert at least two optical
signals into at least two electrical signals and a signal converter
that is configured to convert at least two electrical signals into
at least two optical signals; a first one of the at least two
electrical signals subject to a delay of magnitude T.sub.skew
relative to a second one of the at least two electrical signals;
the signal converter having at least two optical ports and at least
two electrical ports, the at least two optical ports selected from
the group consisting of at least two input ports and at least two
output ports, and the at least two electrical ports selected from
the other type of port in the group consisting of at least two
input ports and at least two output ports; and at least one optical
delay line in optical communication with at least one of the at
least two optical ports, the at least one optical delay line
configured to apply a correction comprising a compensation delay to
a selected one of the first one and the second one of the at least
two electrical signals so that after the correction, the time delay
between the first one and the second one of the at least two
electrical signals is different from T.sub.skew.
[0007] In one embodiment, the delay after correction is less than
T.sub.skew.
[0008] In another embodiment, the delay after correction is greater
than T.sub.skew.
[0009] In yet another embodiment, the at least two optical signals
are phase differentiated and comprise an I component and a Q
component.
[0010] In still another embodiment, the at least two optical
signals are converted from orthogonal polarizations in an optical
carrier.
[0011] In a further embodiment, the at least one optical delay line
is a single mode waveguide.
[0012] In yet a further embodiment, the at least one optical delay
line is a multi-mode waveguide.
[0013] In an additional embodiment, the at least one optical delay
line has an adjustable optical path length.
[0014] In one more embodiment, the adjustable optical path length
is configured to be thermally adjustable.
[0015] In still a further embodiment, the adjustable optical path
length is configured to be adjustable by charge carrier
concentration.
[0016] In another embodiment, the at least one optical delay line
comprises silicon.
[0017] In yet another embodiment, the at least one optical delay
line is a switched delay line.
[0018] In still another embodiment, the at least one optical delay
line is a 1.times.N electro-optic switch combined with N waveguides
having different lengths.
[0019] In a further embodiment, the skew compensation apparatus
further comprises a thermal measurement device and a heater
adjacent the optical delay line.
[0020] In yet a further embodiment, the skew compensation apparatus
is configured to operate using an optical signal having a
wavelength within the range of a selected one of an O-Band, an
E-band, a C-band, an L-Band, an S-Band and a U-band.
[0021] According to another aspect, the invention relates to a
method of compensating skew, comprising the steps of: providing an
apparatus, comprising: a signal converter selected from the group
of signal converters consisting of a signal converter configured to
convert at least two optical signals into at least two electrical
signals and a signal converter that configured to convert at least
two electrical signals into at least two optical signals; a first
one of the at least two electrical signals subject to a delay of
magnitude T.sub.skew relative to a second one of the at least two
electrical signals; the signal converter having at least two
optical ports and at least two electrical ports, the at least two
optical ports selected from the group consisting of at least two
input ports and at least two output ports, and the at least two
electrical ports selected from the other type of port in the group
consisting of at least two input ports and at least two output
ports; and at least one optical delay line in optical communication
with at least one of the at least two optical port, the at least
one optical delay line configured to apply a correction comprising
a compensation delay to a selected one of the first one and the
second one of the at least two electrical signals so that after the
correction, the time delay between the first one and the second one
of the at least two electrical signals is different from
T.sub.skew; and applying at least two input signals to the at least
two input ports of the signal converter; and applying the
compensation delay to a selected one of the first one and the
second one of the at least two electrical signals so that the time
delay between the first one and the second one of the at least two
electrical signals is different from T.sub.skew.
[0022] In one embodiment, the delay after correction is less than
T.sub.skew.
[0023] In another embodiment, the delay after correction is greater
than T.sub.skew.
[0024] In yet another embodiment, the method of compensating skew
further comprises the step of determining the magnitude T.sub.skew
of the delay.
[0025] In still another embodiment, the at least two optical
signals are phase-differentiated and comprise an I component and a
Q component.
