# Patent

## Title
DQPSK/DPSK optical receiver with tunable optical filters

## Abstract

An optical receiver includes a first interferometer having a plurality of arms. The optical receiver further includes first tunable optical filters connected in series with the arms of the first interferometer, where each first tunable optical filter is tuned to filter a region of overlap in the optical frequency spectrum between adjacent optical channels.

## Background
RELATED APPLICATION
This application is a continuation of U.S. patent application Ser. No. 11/953,220 filed on Dec. 10, 2007, the content of which is relied upon and incorporated herein by reference in its entirety.


BACKGROUND
Differential Phase Shift Keying (DPSK) and differential quadrature phase shift keying (DQPSK) are modulation techniques used in optical data transmission. Dense Wavelength Divisional Multiplexing (DWDM) transmission has been demonstrated over substantial optical fiber distances at high rates (e.g., 100 Gb/s) using either DPSK or DQPSK.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a diagram of a network according to an exemplary implementation;
FIG. 2A is a diagram of an exemplary configuration of the DQPSK transmitter of FIG. 1;
FIG. 2B is a diagram of an exemplary configuration of a channel DQPSK unit of the DQPSK transmitter of FIG. 2A;
FIG. 3A is a diagram of an exemplary configuration of the DQPSK receiver of FIG. 1;
FIG. 3B is a diagram of an exemplary configuration of a channel DQPSK unit of the DQPSK receiver of FIG. 3A;
FIG. 4 illustrates an exemplary narrow optical pulse emitted by the pulse modulator of FIG. 2 in the time domain and the frequency domain;
FIG. 5 illustrates the frequency domain spectrum of FIG. 4 broadened with asymmetric blue and red shift due to fiber dispersion and non-linearity;
FIG. 6 illustrates three exemplary optical channels, with wide channel spacing, such that there is very little overlap between the channel spectrums;
FIG. 7A illustrates three exemplary optical channels, with narrow channel spacing, having substantial overlap between the channel spectrums;
FIG. 7B illustrates three exemplary optical channels having narrow channel spacing and center frequencies that have drifted from nominal;
FIG. 7C illustrates the use of tunable optical filters in the DQPSK receiver of FIG. 3A to filter regions of overlap between the channel spectrums of the narrow channel spacing of FIGS. 7B and 7C; and
FIGS. 8A and 8B are flowcharts of an exemplary process for tuning the tunable optical filters in the DQPSK receiver of FIG. 3A.



DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The following detailed description refers to the accompanying drawings. The same reference numbers in different drawings may identify the same or similar elements. The following detailed description does not limit the invention.
Exemplary embodiments described herein use tunable optical filters in an optical receiver's interferometer(s) to reduce channel overlap (e.g., cross talk) between adjacent optical channels in a DQPSK or DPSK modulated system. When narrow channel spacing is employed, spectrum broadening due to fiber dispersion and non-linearity can create regions of substantial channel overlap that reduce system performance. Use of tunable optical filters in the DQPSK/DPSK receiver, as described herein, reduces this channel overlap and improves optical performance in a number of ways that are further described below. Exemplary embodiments described herein have particular applicability to high speed (e.g., 100 Gb/s) DQPSK/DPSK systems that employ narrow channel spacing.
FIG. 1 is a diagram of a network 100 according to an exemplary implementation. Network 100 may include a DQPSK transmitter 110, a DQPSK receiver 120 and an optical network 130. As shown in FIG. 1, DQPSK transmitter 110 may transmit DWDM DQPSK optical signals 140 to DQPSK receiver 120 via optical network 130. DQPSK transmitter 110 may include an optical transmitter that uses DQPSK techniques for modulating optical signals to encode the signals with data and for transmitting the modulated optical signals across optical network 130 to DQPSK receiver 120. DQPSK receiver 120 may include an optical receiver that uses DQPSK techniques for receiving and demodulating optical signals received from DQPSK transmitter 110 via optical network 130 and for extracting the original data from the modulated optical signals. Optical network 130 may include, for example, an optical fiber network which carries optical signals from one or more transmitters to one or more receivers. According to other exemplary embodiments (not shown), transmitter 110 may include a DPSK transmitter, instead of a DQPSK transmitter, and receiver 120 may include a DPSK receiver, instead of a DQPSK receiver.
FIG. 2A illustrates an exemplary configuration of DQPSK transmitter 110. As shown in FIG. 2A, DQPSK transmitter 110 may include one or more channel DQPSK units 200-1 through 200-N (individually and generically referred to herein as a “channel DQPSK unit 200-x”) and a multiplexer 205. Each channel DQPSK unit 200-x may include components for transmitting data via optical pulses over a specific optical channel. Each DQPSK unit 200-x, thus, may transmit data over a different optical channel (e.g., at a different optical wavelength). Multiplexer 205 may include a wavelength division multiplexing device that multiplexes different channels of optical signals into an optical fiber. Multiplexer 205 may include, for example, an optical add-drop multiplexer (OADM). Multiplexer 205 may multiplex optical pulses of the different channels transmitted by channel DQPSK units 200-1 through 200-N to produce a DQPSK signal out.
FIG. 2B is a diagram of an exemplary configuration of a channel DQPSK unit 200-x. As depicted in the exemplary configuration of FIG. 2B, DQPSK unit 200-x may include a continuous wave (CW) laser 210, a pulse modulator 215, a clock driver 220, a data divider and encoder 225, data drivers 230 and 235, and an interferometer 240.
CW laser 210 may include a laser source that produces optical CW carrier signals at a given wavelength (e.g., a single channel). Pulse modulator 215 may pulse modulate the carrier signal from CW laser 210, based on clock signals received from clock driver 220, to generate optical pulses. Data divider and encoder 225 may include circuitry for dividing the data to be transmitted (e.g., data at 100 Gb/s) into two streams of data (e.g., 50 Gb/s for each data stream) and may encode each stream of data for providing to data drivers 230 and 235. Data driver 230 may output signals, based on encoded data received from data divider and encoder 225, to one arm of interferometer 240. Data driver 235 may output signals, based on encoded data received from data divider and encoder 225, to another arm of interferometer 240.
