US20090190924A1 - Method and system for automatic feedback control for fine tuning a delay interferometer - Google Patents
Method and system for automatic feedback control for fine tuning a delay interferometer Download PDFInfo
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- US20090190924A1 US20090190924A1 US12/383,972 US38397209A US2009190924A1 US 20090190924 A1 US20090190924 A1 US 20090190924A1 US 38397209 A US38397209 A US 38397209A US 2009190924 A1 US2009190924 A1 US 2009190924A1
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
- H04B10/60—Receivers
- H04B10/66—Non-coherent receivers, e.g. using direct detection
- H04B10/69—Electrical arrangements in the receiver
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
- H04B10/60—Receivers
- H04B10/66—Non-coherent receivers, e.g. using direct detection
- H04B10/67—Optical arrangements in the receiver
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
- H04B10/60—Receivers
- H04B10/66—Non-coherent receivers, e.g. using direct detection
- H04B10/67—Optical arrangements in the receiver
- H04B10/676—Optical arrangements in the receiver for all-optical demodulation of the input optical signal
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
- H04B10/60—Receivers
- H04B10/66—Non-coherent receivers, e.g. using direct detection
- H04B10/67—Optical arrangements in the receiver
- H04B10/676—Optical arrangements in the receiver for all-optical demodulation of the input optical signal
- H04B10/677—Optical arrangements in the receiver for all-optical demodulation of the input optical signal for differentially modulated signal, e.g. DPSK signals
Definitions
- This disclosure relates generally to the field of optical communication networks and more specifically to a method and system for automatic feedback control for fine tuning a delay interferometer.
- Telecommunications systems, cable television systems, and data communication networks use optical networks to rapidly convey large amounts of information between remote points.
- information is conveyed in the form of optical signals through optical fibers.
- Optical fibers comprise thin strands of glass capable of transmitting the signals over long distances with very little loss.
- the optical signals have at least one characteristic modulated to encode audio, video, textual, real-time, non-real-time, and/or other suitable data.
- the transmitter is a phase modulator that transmits a differentially encoded binary data signal by modulating the phase of the carrier signal.
- the phase modulated signal is further modulated by a bit synchronous sinusoidal clock signal.
- the encoded signal is differentially decoded and detected.
- a method and system for automatic feedback control for fine tuning a delay interferometer are provided.
- the automatic feedback control may be used in an optical receiver that monitors quality of a channel and adjusts differential decoding of the channel to enhance channel quality.
- a method for receiving an optical signal is included where an ingress signal is split into a first portion and a second portion. A relative delay is induced between the first portion and the second portion, which are optically interfered to generate at least one interfered signal. Quality criteria of a monitored signal at least based on the at least one interfered signal is monitored so that a relative delay based in the quality criteria may be adjusted.
- Certain embodiments may provide one or more technical advantages.
- a technical advantage of one embodiment may include improving signal quality at the receiver. More particularly, signal degradation may be reduced or eliminated at the optical receiver by providing for automatic feedback control of a delay interferometer.
- Another technical advantage of one or more embodiments may include accurate and efficient fine-tuning of a delay interferometer by monitoring quality criteria of optical signals.
- Yet another advantage of one or more embodiments may include improving DPSK/IMDPSK systems.
- Another technical advantage of one or more embodiments includes using quality criteria to automatically adjust the optical signal at the optical delay interferometer. Therefore, an optical communications system may be more cost effective since the optical delay interferometer may not be adjusted manually and drift tolerances associated with transmitter lasers may be lessened. Yet another technical advantage of one or more embodiments may include the use of DPSK/IMDPSK technology with Ultra Long Haul (ULH) systems due to improved tolerance to non-linear effects, optical signal-to-noise ratio (OSNR) and dispersion.
- UH Ultra Long Haul
- OSNR optical signal-to-noise ratio
- FIG. 1 is a block diagram of one embodiment of an optical communication system
- FIG. 2 is a block diagram of one embodiment of an optical transmitter for use with the optical communication system of FIG. 1 ;
- FIG. 3 is a block diagram of one embodiment of an optical receiver for use with the optical communication system of FIG. 1 ;
- FIGS. 4A and 4B illustrate embodiments of optical delay interferometers for use in the optical receiver of FIG. 3 ;
- FIGS. 5A-5D illustrate embodiments of feedback control and signal monitoring for use in the optical receiver of FIG. 3 ;
- FIG. 6 is a flowchart illustrating one embodiment of a method for automatic feedback control for fine tuning a delay interferometer.
- FIG. 1 is a block diagram of an embodiment of an optical communication system 10 using automatic feedback control for fine tuning a delay interferometer.
- optical communication system 10 allows optical signals to be transmitted over a common path at disparate wave lengths.
- An optical receiver receives the optical signals to separate and decode the optical information.
- optical communication system 10 is a wavelength division multiplexed (WDM) system such as a dense WDM (DWDM) system, where optical information is transmitted using differential phase shift king (DPSK) with bit synchronous intensity modulation (IM).
- WDM wavelength division multiplexed
- DWDM dense WDM
- DPSK differential phase shift king
- IM bit synchronous intensity modulation
- System 10 may comprise other suitable single channel, multi-channel, or by directional transmission systems, and may use other modulation formats.
- System 10 includes a transmitter 12 , an optical link 16 , and a receiver 14 coupled as shown in FIG. 1 .
- Transmitter 12 includes a plurality of optical transmitters 20 and a multiplexer 22 .
- Optical transmitters 20 may form part of a transponder or other node element.
- Each optical transmitter 20 generates an optical information signal 24 on one of a set of distinct wave lengths .lambda..sub.1, .lambda..sub.2, . . . .lambda..sub.n at a certain channel spacing.
- channel spacing may be 100 Gigahertz (GHz).
- the channel spacing may be selected to avoid or minimize crosstalk between adjacent channels.
- Optical information signals 24 may comprise optical signals with at least one characteristic modulated to encode audio, video, textual, real time, non-real time, or other suitable data.
- optical information signals 24 are encoded using differential phase shift keying DPSK with intensity modulation (IM).
- IM intensity modulation
- Optical information signals 24 are multiplexed by multiplexer 22 into a transport signal 26 for transmission on optical link 16 .
- Optical information signals 24 may be otherwise suitably combined into transmit signal 26 .