[0026] In a further embodiment, the at least two optical signals
are converted from orthogonal polarizations in the optical
carrier.
[0027] In yet a further embodiment, the at least one optical delay
line is a single mode waveguide.
[0028] In an additional embodiment, the at least one optical delay
line is a multi-mode waveguide.
[0029] In one more embodiment, the at least one optical delay line
has an adjustable optical path length.
[0030] In still a further embodiment, the adjustable optical path
length is configured to be thermally adjustable.
[0031] In one embodiment, the adjustable optical path length is
configured to be adjustable by charge carrier concentration.
[0032] In another embodiment, the at least one optical delay line
comprises silicon.
[0033] In yet another embodiment, the at least one optical delay
line is a switched delay line.
[0034] In still another embodiment, the at least one optical delay
line is a 1.times.N electro-optic switch combined with N waveguides
having different lengths.
[0035] In a further embodiment, the method of compensating skew in
an optical system further comprises a thermal measurement device
and a heater adjacent the optical delay line.
[0036] In yet a further embodiment, the input optical signal has a
wavelength within the range of a selected one of an O-Band, an
E-band, a C-band, an L-Band, an S-Band and a U-band.
[0037] The foregoing and other objects, aspects, features, and
advantages of the invention will become more apparent from the
following description and from the claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0038] The objects and features of the invention can be better
understood with reference to the drawings described below, and the
claims. The drawings are not necessarily to scale, emphasis instead
generally being placed upon illustrating the principles of the
invention. In the drawings, like numerals are used to indicate like
parts throughout the various views.
[0039] FIG. 1 is a schematic block diagram of the electrical and
optical components of a prior art coherent optical transceiver.
[0040] FIG. 2A is a graph of XI and XQ signal waveforms versus time
as initially provided with zero skew.
[0041] FIG. 2B is a graph of XI and XQ signal waveforms versus time
with skew introduced, showing how one defines or measures
T.sub.skew.
[0042] FIG. 3A is a diagram that illustrates skew compensation
within a packaged optical receiver using electrical wiring traces
that have different lengths.
[0043] FIG. 3B is a diagram showing an embodiment of the electronic
components in the packaged optical receiver of FIG. 3A.
[0044] FIG. 4 is a diagram that shows an embodiment of the reduced
package size enabled by moving skew compensation from the
electrical domain to the optical domain, according to principles of
the invention.
[0045] FIG. 5 is a diagram that shows an embodiment of the reduced
package size enabled by moving skew compensation from the
electrical domain to the optical domain. The optical domain
introduces timing skew that pre-corrects for the skew introduced
later.
[0046] FIG. 6 is a schematic diagram of an embodiment in which a
signal that requires a skew compensation which may vary with time
or with frequency can be skew pre-compensated using one of N
different skew compensation elements.
[0047] FIG. 7 is a schematic diagram that illustrates the general
principles of the invention.
[0048] FIG. 8 is a schematic diagram of an embodiment of a system
having N channels in parallel in which skew compensation in
provided in a transmitter.
[0049] FIG. 9 is a schematic diagram of an embodiment having N
channels in parallel in which skew compensation in provided in a
receiver.
[0050] FIG. 10 is a schematic diagram of an embodiment in which N
channels each having 4 electrical inputs and 4 electrical outputs
are corrected for skew in a skew compensating module comprising a
polarizing beamsplitter, one or more hybrid mixers, the skew
compensation elements and one or more optoelectronic
converters.
DETAILED DESCRIPTION
Acronyms
[0051] A list of acronyms and their usual meanings in the present
document (unless otherwise explicitly stated to denote a different
thing) are presented below.