Interferometer 240 may include an optical splitter 245, a first phase modulator 250, a second phase modulator 255, a phase shifter 260 and an optical coupler 265. Optical splitter 245 may split an incoming optical pulse into two substantially identical pulses and provide each of the optical pulses to a different arm of interferometer 240. One of the optical pulses from splitter 245 may be provided to phase modulator 250 in a first arm of interferometer 240, and the other to phase modulator 255 in a second arm of interferometer 240. Phase modulator 250 may modulate the phase of the received optical pulse based on signals received from data driver 230. Phase modulator 255 may modulate the phase of the received carrier wave optical pulse based on signals received from data driver 235. Phase modulator 255 may provide the phase modulated optical pulse to phase shifter 260, which may then phase shift the modulated carrier wave signal a specified amount. In one implementation, phase shifter 260 may include a π/2 phase shifter that induces 90 degrees of phase shift in the phase modulated optical pulse. The phase shifted optical pulse output from phase shifter 260 may be provided to optical coupler 265. Optical coupler 265 may include, for example, a 50/50 coupler and may couple the phase modulated optical pulses from phase modulator 250 and from phase modulator 255 to provide an output signal that can be provided to MUX 205 for multiplexing.
FIG. 3A illustrates an exemplary configuration of DQPSK receiver 120. As shown in FIG. 3A, DQPSK receiver 120 may include a demultiplexer 300 and one or more channel DQPSK units 305-1 through 305-N (individually and generically referred to herein as a “channel DQPSK unit 305-x”). Demultiplexer 300 may include a wavelength division demultiplexing device that demultiplexes different channels of optical signals out of an optical fiber for provision to different ones of DQPSK units 305-1 through 305-N. For example, demultiplexer 300 receives the DQPSK signal in, which includes optical signals in multiple channels (e.g., channels 1 through N) and demultiplexes signals from each channel to a respective channel DQPSK unit 305-x. Each of channel DQPSK units 305-1 through 305-N demodulates the received optical signals in a respective optical channel to retrieve the data encoded in the optical signals.
FIG. 3B is a diagram that illustrates an exemplary configuration of a channel DQPSK unit 305-x. As depicted in FIG. 3B, channel DQPSK unit 305-x may include an optical splitter 310, a first interferometer 315, a second interferometer 320 and a data combiner 325. Optical splitter 310 may split each DQPSK optical pulse received from demultiplexer 300 into two DQPSK optical pulses and may provide a first one of the pulses to interferometer 315 and a second one of the pulses to interferometer 320.
Interferometer 315 may include an optical splitter 327 that connects to a first arm 329 and a second arm 330 of interferometer 315. Optical splitter 327 may split an incoming optical pulse into two substantially identical pulses and provide each of the optical pulses to a different arm of interferometer 315 (i.e., one pulse to arm 329 and another pulse to arm 330). Arm 329 may include a tunable optical filter 332 and a photodetector 334. Arm 329 may optionally also include a tunable electrical filter 335 connected to the output of photodetector 334. Arm 330 may include a tunable optical filter 337 and a photodetector 339. Arm 330 may optionally also include a tunable electrical filter 340 connected to the output of photodetector 339. Tunable optical filters 332 and 337 may be appropriately tuned to filter channel overlap regions, as further described below, and reduce channel cross-talk. Optional tunable electrical filters 335 and 340 may also be tuned, in conjunction with the tuning of optical filters 332 and 337, to reduce excessive frequency domain overlapping and thereby reduce frequency overlapping noise. Tunable optical filters 332 and 337 may include tunable optical band reject filters, such as, for example, narrowband optical tunable thin filter filters. Photodetectors 334 and 339 detect each received optical pulse and convert the optical pulse into an output electrical signal. Photodetectors 334 and 339 may include positive intrinsic negative (PIN) diodes, avalanche photo detectors (APDs), and/or photo-multiplier tubes (PMTs). The electrical signal output from photodetectors 334 and 339 may be input into a comparator 355 (or into filters 335 and 340 and then into comparator 345). Arm 330 may induce a delay (e.g., a one pulse delay) in received optical pulses relative to optical pulses received at arm 329 of interferometer 315. Thus, for any given pulse detected at photodetector 334 of arm 329, the previous optical pulse will be detected at photodetector 339 enabling comparator 355 to subtract the electrical signals from one another to determine the phase difference between sequential optical pulses.