- Optical link 16 comprises optical fiber 40 or other suitable medium in which optical signals may be transmitted with low loss.
- optical fiber 40 may comprise Single Mode fiber (SMF).
- SMF Single Mode fiber
- Optical amplifiers 41 amplify transmit signal 26 without the need for optical to electrical conversion.
- Receiver 14 includes a demultiplexer 32 and a plurality of optical receivers 30 .
- Demultiplexer 32 demultiplexes the amplified transmit signal 26 to separate each optical information signal 24 from the others.
- Each optical receiver 30 receives an optical information signal 24 from the multiplexer 32 and recovers the data information corresponding to each signal 24 .
- each refers to each member of a set or each member of a subset of a set.
- optical receiver 30 converts phase modulated (PM) signals into amplitude modulated (AM) signals that are further processed by a direct detection receiver.
- PM to AM conversion may be performed by a delay interferometer such as Mach-Zehnder interferometer (MZI), a polarization maintaining fiber (PMF) between a pair of suitably aligned polarizers, or any other suitable device operable to delay a portion of a signal relative to another portion and to interfere the delayed portion with another portion of the signal.
- MZI Mach-Zehnder interferometer
- PMF polarization maintaining fiber
- optical transmitter 20 In operation, optical transmitter 20 generates an optical information signal 24 at a particular carrier frequency at which a laser device operates.
- Optical information signal 24 may be multiplexed by multiplexer 22 and amplified by one or more optical amplifiers 41 so that optical information signal 24 may be transmitted through an optical link 16 to receiver 14 .
- each optical information signal 24 is demultiplexed using demultiplexer 32 and received at the appropriate optical receiver 30 , where the signals are decoded to recover the encoded data of optical information signal 24 .
- optical information signal 24 may be received as a deviated signal.
- Optical receiver 30 monitors one or more quality criteria of optical information signal 24 to automatically determine adjustments to the delay of the signal received to compensate for the deviation in frequency caused at optical transmitter 20 .
- FIG. 2 An embodiment of an optical transmitter for use with the optical communication system 10 of FIG. 1 is described with reference to FIG. 2 .
- An embodiment of a portion of an optical receiver for use with the optical communication system 10 of FIG. 1 is described with reference to FIG. 3 .
- Embodiments of delay interferometers are more particularly described with reference to FIGS. 4A and 4B .
- Block diagrams of embodiments of feedback control and signal monitoring for use with the optical communication system 10 of FIG. 1 are described with reference to FIGS. 5A-5D .
- FIG. 6 A flowchart of a method for automatic feedback control for fine tuning a delay interferometer is described with reference to FIG. 6 .
- FIG. 2 illustrates an embodiment of an optical transmitter 20 for use with optical communication system 10 of FIG. 1 .
- Optical transmitter 20 may include an optical emitter 50 , a phase modulator 52 , a differential encoder 54 , a clock 56 , and an intensity modulator 58 coupled as shown in FIG. 2 .
- Optical emitter 50 produces an optical carrier that may be modulated to transmit data.
- Optical emitter 50 may comprise any emitter suitable for generating optical signals at a particular carrier frequency.
- optical emitter 50 may comprise a laser diode.
- Phase modulator 52 and differential encoder 54 modulate encoded data using the optical carrier.
- Differential encoder 54 receives a stream of bits that are encoded using DPSK techniques.
- Phase modulator 52 receives the differentially encoded bits to modulate the phase of the optical carrier to produce an optically modulated signal comprising differentially encoded bits.
- phase modulator 52 generates a DPSK signal having binary data encoded as either a zero or .pi. phase swing between adjacent bits.
- phase modulator 52 may be driven to reduce the depth of phase modulation by adjusting the driving voltage. For example, a driving voltage of phase modulator 52 may be reduced to decrease the phase modulation from zero to .pi. to a phase swing of zero to 2.alpha., where the resulting phase modulation may be in the range of 0 ⁇ 2.alpha. ⁇ .pi.. Any other suitable phase swing and/or phase shift difference may be used without departing from the cope of the invention.
- Transmitter 20 provides a synchronization signal that intensity modulator 58 utilizes to modulate the received DPSK signal using bit synchronous intensity modulation.
- Transmitter 20 may include additional or fewer modules depending on the desired application.
- transmitter 20 may include a driver amplifier coupled to phase modulator 52 to drive the phase modulation of an electronic subcarrier received from emitter 50 .
- FIG. 3 is the block diagram of an embodiment illustrating a portion of an optical receiver for use with optical communication system 10 of FIG. 1 .
- optical receiver 30 includes an optical delay interferometer 32 , a signal monitor 34 , and a processor 38 coupled as shown in FIG. 3 .
- a photodiode or other detector may be coupled to the output of the optical delay interferometer, between the optical delay interferometer 32 and the signal monitor 34 or otherwise in receiver 30 .
- signal monitor 34 may monitor the interfered signal, a decoded signal, a recovered signal, or other signal that is at least based on the interfered signal.
- Optical delay interferometer 32 receives ingress optical information signal 24 and demodulates it to generate an interfered signal 28 .
- optical delay interferometer 32 is an optical device operable to split a received signal into two portions. Additionally, optical delay interferometer 32 induces a relative delay, or shift, between the portions of the split signal.
- Optical delay interferometer 32 optically interferes the portions of the split signals into at least one interfered signal 28 .
- optical delay interferometer 32 comprises a Mach-Zehnder interferometer (MZI). Any other optical device suitable for converting phase modulated (PM) signals into interfered signal 28 may be used without departing from the scope of the invention.
- optical delay interferometer 32 may comprise a polarization maintaining fiber (PMF), or a birefringer fiber.
- interfered signal 28 Before being monitored by signal monitor 34 , interfered signal 28 may be detected by a photodetector to generate a detected signal. By photodetecting interfered signal 28 before monitoring, signal monitor 34 may receive a photocurrent and a photovoltage associated with optical information signal 24 . Additionally, as will be described in more detail below, signal monitor 34 and optical delay interferometer 32 may be DC biased to allow a DC current corresponding to the detected signal to be monitored by signal monitor 34 .
- Signal monitor 34 monitors quality criteria corresponding to a detected signal.