[0052] AMR Adabatic Micro-Ring
[0053] APD Avalanche Photodetector
[0054] ARM Anti-Reflection Microstructure
[0055] ASE Amplified Spontaneous Emission
[0056] BER Bit Error Rate
[0057] BOX Buried Oxide
[0058] CMOS Complementary Metal-Oxide-Semiconductor
[0059] CMP Chemical-Mechanical Planarization
[0060] DBR Distributed Bragg Reflector
[0061] DC (optics) Directional Coupler
[0062] DC (electronics) Direct Current
[0063] DCA Digital Communication Analyzer
[0064] DRC Design Rule Checking
[0065] DUT Device Under Test
[0066] ECL External Cavity Laser
[0067] FDTD Finite Difference Time Domain
[0068] FOM Figure of Merit
[0069] FSR Free Spectral Range
[0070] FWHM Full Width at Half Maximum
[0071] GaAs Gallium Arsenide
[0072] InP Indium Phosphide
[0073] LiNO.sub.3 Lithium Niobate
[0074] LIV Light intensity(L)-Current(I)-Voltage(V)
[0075] MFD Mode Field Diameter
[0076] MPW Multi Project Wafer
[0077] NRZ Non-Return to Zero
[0078] PIC Photonic Integrated Circuits
[0079] PRBS Pseudo Random Bit Sequence
[0080] PDFA Praseodymium-Doped-Fiber-Amplifier
[0081] PSO Particle Swarm Optimization
[0082] Q Quality factor
Q = 2 .pi. .times. Energy Stored Energy dissipated per cycle = 2
.pi. f r .times. Energy Stored Power Loss . ##EQU00001##
[0083] QD Quantum Dot
[0084] RSOA Reflective Semiconductor Optical Amplifier
[0085] SOI Silicon on Insulator
[0086] SEM Scanning Electron Microscope
[0087] SMSR Single-Mode Suppression Ratio
[0088] TEC Thermal Electric Cooler
[0089] WDM Wavelength Division Multiplexing
[0090] FIG. 2A is a graph 200 of XI and XQ signal waveforms versus
time as initially provided with zero skew. In FIG. 2A there are
shown a waveform 202 which can be the XI component and a waveform
204 which can be the XQ component of a dual-polarization coherent
communication system.
[0091] FIG. 2B is a graph 201 of XI and XQ signal waveforms versus
time with skew introduced, showing how one defines or measures
T.sub.skew. In FIG. 2B, the waveform 202 is illustrated as being in
the same relative time relation that it had in FIG. 2A. One can
understand this as using waveform 202 as a time baseline. However,
waveform 204' is displaced in time relative to waveform 202, as
indicated by the curved arrow 210, which displacement is called the
skew. The time T.sub.skew 220 which is a measure of the amount of
displacement is the difference between the relative time offset
T.sub.0 205 in FIG. 2A and the relative time offset T.sub.1 206 in
FIG. 2B. T.sub.skew 220 may be represented by the relation
T.sub.skew=Absolute value (T.sub.1-T.sub.0).
[0092] The skew in FIG. 2A is illustrated for the XQ component
relative to the XI component. However, in real systems it is
possible to have skew defined as the offset of the XI component
relative to the XQ component (e.g., the XQ component is used as the
time base for measurement). In similar fashion, the YI and YQ
components of the dual-polarization coherent communication system
can also exhibit skew, using either the YI component or the YQ
component as the time base for measurement.
[0093] The skew is compensated by applying a delay of magnitude
T.sub.skew to the signal that is not skewed, so that both signal in
a pair of signals XI. XQ and YI, YQ have equal delays, and are
therefore in the original time relation that existed prior to the
optical to electrical conversion. The compensation is applied in
the optical domain as a compensation according to the principles of
the invention, rather than in the electrical domain as a
post-compensation relative to the optical to electrical
conversion.
[0094] FIG. 3A is a diagram 300 that illustrates skew compensation
within a packaged optical receiver using electrical wiring traces
310 that have different lengths. In the embodiment illustrated,
significant area is needed to time-align the signals from a set of
transimpedance amplifiers (TIAs) to the electrical pins on the
package. The optical receiver components are indicated by the block
320, which is shown in greater detail in FIG. 3B. The optical
receiver has a signal input 302 that can receive an optical input
signal. In some embodiments the optical input signal is part of a
dual-polarization coherent communication system. The optical
receiver has a local oscillator 304, for example a laser or a laser
diode, which provides a frequency signal.