Interferometer 320 may include an optical splitter 350 that connects to a first arm 352 and a second arm 354 of interferometer 320. Optical splitter 350 may split an incoming optical pulse into two substantially identical pulses and provide each of the optical pulses to a different arm of interferometer 320 (i.e., one pulse to arm 352 and another pulse to arm 354). Arm 352 may include a tunable optical filter 355 and a photodetector 357. Arm 352 may optionally also include a tunable electrical filter 359 connected to the output of photodetector 357. Arm 354 may include a tunable optical filter 360 and a photodetector 362. Arm 354 may optionally also include a tunable electric filter 364 connected to the output of photodetector 362. Tunable optical filters 355 and 360 may be appropriately tuned to filter channel overlap regions, as further described below, and reduce channel cross-talk. Optional tunable electrical filters 359 and 364 may also be tuned, in conjunction with the tuning of optical filters 355 and 360, to reduce excessive frequency domain overlapping and thereby reduce frequency overlapping noise. Tunable optical filters 355 and 360 may include tunable optical band reject filters, such as, for example, narrowband optical tunable thin filter filters. Photodetectors 357 and 362 may detect each received optical pulse and convert the optical pulse into an output electrical signal. Photodetectors 357 and 362 may include positive intrinsic negative (PIN) diodes, avalanche photo detectors (APDs), and/or photo-multiplier tubes (PMTs). The electrical signal output from photodetectors 357 and 362 may be input into a comparator 365 (or into filters 359 and 364 and then into comparator 365). Arm 354 may induce a delay (e.g., a one pulse delay) in received optical pulses relative to optical pulses received at arm 352 of interferometer 320. Thus, for any given pulse detected at photodetector 357 of arm 352, the previous optical pulse will be detected at photodetector 362 enabling comparator 365 to subtract the electrical signals from one another to determine the phase difference between sequential optical pulses.
Data combiner 325 may receive the phase difference signals from comparators 345 and 365 and may determine the data values (e.g., 00, 01, 10, 11) that correspond to those phase difference signals.
FIG. 4 depicts an example of an optical pulse 400, in the time domain, which may be emitted by pulse modulator 215 of a channel DQPSK unit 200-x of DQPSK transmitter 110. As further shown in FIG. 4, the time domain optical pulse 400 may have a corresponding optical spectrum 410 in the frequency domain. As additionally depicted in FIG. 5, the optical spectrum 410 associated with optical pulse 400 can undergo asymmetric broadening, with asymmetric blue and red shifts, as optical pulse 400 traverses optical network 130 from transmitter 110 to receiver 120 due to fiber dispersion and non-linearity.
FIG. 6 further depicts the use of multiple channels (e.g., multiple different optical wavelengths) for transmitting data between transmitter 110 and receiver 120 via optical network 130. In the example of FIG. 6, the channel spacing is set relatively far apart, resulting in very little channel spectrum and phase overlap. As shown in FIG. 6, a multi-channel frequency spectrum 600 may include channel 1 610, channel 2 620 and channel 3 630, with each having respective center frequencies fc1, fc2 and fc3. As further shown in FIG. 6, overlap regions exist (e.g., overlap region 640 between channel 1 610 and channel 2 620 and overlap region 650 between channel 2 620 and channel 3 630) that have minimal overlap between the spectrums of the adjacent channels.
In the case where narrow channel spacing is employed, such as in the example shown in FIG. 7A, a relatively large amount of undesirable channel spectrum and phase overlap occurs. As depicted in FIG. 7A, a multi-channel frequency spectrum 700 may include channel 1 710, channel 2 720 and channel 3 730, with each having respective center frequencies fc1, fc2 and fc3. As further shown in FIG. 7A, overlap regions exist (e.g., overlap region 740 between channel 1 710 and channel 2 720, and overlap region 750 between channel 2 720 and channel 3 730) that have substantial overlap between the spectrums of the adjacent channels (e.g., substantial cross-talk between channels). Overlap regions between adjacent channels that have been asymmetrically broadened may have significantly increased channel overlap. For example, overlap region 740 between channel 1 710 and channel 2 720 includes a large channel overlap due to asymmetric broadening of the channel spectrum.