- the quality criteria may comprise a Bit Error Rate (BER), extinction ratio, and an optical power. Any other suitable criteria indicative of the quality of a detected or other signal may be used.
- Signal monitor 34 may comprise an RF monitor, a DC monitor, a trans-impedence amplifier, a controller, a forward error correction unit (FEC), or any other device suitable for monitoring a signal. Embodiments of signal monitor 34 will be described in more detail with respect to FIGS. 5A-5D .
- Processor 38 receives monitored quality criteria information to adjust the relative delay of optical delay interferometer 32 .
- the information may be, for example, the value of a quality criteria, an indication of any change, a value of any change, an indication that a level or limit has been exceeded or the like.
- processor 38 may receive a measurement of optical power, which processor 38 may use to generate a feedback signal 42 .
- processor 38 may control and adjust the relative delay induced by optical delay interferometer 32 .
- processor 38 uses feedback signal 42 to adjust the temperature of optical delay interferometer 32 in order to adjust the relative delay of the split signal portions that combine to form interfered signal 28 .
- the relative delay may be adjusted mechanically such as with a free-space optics system.
- Processor 38 may comprise any suitable logic, whether hardware and/or software, operable to control the relative delay at optical delay interferometer 32 based on information from signal monitor 34 .
- a Bias-T module may be coupled to optical delay interferometer 32 and signal monitor 34 to perform DC biasing.
- functions may be performed using any suitable logic compressing software, hardware, other logic, or any suitable combination of the proceeding.
- FIGS. 4A and 4B illustrate embodiments of optical delay interferometer 32 for use in optical communications system 10 of FIG. 1 .
- FIG. 4A illustrates one embodiment of a Mach-Zehnder interferometer (MZI) 60 .
- MZI 60 includes an upper arm 62 and a lower arm 64 .
- MZI 60 converts a phase modulated (PM) signal into an amplitude modulated (AM) signal by optically delaying one portion of the signal with respect to the other and applying additional phase shift between the two arms of the MZI.
- the amount of relative delay equals approximately the symbol period, or can be equal to any integer multiple of the symbol period.
- the relative phase shift between the upper arm and the lower arm may vary from .pi./2 to ⁇ .pi./2.
- the upper arm signal 62 and lower arm signal 64 are interferometrically combined.
- MZI 60 achieves constructive interference if the phase of the upper arm signal 62 or lower arm signal 64 are in phase.
- MZI 60 achieves destructive interference if the phase difference between upper arm 62 and lower arm 64 is .pi./2.
- the IMDPSK signal, or optical information signal 24 received at MZI 60 , is split into two arms with a splitting ratio of 0.5.
- upper arm signal 62 and lower arm signal 64 may be combined using a 3 dB coupling.
- the relative phase shift and relative delay may be adjusted by controlling the temperature of a waveguide associated with MZI 60 .
- MZI 60 may be implemented using planar lightwave circuit (PLC) technology.
- the PLC may be implemented on a silica substrate, and the relative phase shift between the two arms may be controlled by changing the substrate temperature.
- MZI 60 may include egress optical port A and egress optical port B, which may be complimentary to each other. For example, if upper arm signal 62 and lower arm signal 64 have a phase difference of 180 degrees, MZI 60 achieves destructive interference of the current received bit and the previous received bit resulting in optical port A generating a low level signal.
- Equation (1) The above-stated example may be more particularly represented by describing a transmitted IMDPSK signal s(t) by Equation (1):
- Equation (2) a signal at optical port A of MZI 60 may be described by Equation (2):
- a ( t ) ( A/ 2)sin(2. pi.rt )[cos(2. pi.ft+.phi .( t )+. pi.+ cos(2. pi.ft+.phi .( t ⁇ T )+. T ⁇ HETA . ⁇ 2. pi.fT )] (2)
- T 1/r and defines the bit period
- .THETA. represents the phase difference between upper arm 62 and lower arm 64
- 2.pi.fT represents a relative phase difference introduced due to the relative delay of one bit period.
- Equation (3) the relative phase difference 2.pi.fT introduced due to bit delay depends on the carrier frequency f for a given bit rate r. If the frequency drifts from f to f+.DELTA., the relative phase difference in the delay interferometer would be changed as described by Equation (3):
- the relative phase difference .DELTA..theta. may be cancelled by adjusting the phase change .THETA.. This can be achieved, for example, by tuning the temperature of the PLC substrate. If the geometrical path lengths of the upper and lower arms are L.sub.U and L.sub.L, respectively, and the temperature coefficient of the effective refractive index of the waveguides is k (/degree), the required temperature change .DELTA.T to provide the phase change .THETA. may be described by Equation (4):
- the frequency drift may be tracked so that an adjustment to the relative phase difference or delay of upper arm 62 and lower arm 64 may be adjusted. Feedback control and monitoring signal quality criteria will be described with reference to FIG. 5 .
- FIG. 4B illustrates an embodiment of a polarization maintaining fiber (PMF) 65 for use as optical delay interferometer 32 .
- a polarization controller 67 may be used to receive IMDPSK signals and control the polarization of the IMDPSK signal to a 45 degree linear polarization.
- PMF 65 carries polarized signal 68 over a fiber of length L and birefringence of (n1 ⁇ n2).
- PMF 65 introduces a relative delay between two signals. This relative delay may be described by Equation (5):
- L is the length of PMF 65
- (n1 ⁇ n2) represents the birefringence of PMF 65 .
- PMF 65 carries polarized light, where the orthogonal polarization components may be interfered and received by analyzer 69 .
- Analyzer 69 receives polarized signal 68 and monitors the quality criteria of the interfered signal.
- analyzer 69 may comprise a polarization beam splitter (PBS) to split polarized signal 68 into two linearly polarized signals orthogonal to each other that may be used when monitoring signals from more than one port of optical delay interferometer 32 .
- PBS polarization beam splitter
- FIGS. 5A through 5D illustrate embodiments of feedback control and signal monitoring that may be used with the optical communication system 10 of FIG. 1 .
- FIG. 5A illustrates one embodiment of automatic feedback control using monitoring of extinction ratio
- FIGS. 5B through 5D illustrate embodiments of feedback control using optical power monitoring.
- Other suitable circuits may be used without departing from the scope of the present invention.
- FIG. 5A is a block diagram of an embodiment illustrating an automatic feedback control using monitoring of extinction ratio.