[0095] Each component in the signal paths adds some skew to the
signal. This amount of skew should be minimized. The size of
electro-optical modules implementing dual-polarization IQ
modulators and receivers is affected by the amount of space needed
to compensate for electrical skews. Electrical delays are needed in
order to fan-out the electrical trace from some small component,
such as an amplifier, to the pins on a package surrounding the
device. As can be clearly seen, significant area is required for
electrical skew compensation. It is well known that a foot
(approximately 30 centimeters) of electrical wiring adds a delay of
approximately one nanosecond to an electrical signal. Therefore,
depending on the amount of skew (e.g., the value of T.sub.skew)
that has to be compensated by delaying the components that have not
suffered skew, wiring of significant length may be required.
[0096] FIG. 3B is a diagram showing an embodiment of the electronic
components in the packaged optical receiver 320 of FIG. 3A. As
shown in the embodiment of FIG. 3B, the input signal enters at port
302. A small portion of the input optical signal (typically less
than 5%) is split off and sent to photodiode 314, which generates
an electrical signal that can be used to monitor properties of the
input optical signal, such as its power content. In other
embodiments, the power can be monitored using different hardware.
The remainder of the input optical signal is sent through a
variable optical attenuator 312, which can adjust the signal
intensity, and is split by a polarized beam splitter (PBS) 315 into
x-polarized (X-Pol) and y-polarized (Y-Pol) components. The X-Pol
component is sent to a 90 degree hybrid mixer 306, and the Y-Pol
component is sent to a 90 degree hybrid mixer 308. The local
oscillator 304 provides a signal that is split by beam splitter 313
and components of the local oscillator signal are sent to each of
90 degree hybrid mixers 306 and 308. The 90 degree hybrid mixers
306 and 308 are optical components that each generate two phase
differentiated optical signals, the XI and XQ signals and the YI
and YQ signals, respectively. Finally, each of the four phase
differentiated signals are converted to electrical signals by
respective photodiodes and electrical amplifiers (collectively
indicated by the numerals 316, 316', 316'' and 316'''). In some
embodiments, the electrical amplifiers are transimpedance
amplifiers, but in principle other kinds of amplifiers can also be
used. The electrical signals are then provided at four respective
output terminals (which may be single-sided signals referenced to a
common ground or may be differential signals). Because skew is
generated during the process of converting the input optical signal
into the output electrical signals XI, XQ, YI and YQ; as well as in
the length of electrical wiring due to the fan-out from the circuit
to the package pins, post-compensation for the skew is then applied
as shown in FIG. 3A. In principle, the systems and methods of the
invention can also be applied to signals that are differentiated in
amplitude or frequency
[0097] In the systems and methods of the present invention, optical
delay lines are used in order to pre-compensate for any
electrical-domain skews in the optical signal paths. Optical delay
lines can be integrated onto the same photonic integrated circuit
that performs polarization splitting and the 90.degree. mixing
without enlarging the size of the chip. It is believed that an
advantage of eliminating the need for electrical skew compensation
is a reduction in the size of the larger package. In addition,
optical compensation delay lines can be used to compensate for
skews outside the package in any of the components and in wiring
between a signal source or receiver, in either the transmit or
receive path.
[0098] FIG. 4 is a diagram 400 that shows an embodiment of the
reduced package size in a packaged optical receiver enabled by
moving skew compensation from the electrical domain to the optical
domain. The optical domain introduces timing skew that pre-corrects
for the skew introduced later. The pre-correction is accomplished
by applying a skew of the opposite sense to a signal that will
suffer skew during the optical to electrical conversion process, so
that the net skew for that signal is zero, or is as close to zero
as is practical. As shown in FIG. 4, the packaged optical receiver
420 has all of the components present and described in in the
packaged optical receiver 320 of FIG. 3B. In addition, the packaged
optical receiver 420 has skew compensation elements 410, 410',
410'' and 410''' (which can be optical delay lines) situated after
the outputs of the X-Pol and Y-Pol 90 degree hybrid mixers 306 and
308.