FIG. 7B illustrates additional channel spectrum and phase overlap that may occur due to center frequency drift associated with each channel. The center frequency drift may occur as a result of frequency error in CW laser 210 for a respective channel, or may occur due to frequency drift that may be induced by multiplexer 205 of DQPSK transmitter 110 or demultiplexer 300 of DQPSK receiver 120. As shown in FIG. 7B, each channel may have a drift in the center frequency from a nominal center frequency. For example, channel 1 710 may have an actual center frequency “fc1 actual” that represents a frequency drift Δfc1 from the nominal center frequency “fc1 nominal.” As another example, channel 2 720 may have an actual center frequency “fc2 actual” that represents a frequency drift Δfc2 from the nominal center frequency “fc2 nominal.” As an additional example, channel 3 730 may have an actual center frequency “fc3 actual” that represents a frequency drift Δfc3 from the nominal center frequency “fc3 nominal.”
To reduce optical cross-talk in the optical spectrum 700 depicted in FIG. 7A or 7B, tunable optical filters may be used in receiver 120. For example, tunable optical filters 332, 337, 355 and 360 of a channel DQPSK unit 305-x of receiver 120 may be used to decrease the cross-talk (e.g., filter the cross-talk) in the overlap regions 740 and 750 of spectrum 700 when the channels are narrowly spaced. As shown in FIG. 7C, tunable optical filters 332, 337, 355 and 360 of receiver 120 may act to filter regions 760 and 770 to eliminate or reduce channel overlap. Use of tunable optical filters in receiver 120, as described herein, improves optical performance particularly when employing narrow channel spacing (e.g., reduced adjacent channel cross-talk), improves optical pulse asymmetrical broadening in the frequency domain, improves polarization mode dispersion (PMD) and residual dispersion correction and tolerances, improves the margins for the DWDM tunable laser used in the DQPSK transmitter (e.g., increased tolerance in the laser central frequency drift from International Telecommunication Union (ITU) grid), relaxes the specifications in the design of the DQPSK transmitter, and may improve Optical Signal to Noise Ratio (OSNR) by, for example, 1-3 dB. The use of tunable optical filters in the optical receiver, as described herein, has particular applicability to narrow channel spacing Dense Wavelength Division Multiplexing (DWDM) systems as compared to wide channel spacing or coarse wave length division multiplexing (CWDM) systems. Optimum settings of each of tunable optical filters 332, 337, 355 and 360 of receiver 120 may be found through performance testing. Settings of each of the tunable filters may be determined that provide the desired improvements in performance at receiver 120.
FIGS. 8A and 8B are flow diagrams of an exemplary process for tuning the tunable optical filters in receiver 120 to provide desired receiver performance improvements (e.g., reduction of channel crosstalk). The exemplary process of FIGS. 8A and 8B may be manually performed or may be performed by an automated performance testing system. The flow diagrams of FIGS. 8A and 8B refer to an “optical filter A” and an “optical filter B.” “Optical filter A” may correspond to a tunable optical filter in a first arm of an interferometer of a channel DQPSK unit 305-x, and “optical filter B” may correspond to a tunable optical filter in a second arm of an interferometer of a channel DQPSK unit 305-x. For example, “optical filter A” may correspond to tunable optical filter 332 in arm 329 of interferometer 315 and “optical filter B” may correspond to tunable optical filter 337 in arm 330 of interferometer 315. As another example, “optical filter A” may correspond to tunable optical filter 355 in arm 352 of interferometer 315 and “optical filter B” may correspond to tunable optical filter 360 in arm 354 of interferometer 315.
The exemplary process may begin with the tuning of optical filter A and optical filter B to a same center frequency (block 800). The same center frequency may correspond to the nominal center frequency for the channel associated with the respective channel DQPSK unit 305-x. Optical filter A may then be tuned to selected positions left and right of the center frequency, leaving optical filter B fixed at the center frequency, and a signal bit error rate (BER) and/or Q value may be measured and recorded at each filter position (block 805). Optical filter B may then be tuned to selected positions left and right of the center frequency, leaving optical filter A fixed at the center frequency, and a signal BER and/or Q value may be measured and recorded at each filter position (block 810).