- the extinction ratio of an amplitude modulated (AM) signal may be defined as the ratio of the power levels corresponding to level 1 and level 0 .
- the extinction ratio is maximum.
- Photodetector 70 receives interfered signal 28 from optical delay interferometer 32 and generates a detected signal 29 .
- Detected signal 29 is split by a Bias-T module 72 , which is operable to split a high frequency component from a low frequency component of the signal.
- the high frequency component, or RF signal may be received by RF monitor 74 and the low frequency component, or DC component, may be received by DC monitor 76 .
- RF monitor 74 can detect reductions in RF power of detected signal 29 and therefore reductions in RF power of optical information signal 24 .
- RF monitor 74 provides the RF power measurements to processor 38 where the RF power measurements are normalized using a DC power measurement obtained by DC monitor 76 . By normalizing the measured RF power, fluctuations caused by changing input optical power may be eliminated.
- Processor 38 may determine from a decrease in RF power and normalized power, that an extinction ratio decreases. Accordingly, processor 38 may adjust the relative delay of optical delay interferometer 32 in order to improve the extinction ratio. In one embodiment, the relative delay may be increased by increasing the extinction ratio.
- FIG. 5B is a block diagram illustrating an automatic feedback control using optical power monitoring.
- the optical power of an output of optical delay interferometer 32 may be used to adjust or optimize the phase difference or the relative delay of optical delay interferometer 32 .
- the phase modulation depth may be reduced at the transmitter by decreasing the driving voltage of phase modulator 52 as was described with reference to FIG. 2 . Although reducing the driving voltage may also lead to an eye opening penalty, reducing the phase modulation depth may increase the degree of control corresponding to monitoring optical power to adjust the relative phase difference at the optical delay interferometer 32 .
- an eye opening penalty may be less than 0.2 dB for a reduction in driving voltage at the transmitter resulting in a reduction of phase modulation depth of 2.alpha.>150 degrees. Any other suitable reduction in driving voltage may be selected to monitor the optical power without departing from the scope of the present invention.
- Optical power is monitored based on a DC current received by DC monitor 76 after a Bias-T module 72 splits detected signal 29 .
- the RF component of detected signal 29 may be received by trans-impedance amplifier 78 for conversion and amplification of the signal into an electrical signal that may be used by controller 80 to receive the data.
- DC monitor 76 measures an average optical power corresponding to the detected signal 29 from a single port at optical delay interferometer 32 .
- Processor 38 receives the measured average optical power and generates a feedback signal 42 to adjust the relative delay and to maximize the detected signal 29 .
- the controlling algorithm to determine the feedback signal 42 so that the detected signal 29 be maximized may be found in the public domain.
- FIG. 5C is a block diagram illustrating another embodiment of an automatic feedback control using optical power monitoring.
- both optical port outputs of optical delay interferometer 32 are detected by photodetectors 70 a and 70 b so that a balanced photodetected signal 71 may be monitored.
- a Bias-T module 72 may be used to split a signal into an RF component and DC component.
- the DC current associated with balanced photodetected signal 71 is received by DC monitor 76 to measure the optical power.
- DC monitor 76 measures the DC voltage which may be proportional to the average optical power difference of the signals detected by each photodetector 70 a and 70 b .
- processor 38 may determine feedback signal 42 . As was discussed with reference to FIG. 5B , the average power difference between the two output ports as measured by DC monitor 76 is used by processor 38 to determine if the relative delay at optical delay interferometer 32 may be adjusted. For example, maintaining an average optical power difference at or near a maximum may be accomplished by processor 38 adjusting the relative phase difference to as close as possible to 180 degrees.
- FIG. 5D is a block diagram illustrating yet another embodiment of an automatic feedback control using optical power monitoring of two optical ports of optical delay interferometer 32 .
- the optical port signals of optical delay interferometer 32 are each detected by photodetectors 70 a and 70 b .
- detected signals 73 a and 73 b are each used for a different purpose.
- detected signal 73 a is used for signal reception by directing its RF signal component trans-impedance amplifier 78 and controller 80 .
- detected signal 73 b may be dedicated to measuring optical power by DC monitor 76 .
- DC monitor 76 measures the optical power of detected signal 73 b and compares it to an average optical power so that processor 38 may determine feedback signal 42 that adjusts the relative delay at optical delay interferometer 32 .
- a Bias-T module 72 is shown to include a capacitor C and an inductor L, any other, or additional components suitable for splitting a signal into an RF signal and DC current may be used without departing from the scope of the invention.
- DC monitor 76 may be modified to include a forward error correction (FEC) unit to monitor a bit error rate corresponding to the detected signal 29 that may be used by processor 38 to adjust the relative delay at optical delay interferometer 32 .
- FEC forward error correction
- functions may be performed using any suitable logic comprising software, hardware, other logic, or any suitable combination of the preceding.
- FIG. 6 is a flowchart of a method for automatic feedback control for fine tuning a delay interferometer. It will be understood that the method may be used in connection with any number of optical receivers 30 and any number of optical information signals 24 .
- optical delay interferometer 32 receives a modulated signal.
- the modulated signal comprises an intensity modulated DPSK signal (IMDPSK).
- IMDPSK intensity modulated DPSK signal
- optical delay interferometer 32 may split a signal into a plurality of signals, delay one of the signals relative to the other, and interfere the plurality of signals.
- MZI 60 splits the signal into an upper arm signal 62 and a lower arm signal 64 and introduces a relative delay at upper arm signal 62 with reference to lower arm signal 62 .
- step 104 signal monitor 34 monitors signal quality criteria of at least one interfered signal.
- signal monitor 34 monitors a detected signal using a bit error rate (BER), an extinction ratio, and an optical power.
- processor 38 determines the signal adjustment based on the monitored quality criteria. For example, if signal monitor 38 monitors the BER of a detected signal, processor 38 determines feedback signal 42 comprising forward error correction information based on the monitored BER.
- step 108 processor 38 adjusts the phase shift of the interfered signals based on the monitored quality criteria.
- processor 38 may use a feedback signal 42 comprising forward error correction information to adjust the relative delay between the upper arm 62 and lower arm 64 of optical delay interferometer 32 .
- the method terminates.