[0099] In the systems and methods of the present invention, optical
delay lines are used in order to pre-compensate for any
electrical-domain skews in the optical signal paths. However, it
may also be advantageous to increase the skew or reduce the skew to
some other non-zero skew for the purposes of constructing a
feed-forward, feed-backward, or equalizing filter.
[0100] It is believed that in various embodiments, the optical
delay lines can be implemented using silicon optical waveguides on
the same substrate as other optical and electro-optical components
in the receiver path. Silicon waveguides can be very tightly
confining, and delay lines up to many picoseconds can be
accommodated without any impact on the total area requirement of
the photonic integrated circuit.
[0101] While FIG. 4 shows an embodiment in which a skew
compensation element is provided on each input of the 90 degree
hybrid mixers 306 and 308, it should be understood that the skew
compensation described herein can be implemented in a simpler, less
capable system having only two electrical output components, by
providing only one 90 degree hybrid mixer and only one skew
compensation element in optical communication with one of the two
optical inputs of the one 90 degree hybrid mixer. Such a system
would not be effective in a full dual polarization coherent
communication system in all instances, but it would be effective
for a less capable coherent communication system having only two
electrical output components.
[0102] While FIG. 4 shows an embodiment of a packaged optical
receiver that comprises pre-skew compensation, it should be
understood that one can equally provide the "mirror image" optical
compensation for skew in a packaged optical transmitter. This may
be readily envisioned by reversing the sense of the electrical
signals to input signals in FIG. 4, reversing the sense of the
electrical amplifiers 316, 316', 316'' and 316''' by replacing them
with optical modulators, replacing the 90 degree hybrid mixers with
polarization rotators and splitters (PSRs) as described in
co-pending U.S. provisional patent application No. 62/118,420 and
in co-pending U.S. provisional patent application No. 62/132,742
(which polarization rotators and splitters are operated in the
combining sense), and applying the skew correction described to the
outputs of the PSRs before combining all the signals and providing
them as output at an output port (e.g., the converse of the input
port 302). Thereby providing an optical transmitter with optical
skew compensation.
[0103] FIG. 5 is a diagram that shows an embodiment of the reduced
package size enabled by moving skew compensation from the
electrical domain to the optical domain. In some embodiments, the
optical domain introduces timing skew that pre-corrects for the
skew introduced later. In FIG. 5, in the XI portion of the
receiver, a skew compensation element (510, 511) is provided
between the 90 degree hybrid mixer 306 and the electrical amplifier
316. Similar skew compensation elements are also shown in each of
the other XQ, YI and YQ portions of the receiver. The components of
the embodiment of FIG. 5 that are not explicitly identified with
numerals are the equivalents of the corresponding components shown
in FIG. 3 and FIG. 4.
[0104] FIG. 6 is a schematic diagram 600 of an embodiment in which
a signal that requires a skew compensation which may vary with time
or with frequency can be skew pre-compensated using one of N
different skew compensation elements. In FIG. 6, the optical signal
enters on input port 610 and is switched by 1.times.N optical
switch 620 to a respective one of N different skew compensation
elements (631, 632, . . . , 63N) and then is switched by N.times.1
optical switch 640 to an output port 650. As long as 1.times.N
optical switch 620 and N.times.1 optical switch 640 are operated to
connect the same skew compensation element between the input port
610 and the output port 650 at any given time, the embodiment of
FIG. 6 can be used to provide a selected one of N different skew
compensation values to an optical signal.