Optical filter A may be tuned to an offset from the center frequency that is less than the center frequency and optical filter B may be tuned to an offset from the center frequency that is greater than the center frequency (block 815). Optical filters A and B may then be tuned simultaneously, from their respective offsets, towards the center frequency and the signal BER and/or Q value may be measured and recorded at each filter position (block 820).
Optical filter A and optical filter B may be tuned to a same center frequency (block 825). Optical filter A and optical filter B may then be tuned simultaneously away from the center frequency and the signal BER and/or Q value may be measured and recorded at each filter position (block 830). In this block, optical filter A and optical filter B may be simultaneously tuned away from the center frequency in opposite directions. Thus, optical filter A may be tuned towards decreasing frequencies while optical filter B may be tuned towards increasing frequencies, and then optical filter A may be tuned towards increasing frequencies while optical filter B may be tuned towards decreasing frequencies.
Optical filter A and optical filter B may be tuned to an offset from the center frequency that is less than the center frequency (block 835). Optical filter A and optical filter B may then be simultaneously tuned towards increasing frequencies and the signal BER and/or Q value may be measured and recorded at selected filter positions (block 840).
Optical filter A and optical filter B may be tuned to an offset from the center frequency that is greater than the center frequency (block 845). Optical filter A and optical filter B may then be simultaneously tuned towards decreasing frequencies and the signal BER and/or Q value may be measured and recorded at selected filter positions (block 850).
The filter positions of optical filters A and B that maximize BER and/or Q value performance may be determined based on previous filter tunings and a comparison of recorded BER and/or Q values (block 855). Recorded BER and/or Q values associated with each of the filter tuning blocks (e.g., blocks 805, 810, 820, 830, 840 and 850 of FIGS. 8A and 8B) may be compared with one another to deduce the optimum filter positions of optical filters A and B that maximize BER and/or Q value performance. For example, the recorded BER values associated with each of the filter tuning blocks of FIGS. 8A and 8B may be compared with one another to deduce the optimum filter positions of optical filters A and B that produce the smallest BER.
The blocks of the flow diagram of FIGS. 8A and 8B have been described with respect to tuning optical filters A and B, which may correspond to tunable optical filters 332 and 337 or 355 and 360 of FIG. 3B. In additional exemplary embodiments, tunable electrical filters 335 and 340 or 359 and 364 may be tuned in conjunction with the tuning of optical filters 332 and 337 or 355 and 360 to maximize BER and/or Q value performance.
In the preceding specification, various preferred embodiments have been described with reference to the accompanying drawings. It will, however, be evident that various modifications and changes may be made thereto, and additional embodiments may be implemented, without departing from the broader scope of the invention as set forth in the claims that follow. The specification and drawings are accordingly to be regarded in an illustrative rather than restrictive sense. Modifications and variations are possible in light of the specification, or may be acquired from practice of the invention. For example, embodiments have been described herein with respect to a DQPSK receiver. However, tunable optical filters (and tunable electrical filters) may be similarly employed in the DPSK receiver of a DPSK system for similar improvements in optical performance, as described herein.
It will be apparent that embodiments, as described above, may be implemented in many different forms of software, firmware, and hardware in the implementations illustrated in the figures. The actual software code or specialized control hardware used to implement embodiments is not limiting of the invention. Thus, the operation and behavior of the embodiments have been described without reference to the specific software code, it being understood that software and control hardware may be designed based on the description herein.
No element, act, or instruction used in the present application should be construed as critical or essential to the invention unless explicitly described as such. Also, as used herein, the article “a” is intended to include one or more items. Where only one item is intended, the term “one” or similar language is used. Further, the phrase “based on” is intended to mean “based, at least in part, on” unless explicitly stated otherwise.