- the method may be repeated continuously, or periodically, for example, once a second, a few times a minute, once per bit period T, or otherwise.
- Steps may be added, omitted, modified, or performed in any suitable order without departing from the scope of the invention.
- a step of detecting at a photodetector an optical signal may be added between the steps of generating interferometer paths at step 102 and monitoring signal properties at step 104 .
- a step may be added after generating interferometer paths according to a phase shift at step 102 , where the interferometer paths are optically interfered to yield an interfered signal.
- Certain embodiments may provide one or more technical advantages.
- a technical advantage of one embodiment may include improving signal quality at the receiver. More particularly, signal degradation may be reduced or eliminated at the optical receiver by providing for automatic feedback control of a delay interferometer.
- Another technical advantage of one or more embodiments may include accurate and efficient fine-tuning of a delay interferometer by monitoring quality criteria of optical signals.
- Yet another advantage of one or more embodiments may include improving DPSK/IMDPSK systems.
- Another technical advantage of one or more embodiments includes using quality criteria to automatically adjust the optical signal at the optical delay interferometer. Therefore, an optical communications system may be more cost effective since the optical delay interferometer may not be adjusted manually and drift tolerances associated with transmitter lasers may be lessened. Yet another technical advantage of one or more embodiments may include the use of DPSK/IMDPSK technology with Ultra Long Haul (ULH) systems due to improved tolerance to non-linear effects, optical signal-to-noise ratio (OSNR) and dispersion.
- UH Ultra Long Haul
- OSNR optical signal-to-noise ratio
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Abstract
A method for receiving an optical signal is included where an ingress signal is split into a first portion and a second portion. A relative delay is induced between the first portion and the second portion, which are optically interfered to generate at least one interfered signal. Quality criteria of a monitored signal at least based on the at least one interfered signal is monitored so that a relative delay based in the quality criteria may be adjusted.
Description
- This application is a continuation of U.S. patent application Ser. No. 10/651,125, filed Aug. 28, 2003, the entire text of which is hereby incorporated by reference herein.
- This disclosure relates generally to the field of optical communication networks and more specifically to a method and system for automatic feedback control for fine tuning a delay interferometer.
- Telecommunications systems, cable television systems, and data communication networks use optical networks to rapidly convey large amounts of information between remote points. In an optical network, information is conveyed in the form of optical signals through optical fibers. Optical fibers comprise thin strands of glass capable of transmitting the signals over long distances with very little loss. The optical signals have at least one characteristic modulated to encode audio, video, textual, real-time, non-real-time, and/or other suitable data.
- For an intensity modulated differential phase shift keying (IMDPSK) system, the transmitter is a phase modulator that transmits a differentially encoded binary data signal by modulating the phase of the carrier signal. The phase modulated signal is further modulated by a bit synchronous sinusoidal clock signal. At the receiver, the encoded signal is differentially decoded and detected.
- A method and system for automatic feedback control for fine tuning a delay interferometer are provided. The automatic feedback control may be used in an optical receiver that monitors quality of a channel and adjusts differential decoding of the channel to enhance channel quality.
- According to one embodiment, a method for receiving an optical signal is included where an ingress signal is split into a first portion and a second portion. A relative delay is induced between the first portion and the second portion, which are optically interfered to generate at least one interfered signal. Quality criteria of a monitored signal at least based on the at least one interfered signal is monitored so that a relative delay based in the quality criteria may be adjusted.
- Certain embodiments may provide one or more technical advantages. A technical advantage of one embodiment may include improving signal quality at the receiver. More particularly, signal degradation may be reduced or eliminated at the optical receiver by providing for automatic feedback control of a delay interferometer. Another technical advantage of one or more embodiments may include accurate and efficient fine-tuning of a delay interferometer by monitoring quality criteria of optical signals. Yet another advantage of one or more embodiments may include improving DPSK/IMDPSK systems.
- Another technical advantage of one or more embodiments includes using quality criteria to automatically adjust the optical signal at the optical delay interferometer. Therefore, an optical communications system may be more cost effective since the optical delay interferometer may not be adjusted manually and drift tolerances associated with transmitter lasers may be lessened. Yet another technical advantage of one or more embodiments may include the use of DPSK/IMDPSK technology with Ultra Long Haul (ULH) systems due to improved tolerance to non-linear effects, optical signal-to-noise ratio (OSNR) and dispersion.
- Certain embodiments may include none, some, or all of the above technical advantages. One or more other technical advantages may be readily apparent to one skilled in the art from the figures, descriptions, and claims included herein.