[0105] FIG. 7 is a schematic diagram 700 that illustrates the
general principles of the invention. In the generic embodiment
illustrated in FIG. 7, a signal input port (input signal waveguide)
710 and a reference signal input port 712 (LO or local oscillator
waveguide) are provided. The two signals are processed in an
optical hybrid element 720 to generate optical signals having
different components. Optical skew compensation elements 730, 732,
734, 736 are provided to apply a pre-skew to each optical
component. Opto-electronic conversion elements 740, 742 convert the
optical signals into electrical signals that are transmitted and
that will experience skews 750, 752, 754 and 756 during the
electrical transmission. In FIG. 7, the electrical skews 750 and
752 are shown using the same schematic elements as pre-skews 732
and 730, respectively, which is intended to indicate that the
pre-skew 730 when added to the skew 750 is the same total skew as
the sum of pre-skew 732 and skew 752. Similarly, FIG. 7 is intended
to indicate that the sum of pre-skew 734 and skew 754 equals the
sum of pre-skew 736 and skew 756. By such skew compensation, the
signals on the respective pairs of transmission lines arrive at
their destination with zero relative skew to each other.
[0106] FIG. 8 is a schematic diagram 800 of an embodiment of a
system having N channels in parallel in which skew compensation in
provided in a transmitter.
[0107] In FIG. 8, the first channel has an electrical input 812, an
electrical transmission medium 810, an optical transmitter 820
comprising an electro-optic converter 821 and a skew compensation
element 815, an optical transmission medium 830, an optical
receiver 840 and an electrical transmission medium 850 that
provides an electrical signal at an electrical output port 819.
[0108] Channels 2, . . . , N have substantially identical elements
810, 820, 830, 840 and 850 as are present in Channel 1. However,
each respective channel 2, . . . , N has a respective electrical
input 822, . . . , 8N2, a respective skew compensation element 825,
. . . , 8N5, and a respective electrical output port 829, . . . ,
8N9.
[0109] Skew between channels 1, 2, . . . , N may be introduced in
propagation through the transmission medium, the optical receiver,
and the electrical transmission medium. The skew may be
pre-compensated in the optical transmitter for the skews introduced
in the aforementioned sources. The skews introduced may be a
function of frequency. In some embodiments, the net skew introduced
by the aforementioned sources is pre-compensated in the optical
transmitter.
[0110] FIG. 9 a schematic diagram 900 of an embodiment having N
channels in parallel in which skew compensation in provided in a
receiver.
[0111] In FIG. 9, the first channel has an electrical input 912, an
electrical transmission medium 910, an optical transmitter 920, an
optical transmission medium 930, an optical receiver 940 comprising
a skew compensation element 915 and an electro-optic converter 942
and an electrical transmission medium 950 that provides an
electrical signal at an electrical output port 919.
[0112] Channels 2, . . . , N have substantially identical elements
910, 920, 930, 940 and 950 as are present in Channel 1. However,
each respective channel 2, . . . , N has a respective electrical
input 922, . . . , 9N2, a respective skew compensation element 925,
. . . , 9N5, and a respective electrical output port 929, . . . ,
9N9.
[0113] Skew between channels 1, 2, . . . , N may be introduced in
propagation through the transmission medium, the optical
transmitter, and the electrical transmission medium. The skew may
be pre-compensated in the optical receiver for the skews introduced
in the aforementioned sources. The skews introduced may be a
function of frequency. In some embodiments, the net skew introduced
by the aforementioned sources is pre-compensated in the optical
receiver.
[0114] FIG. 10 is a schematic diagram 1000 of an embodiment in
which N channels each having 4 electrical inputs and 4 electrical
outputs are corrected for skew in a skew compensating module
comprising a polarizing beamsplitter, one or more hybrid mixers,
the skew compensation elements and one or more optoelectronic
converters.
[0115] In the embodiment of FIG. 10, each of N channels includes a
4.times.N electrical input 1005, a first electrical transmission
medium 1010, an optical transmitter 1020, an optical transmission
medium 1030, a skew compensating module 1040 comprising a
polarizing beamsplitter (PBS) 1042, one or more hybrid mixers 1044,
the skew compensation elements 1046 and one or more optoelectronic
converters 1048, and a first electrical transmission medium 1050
that sends signals out through a 4.times.N electrical output
1055.