## Claims

1. A method comprising:
tuning an optical filter in an arm of an interferometer,
tuning the optical filter including:
tuning the optical filter to a first plurality of filter positions,
one of the first plurality of filter positions being left of a center frequency and another one of the first plurality of filter positions being right of the center frequency, and
another optical filter in another arm of the interferometer being positioned at the center frequency during the tuning of the optical filter;



measuring at least one of bit error rates (BERs) or Q-values associated with the interferometer during the tuning of the optical filter; and
maximizing, based on the measuring, at least one of BER performance or Q-value performance of the interferometer.

2. The method of claim 1, where tuning the optical filter further includes:
tuning the other optical filter to a second plurality of filter positions,
where one of the second plurality of filter positions is left of the center frequency and another one of the second plurality of filter positions is right of the center frequency, and
where the optical filter is positioned at the center frequency during the tuning of the other optical filter.

3. The method of claim 1, where tuning the optical filter further includes:
tuning, from a first offset from a center frequency, and towards the center frequency, the optical filter, where the first offset is less than the center frequency.

4. The method of claim 1, where maximizing the at least one of the BER performance or the Q-value performance includes:
determining, based on the measured at least one of BERs or the Q-values, a filter position, where the determined filter position produces a smallest BER for the interferometer.

5. A method comprising:
tuning an optical filter in an arm of an interferometer,
tuning the optical filter including:
simultaneously tuning the optical filter and another optical filter,
the other optical filter being positioned in another arm of the interferometer,
the optical filter being tuned from a first offset from a center frequency towards the center frequency,
the other optical filter being tuned from a second offset from the center frequency towards the center frequency, and
the first offset being less than the center frequency and the second offset being greater than the center frequency;



measuring at least one of bit error rates (BERs) or Q-values associated with the interferometer during the tuning of the optical filter; and
adjusting, based on the measuring, at least one of BER performance or Q-value performance of the interferometer.

6. The method of claim 5, where tuning the optical filter further includes:
simultaneously tuning the optical filter and the other optical filter, where the optical filter is tuned in a first direction away from the center frequency and the other optical filter is tuned in a second direction away from the center frequency, where the second direction is opposite to the first direction; and
simultaneously tuning, from the center frequency, the optical filter and the other optical filter, where the optical filter is tuned in the second direction and the other optical filter is tuned in the first direction.

7. The method of claim 5, where adjusting the at least one of the BER performance or the Q-value performance includes:
determining, based on the measured at least one of the BERs or the Q-values, a filter position,
where the determined filter position produces a smallest BER for the interferometer.

8. A system comprising:
one or more devices to:
tune an optical filter in an arm of an interferometer to a plurality of first positions,
the optical filter being tuned to reduce channel overlap between adjacent optical channels,
one of the first plurality of filter positions being left of a center frequency,
another one of the first plurality of filter positions being right of the center frequency, and
another optical filter in another arm of the interferometer being positioned at the center frequency during the tuning of the optical filter,


measure at least one of bit error rates (BERs) or Q-values associated with the interferometer during the tuning of the optical filter, and
adjust, based on the measuring, at least one of BER performance or Q-value performance of the interferometer.

9. The system of claim 8, where the other tunable filter is to reduce frequency domain overlap between the adjacent optical channels.

10. The system of claim 8, where, when tuning the optical filter, the one or more devices are further to:
correct for asymmetrical broadening of optical spectrums associated with one or more of the adjacent optical channels.

11. The system of claim 8, where, when tuning the optical filter, the one or more devices are further to:
correct for a center frequency drift associated with one or more of the adjacent optical channels.

12. The system of claim 8, where the tunable filter is associated with an optical receiver.

13. The system of claim 8, where the adjacent optical channels comprise a plurality of optical channels having a narrow channel spacing.

14. The system of claim 8, where, when tuning the optical filter, the one or more devices are further to:
filter out adjacent channel cross talk.

15. The system of claim 8, where, when tuning the optical filter, the one or more devices are further to:
tune the other optical filter to a second plurality of filter positions,
one of the second plurality of filter positions being left of the center frequency, and
another one of the second plurality of filter positions being right of the center frequency.

16. The system of claim 15, where, when tuning the other optical filter, the one or more devices are further to:
position the optical filter at the center frequency during the tuning of the other optical filter.

17. The system of claim 8, where, when maximizing the at least one of the BER performance or the Q-value performance, the one or more devices are further to:
determine, based on the measured at least one of BERs or the Q-values, a filter position that produces a smallest BER for the interferometer.

18. A system comprising:
a device to:
simultaneously tune an optical filter in an arm of an interferometer and another optical filter in another arm of the interferometer, to filter a region of overlap between adjacent optical channels,
the optical filter being tuned from a first offset from a center frequency towards the center frequency,
the other optical filter being tuned from a second offset from the center frequency towards the center frequency, and
the first offset being less than the second frequency and the second offset being greater than the center frequency,

measure at least one of bit error rates (BERs) or Q-values associated with the interferometer during the tuning of the optical filter, and

maximize, based on the measuring, at least one of BER performance or Q-value performance of the interferometer.

19. The system of claim 18, where simultaneously tuning the optical filter and the other optical filter reduces a frequency domain overlap between the adjacent optical channels.

20. The system of claim 18, where, when maximizing the at least one of the BER performance or the Q-value performance, the device is further to:
determine, based on the measured at least one of BERs or the Q-values, a filter position that produces a smallest BER for the interferometer.