- For a more complete understanding of the present disclosure and its features and advantages, reference is now made to the following description, taken in conjunction with the accompanying drawings, in which:
-
FIG. 1 is a block diagram of one embodiment of an optical communication system; -
FIG. 2 is a block diagram of one embodiment of an optical transmitter for use with the optical communication system ofFIG. 1 ; -
FIG. 3 is a block diagram of one embodiment of an optical receiver for use with the optical communication system ofFIG. 1 ; -
FIGS. 4A and 4B illustrate embodiments of optical delay interferometers for use in the optical receiver ofFIG. 3 ; -
FIGS. 5A-5D illustrate embodiments of feedback control and signal monitoring for use in the optical receiver ofFIG. 3 ; and -
FIG. 6 is a flowchart illustrating one embodiment of a method for automatic feedback control for fine tuning a delay interferometer. -
FIG. 1 is a block diagram of an embodiment of anoptical communication system 10 using automatic feedback control for fine tuning a delay interferometer. In general,optical communication system 10 allows optical signals to be transmitted over a common path at disparate wave lengths. An optical receiver receives the optical signals to separate and decode the optical information. In operation,optical communication system 10 is a wavelength division multiplexed (WDM) system such as a dense WDM (DWDM) system, where optical information is transmitted using differential phase shift king (DPSK) with bit synchronous intensity modulation (IM). It is understood thatsystem 10 may comprise other suitable single channel, multi-channel, or by directional transmission systems, and may use other modulation formats.System 10 includes atransmitter 12, anoptical link 16, and areceiver 14 coupled as shown inFIG. 1 . -
Transmitter 12 includes a plurality ofoptical transmitters 20 and amultiplexer 22.Optical transmitters 20 may form part of a transponder or other node element. Eachoptical transmitter 20 generates anoptical information signal 24 on one of a set of distinct wave lengths .lambda..sub.1, .lambda..sub.2, . . . .lambda..sub.n at a certain channel spacing. For example, in a particular embodiment, channel spacing may be 100 Gigahertz (GHz). The channel spacing may be selected to avoid or minimize crosstalk between adjacent channels.Optical information signals 24 may comprise optical signals with at least one characteristic modulated to encode audio, video, textual, real time, non-real time, or other suitable data. According to the illustrated embodiment,optical information signals 24 are encoded using differential phase shift keying DPSK with intensity modulation (IM).Optical information signals 24 are multiplexed bymultiplexer 22 into a transport signal 26 for transmission onoptical link 16.Optical information signals 24 may be otherwise suitably combined into transmit signal 26. -
Optical link 16 comprisesoptical fiber 40 or other suitable medium in which optical signals may be transmitted with low loss. According to one embodiment,optical fiber 40 may comprise Single Mode fiber (SMF). Interposed alongoptical length 16 are one or moreoptical amplifiers 41.Optical amplifiers 41 amplify transmit signal 26 without the need for optical to electrical conversion. -
Receiver 14 includes ademultiplexer 32 and a plurality ofoptical receivers 30. Demultiplexer 32 demultiplexes the amplified transmit signal 26 to separate eachoptical information signal 24 from the others. Eachoptical receiver 30 receives anoptical information signal 24 from themultiplexer 32 and recovers the data information corresponding to eachsignal 24. As used in this document, “each” refers to each member of a set or each member of a subset of a set. - According to the illustrated embodiment,
optical receiver 30 converts phase modulated (PM) signals into amplitude modulated (AM) signals that are further processed by a direct detection receiver. In one embodiment, PM to AM conversion may be performed by a delay interferometer such as Mach-Zehnder interferometer (MZI), a polarization maintaining fiber (PMF) between a pair of suitably aligned polarizers, or any other suitable device operable to delay a portion of a signal relative to another portion and to interfere the delayed portion with another portion of the signal. - In operation,
optical transmitter 20 generates anoptical information signal 24 at a particular carrier frequency at which a laser device operates.Optical information signal 24 may be multiplexed bymultiplexer 22 and amplified by one or moreoptical amplifiers 41 so thatoptical information signal 24 may be transmitted through anoptical link 16 toreceiver 14. Atreceiver 14, eachoptical information signal 24 is demultiplexed usingdemultiplexer 32 and received at the appropriateoptical receiver 30, where the signals are decoded to recover the encoded data ofoptical information signal 24. When a laser frequency drifts at anoptical transmitter 20,optical information signal 24 may be received as a deviated signal.Optical receiver 30 monitors one or more quality criteria ofoptical information signal 24 to automatically determine adjustments to the delay of the signal received to compensate for the deviation in frequency caused atoptical transmitter 20. - An embodiment of an optical transmitter for use with the
optical communication system 10 ofFIG. 1 is described with reference toFIG. 2 . An embodiment of a portion of an optical receiver for use with theoptical communication system 10 ofFIG. 1 is described with reference toFIG. 3 . Embodiments of delay interferometers are more particularly described with reference toFIGS. 4A and 4B . Block diagrams of embodiments of feedback control and signal monitoring for use with theoptical communication system 10 ofFIG. 1 are described with reference toFIGS. 5A-5D . A flowchart of a method for automatic feedback control for fine tuning a delay interferometer is described with reference toFIG. 6 . -
FIG. 2 illustrates an embodiment of anoptical transmitter 20 for use withoptical communication system 10 ofFIG. 1 .Optical transmitter 20 may include anoptical emitter 50, aphase modulator 52, adifferential encoder 54, aclock 56, and anintensity modulator 58 coupled as shown inFIG. 2 . -
Optical emitter 50 produces an optical carrier that may be modulated to transmit data.Optical emitter 50 may comprise any emitter suitable for generating optical signals at a particular carrier frequency. For example,optical emitter 50 may comprise a laser diode. -
Phase modulator 52 anddifferential encoder 54 modulate encoded data using the optical carrier.Differential encoder 54 receives a stream of bits that are encoded using DPSK techniques.Phase modulator 52 receives the differentially encoded bits to modulate the phase of the optical carrier to produce an optically modulated signal comprising differentially encoded bits. According to the illustrated embodiment,phase modulator 52 generates a DPSK signal having binary data encoded as either a zero or .pi. phase swing between adjacent bits. In one embodiment,phase modulator 52 may be driven to reduce the depth of phase modulation by adjusting the driving voltage. For example, a driving voltage ofphase modulator 52 may be reduced to decrease the phase modulation from zero to .pi. to a phase swing of zero to 2.alpha., where the resulting phase modulation may be in the range of 0<2.alpha.<.pi.. Any other suitable phase swing and/or phase shift difference may be used without departing from the cope of the invention. -
Clock 56 provides a synchronization signal thatintensity modulator 58 utilizes to modulate the received DPSK signal using bit synchronous intensity modulation.Transmitter 20 may include additional or fewer modules depending on the desired application. For example,transmitter 20 may include a driver amplifier coupled tophase modulator 52 to drive the phase modulation of an electronic subcarrier received fromemitter 50. -