[0116] Skew between channels 1, 2, . . . , N may be introduced in
propagation through the first electrical transmission medium 1010,
the optical transmitter 1020, and the optical transmission medium
1030. The skew may be pre-compensated in the skew compensating
module 1040 for the skews introduced in the aforementioned sources.
The skews introduced may be a function of frequency. In some
embodiments, the net skew introduced by the aforementioned sources
is pre-compensated in the skew compensating module 1040. For
phase-differentiated signals, the skews need to be compensated
after the hybrid mixer 1044.
Skew Compensation Embodiments
Silicon Single-Mode Waveguides for Short Skew Compensation
[0117] In one embodiment, a 500 nm width and 220 nm height silicon
waveguide clad in oxide approximately 75 .mu.m of length
corresponds to 1 picosecond of delay in the optical signal passing
through the waveguide. This type of waveguide has on the order of 1
to 2 dB of optical loss per centimeter. Thus, relatively short
skews of a few picoseconds can be compensated with a single-mode
waveguide without significant excess loss.
Wide Multi-Mode Waveguides for Large Skew Compensation
[0118] In other embodiments, 1.2 .mu.m width by 220 nm height
silicon waveguides clad in oxide are multi-modal for illumination
at 1550 nm wavelength, but can be adiabatically coupled into from
single mode waveguides. The lowest propagation mode of wide
waveguides typically has a very low insertion loss, typically on
the order of 0.1 to 0.5 dB per centimeter. Thus, these types of
waveguides are ideal for compensating large amounts of skew.
Periodic Mode Throttlers for Spectral Smoothness
[0119] A common problem in long waveguides is ripples that appear
in the transmission spectrum. These ripples are caused in part by
back-reflected light in higher order modes. A mode throttle is a
waveguide-integrated device that passes the lowest order mode and
attenuates higher order modes. If a long waveguide section has
periodic mode throttlers integrated therein, the transmission
spectrum may be smoothed. Thus, in some embodiments, the need for
skew compensation is alleviated with the use of periodic mode
throttlers in applications or systems that use both single- and
multi-mode waveguides. The design and implementation of mode
throttlers is described in greater detail in co-pending U.S.,
patent application Ser. No. 14/788,608, now U.S. Patent Application
Publication No. ______.
Silicon Nitride Waveguides in the Front-End and Back-End Stack
[0120] Silicon nitride is another material that can be integrated
on a SOI platform. Single-mode waveguides can be built in SiN and
coupled to and from single-mode waveguides in silicon. It is
believed that in various embodiments, these waveguides can also be
used for skew compensation.
[0121] Additionally, it is possible to use the silicon nitride
layers higher in the metal stack for optical routing. This is
described in greater detail in co-pending U.S. patent application
Ser. No. 14/798,780, published as U.S. Patent Application
Publication No. ______. Similarly, it is believed that these
waveguides may be used for skew compensation in various
embodiments.
Tunable Skew Compensation
[0122] It is often desirable to have variable skew compensation.
The optical path length of a silicon waveguide can be adjusted by
integrating heating resistors next to or in the waveguide. It is
believed that long runouts of multi-mode waveguides with heaters
can be used to create a very large tuning range. In some
embodiments, a thermal measurement device is provided, whether a pn
junction, a photodetector, an electro-absorption modulator, or some
other electro-optical device. The thermal measurement device may be
any convenient device. In some embodiments the thermal measurement
device is a Proportional to Absolute Temperature (PTAT) device.
Examples of prior art heaters and PTAT circuits are described in
co-pending U.S. patent application Ser. No. 14/864,760, published
as U.S. Patent Application Publication No. ______, and in U.S. Pat.
No. 8,274,021, and are believed to be suitable for use in the
present invention.
[0123] In some embodiments, it is believed that it is possible to
use the systems and methods described herein to increase the skew
between two signals, for example for purposes of signal
processing.
Feedforward and Feedback Control
[0124] In some embodiments, a feedback loop and/or a feed forward
loop is provided to control skew observed between two signals. For
example in a feedback control system, one can measure the net skew
and control the corrective delay to achieve a desired amount of
skew. In a feedforward system, if one has experience with specific
circuits or devices and has a reasonable expectation of the
uncorrected skew that may be expected, one can apply a compensation
by way of a corrective delay to achieve an expected net skew, in
the absence of making a measurement of the skew, either before or
after the corrective delay is applied. Both feedback and
feed-forward loops used to control or regulate signals are well
known in the art.
Switched Delay Lines
[0125] An even larger distribution of skews can be accommodated
through the use of switched delay lines. A 1.times.N electro-optic
switch can be used to switch between N different sets of waveguide
lengths. Furthermore, each individual waveguide runout within the
switch may have a tunable length as described hereinabove to
provide a continuously tunable large delay adjustment.
Operating Ranges
[0126] It is believed that apparatus constructed using principles
of the invention and methods that operate according to principles
of the invention can be used in the wavelength ranges described in
Table I.
TABLE-US-00001 TABLE I Band Description Wavelength Range O band
original 1260 to 1360 nm E band extended 1360 to 1460 nm S band
short wavelengths 1460 to 1530 nm C band conventional ("erbium
window") 1530 to 1565 nm L band long wavelengths 1565 to 1625 nm U
band ultralong wavelengths 1625 to 1675 nm
[0127] It is believed that in various embodiments, apparatus as
previously described herein can be fabricated that are able to
operate at a wavelength within the range of a selected one of an
O-Band, an E-band, a C-band, an L-Band, an S-Band and a U-band.
[0128] It is believed that apparatus constructed using principles
of the invention and methods that operate according to principles
of the invention can be fabricated using materials systems other
than silicon or silicon on insulator. Examples of materials systems
that can be used include materials such as compound semiconductors
fabricated from elements in Groups III and V of the Periodic Table
(e.g., compound semiconductors such as GaAs, AlAs, GaN, GaP, InP,
and alloys and doped compositions thereof).
Design and Fabrication
[0129] Methods of designing and fabricating devices having elements
similar to those described herein, including high index contrast
silicon waveguides, are described in one or more of U.S. Pat. Nos.
7,200,308, 7,339,724, 7,424,192, 7,480,434, 7,643,714, 7,760,970,
7,894,696, 8,031,985, 8,067,724, 8,098,965, 8,203,115, 8,237,102,
8,258,476, 8,270,778, 8,280,211, 8,311,374, 8,340,486, 8,380,016,
8,390,922, 8,798,406, and 8,818,141.
Definitions
[0130] As used herein, the term "optical communication channel" is
intended to denote a single optical channel, such as light that can
carry information using a specific carrier wavelength in a
wavelength division multiplexed (WDM) system.
[0131] As used herein, the term "optical carrier" is intended to
denote a medium or a structure through which any number of optical
signals including WDM signals can propagate, which by way of
example can include gases such as air, a void such as a vacuum or
extraterrestrial space, and structures such as optical fibers and
optical waveguides.
Theoretical Discussion
[0132] Although the theoretical description given herein is thought
to be correct, the operation of the devices described and claimed
herein does not depend upon the accuracy or validity of the
theoretical description. That is, later theoretical developments
that may explain the observed results on a basis different from the
theory presented herein will not detract from the inventions
described herein.
INCORPORATION BY REFERENCE
[0133] Any material, or portion thereof, that is said to be
incorporated by reference herein, but which conflicts with existing
definitions, statements, or other disclosure material explicitly
set forth herein is only incorporated to the extent that no
conflict arises between that incorporated material and the present
disclosure material. In the event of a conflict, the conflict is to
be resolved in favor of the present disclosure as the preferred
disclosure.
[0134] While the present invention has been particularly shown and
described with reference to the preferred mode as illustrated in
the drawing, it will be understood by one skilled in the art that
various changes in detail may be affected therein without departing
from the spirit and scope of the invention as defined by the
claims.
* * * * *