FIG. 3 is the block diagram of an embodiment illustrating a portion of an optical receiver for use withoptical communication system 10 ofFIG. 1 . According to the illustrated embodiment,optical receiver 30 includes anoptical delay interferometer 32, asignal monitor 34, and aprocessor 38 coupled as shown inFIG. 3 . A photodiode or other detector (not shown) may be coupled to the output of the optical delay interferometer, between theoptical delay interferometer 32 and the signal monitor 34 or otherwise inreceiver 30. Thus, signal monitor 34 may monitor the interfered signal, a decoded signal, a recovered signal, or other signal that is at least based on the interfered signal. -
Optical delay interferometer 32 receives ingressoptical information signal 24 and demodulates it to generate an interferedsignal 28. According to one embodiment,optical delay interferometer 32 is an optical device operable to split a received signal into two portions. Additionally,optical delay interferometer 32 induces a relative delay, or shift, between the portions of the split signal.Optical delay interferometer 32 optically interferes the portions of the split signals into at least one interferedsignal 28. According to one embodiment,optical delay interferometer 32 comprises a Mach-Zehnder interferometer (MZI). Any other optical device suitable for converting phase modulated (PM) signals into interferedsignal 28 may be used without departing from the scope of the invention. For example,optical delay interferometer 32 may comprise a polarization maintaining fiber (PMF), or a birefringer fiber. - Before being monitored by
signal monitor 34, interferedsignal 28 may be detected by a photodetector to generate a detected signal. By photodetecting interferedsignal 28 before monitoring, signal monitor 34 may receive a photocurrent and a photovoltage associated withoptical information signal 24. Additionally, as will be described in more detail below, signalmonitor 34 andoptical delay interferometer 32 may be DC biased to allow a DC current corresponding to the detected signal to be monitored bysignal monitor 34. - Signal monitor 34 monitors quality criteria corresponding to a detected signal. According to one embodiment, the quality criteria may comprise a Bit Error Rate (BER), extinction ratio, and an optical power. Any other suitable criteria indicative of the quality of a detected or other signal may be used. Signal monitor 34 may comprise an RF monitor, a DC monitor, a trans-impedence amplifier, a controller, a forward error correction unit (FEC), or any other device suitable for monitoring a signal. Embodiments of signal monitor 34 will be described in more detail with respect to
FIGS. 5A-5D . -
Processor 38 receives monitored quality criteria information to adjust the relative delay ofoptical delay interferometer 32. The information may be, for example, the value of a quality criteria, an indication of any change, a value of any change, an indication that a level or limit has been exceeded or the like. For example,processor 38 may receive a measurement of optical power, whichprocessor 38 may use to generate afeedback signal 42. Usingfeedback signal 42,processor 38 may control and adjust the relative delay induced byoptical delay interferometer 32. In one embodiment,processor 38 usesfeedback signal 42 to adjust the temperature ofoptical delay interferometer 32 in order to adjust the relative delay of the split signal portions that combine to form interferedsignal 28. In another embodiment, the relative delay may be adjusted mechanically such as with a free-space optics system.Processor 38 may comprise any suitable logic, whether hardware and/or software, operable to control the relative delay atoptical delay interferometer 32 based on information fromsignal monitor 34. - Modifications, additions, or omissions may be made to the optical receiver portion without departing from the scope of the invention. For example, a Bias-T module may be coupled to
optical delay interferometer 32 and signal monitor 34 to perform DC biasing. Additionally, functions may be performed using any suitable logic compressing software, hardware, other logic, or any suitable combination of the proceeding. -
FIGS. 4A and 4B illustrate embodiments ofoptical delay interferometer 32 for use inoptical communications system 10 ofFIG. 1 .FIG. 4A illustrates one embodiment of a Mach-Zehnder interferometer (MZI) 60. According to the illustrated embodiment,MZI 60 includes anupper arm 62 and alower arm 64.MZI 60 converts a phase modulated (PM) signal into an amplitude modulated (AM) signal by optically delaying one portion of the signal with respect to the other and applying additional phase shift between the two arms of the MZI. The amount of relative delay equals approximately the symbol period, or can be equal to any integer multiple of the symbol period. The relative phase shift between the upper arm and the lower arm may vary from .pi./2 to −.pi./2. - The
upper arm signal 62 andlower arm signal 64 are interferometrically combined. For example,MZI 60 achieves constructive interference if the phase of theupper arm signal 62 orlower arm signal 64 are in phase.MZI 60 achieves destructive interference if the phase difference betweenupper arm 62 andlower arm 64 is .pi./2. Typically, the IMDPSK signal, oroptical information signal 24, received atMZI 60, is split into two arms with a splitting ratio of 0.5. Additionally,upper arm signal 62 andlower arm signal 64 may be combined using a 3 dB coupling. According to one embodiment, the relative phase shift and relative delay may be adjusted by controlling the temperature of a waveguide associated withMZI 60. According to one embodiment,MZI 60 may be implemented using planar lightwave circuit (PLC) technology. The PLC may be implemented on a silica substrate, and the relative phase shift between the two arms may be controlled by changing the substrate temperature. - According to the illustrated embodiment,
MZI 60 may include egress optical port A and egress optical port B, which may be complimentary to each other. For example, ifupper arm signal 62 andlower arm signal 64 have a phase difference of 180 degrees,MZI 60 achieves destructive interference of the current received bit and the previous received bit resulting in optical port A generating a low level signal. - The above-stated example may be more particularly represented by describing a transmitted IMDPSK signal s(t) by Equation (1):
-
s(t)=A sin(2.pi.rt)cos(2.pi.ft+.phi.(t)) (1) - where r is the bit rate, f is the carrier frequency for the bit rate r, .phi.(t) is the data encoded as a phase of the carrier signal, using 180 degrees for
bit 1 and zero degrees for bit 0. From Equation (1) a signal at optical port A ofMZI 60 may be described by Equation (2): -
A(t)=(A/2)sin(2.pi.rt)[cos(2.pi.ft+.phi.(t)+.pi.+cos(2.pi.ft+.phi.(t−T)+.T−HETA.−2.pi.fT)] (2) - where T=1/r and defines the bit period, .THETA. represents the phase difference between
upper arm 62 andlower arm 64, and 2.pi.fT represents a relative phase difference introduced due to the relative delay of one bit period. - From Equation (2) the relative phase difference 2.pi.fT introduced due to bit delay depends on the carrier frequency f for a given bit rate r. If the frequency drifts from f to f+.DELTA., the relative phase difference in the delay interferometer would be changed as described by Equation (3):
-
DELTA..theta.=2.pi.(f+.DELTA.f)T−2.pi.(f)T=2.pi.fT (3) - Since the frequency of the transmitter carrier drifts slowly with time, the relative phase difference .DELTA..theta. may be cancelled by adjusting the phase change .THETA.. This can be achieved, for example, by tuning the temperature of the PLC substrate. If the geometrical path lengths of the upper and lower arms are L.sub.U and L.sub.L, respectively, and the temperature coefficient of the effective refractive index of the waveguides is k (/degree), the required temperature change .DELTA.T to provide the phase change .THETA. may be described by Equation (4):
-
.THETA.=(L.sub.U−L.sub.L)k.DELTA.T/c (4) - where c is the speed of light in a vacuum.
- By monitoring quality criteria of the signals in at least one of the output ports of
MZI 60, the frequency drift may be tracked so that an adjustment to the relative phase difference or delay ofupper arm 62 andlower arm 64 may be adjusted. Feedback control and monitoring signal quality criteria will be described with reference toFIG. 5 . -
FIG. 4B illustrates an embodiment of a polarization maintaining fiber (PMF) 65 for use asoptical delay interferometer 32. Apolarization controller 67 may be used to receive IMDPSK signals and control the polarization of the IMDPSK signal to a 45 degree linear polarization.PMF 65 carriespolarized signal 68 over a fiber of length L and birefringence of (n1−n2). Similarly to the embodiment ofMZI 60,PMF 65 introduces a relative delay between two signals. This relative delay may be described by Equation (5): -
L(n1−n2)/c (5) - where L is the length of
PMF 65, (n1−n2) represents the birefringence ofPMF 65. -
PMF 65 carries polarized light, where the orthogonal polarization components may be interfered and received byanalyzer 69.Analyzer 69 receives polarizedsignal 68 and monitors the quality criteria of the interfered signal. According to another embodiment,analyzer 69 may comprise a polarization beam splitter (PBS) to splitpolarized signal 68 into two linearly polarized signals orthogonal to each other that may be used when monitoring signals from more than one port ofoptical delay interferometer 32. -
FIGS. 5A through 5D illustrate embodiments of feedback control and signal monitoring that may be used with theoptical communication system 10 ofFIG. 1 . For example,FIG. 5A illustrates one embodiment of automatic feedback control using monitoring of extinction ratio, whileFIGS. 5B through 5D illustrate embodiments of feedback control using optical power monitoring. Other suitable circuits may be used without departing from the scope of the present invention. -
FIG. 5A is a block diagram of an embodiment illustrating an automatic feedback control using monitoring of extinction ratio. As used in this document, the extinction ratio of an amplitude modulated (AM) signal may be defined as the ratio of the power levels corresponding tolevel 1 and level 0. According to one embodiment, when the relative phase difference between the two arms of theoptical delay interferometer 32 is optimum, the extinction ratio is maximum. -
Photodetector 70 receives interferedsignal 28 fromoptical delay interferometer 32 and generates a detectedsignal 29. Detectedsignal 29 is split by a Bias-T module 72, which is operable to split a high frequency component from a low frequency component of the signal. The high frequency component, or RF signal, may be received by RF monitor 74 and the low frequency component, or DC component, may be received by DC monitor 76. RF monitor 74 can detect reductions in RF power of detectedsignal 29 and therefore reductions in RF power ofoptical information signal 24. RF monitor 74 provides the RF power measurements toprocessor 38 where the RF power measurements are normalized using a DC power measurement obtained by DC monitor 76. By normalizing the measured RF power, fluctuations caused by changing input optical power may be eliminated. -
Processor 38 may determine from a decrease in RF power and normalized power, that an extinction ratio decreases. Accordingly,processor 38 may adjust the relative delay ofoptical delay interferometer 32 in order to improve the extinction ratio. In one embodiment, the relative delay may be increased by increasing the extinction ratio. -
FIG. 5B is a block diagram illustrating an automatic feedback control using optical power monitoring. In operation, the optical power of an output ofoptical delay interferometer 32 may be used to adjust or optimize the phase difference or the relative delay ofoptical delay interferometer 32. In order to enable this function, the phase modulation depth may be reduced at the transmitter by decreasing the driving voltage ofphase modulator 52 as was described with reference toFIG. 2 . Although reducing the driving voltage may also lead to an eye opening penalty, reducing the phase modulation depth may increase the degree of control corresponding to monitoring optical power to adjust the relative phase difference at theoptical delay interferometer 32. In one embodiment, an eye opening penalty may be less than 0.2 dB for a reduction in driving voltage at the transmitter resulting in a reduction of phase modulation depth of 2.alpha.>150 degrees. Any other suitable reduction in driving voltage may be selected to monitor the optical power without departing from the scope of the present invention. - Optical power is monitored based on a DC current received by DC monitor 76 after a Bias-
T module 72 splits detectedsignal 29. The RF component of detectedsignal 29 may be received by trans-impedance amplifier 78 for conversion and amplification of the signal into an electrical signal that may be used bycontroller 80 to receive the data. DC monitor 76 measures an average optical power corresponding to the detectedsignal 29 from a single port atoptical delay interferometer 32.Processor 38 receives the measured average optical power and generates afeedback signal 42 to adjust the relative delay and to maximize the detectedsignal 29. The controlling algorithm to determine thefeedback signal 42 so that the detectedsignal 29 be maximized may be found in the public domain. -
FIG. 5C is a block diagram illustrating another embodiment of an automatic feedback control using optical power monitoring. In this embodiment, both optical port outputs ofoptical delay interferometer 32 are detected byphotodetectors FIGS. 5A and 5B , a Bias-T module 72 may be used to split a signal into an RF component and DC component. The DC current associated with balanced photodetected signal 71 is received by DC monitor 76 to measure the optical power. According to the illustrated embodiment, DC monitor 76 measures the DC voltage which may be proportional to the average optical power difference of the signals detected by eachphotodetector processor 38 may determinefeedback signal 42. As was discussed with reference toFIG. 5B , the average power difference between the two output ports as measured by DC monitor 76 is used byprocessor 38 to determine if the relative delay atoptical delay interferometer 32 may be adjusted. For example, maintaining an average optical power difference at or near a maximum may be accomplished byprocessor 38 adjusting the relative phase difference to as close as possible to 180 degrees. -
FIG. 5D is a block diagram illustrating yet another embodiment of an automatic feedback control using optical power monitoring of two optical ports ofoptical delay interferometer 32. Similarly to the embodiment described with reference toFIG. 5C , the optical port signals ofoptical delay interferometer 32 are each detected byphotodetectors signal 73 a is used for signal reception by directing its RF signal component trans-impedance amplifier 78 andcontroller 80. In contrast, detectedsignal 73 b may be dedicated to measuring optical power by DC monitor 76. As was described with reference toFIG. 5C , DC monitor 76 measures the optical power of detectedsignal 73 b and compares it to an average optical power so thatprocessor 38 may determinefeedback signal 42 that adjusts the relative delay atoptical delay interferometer 32. - Modifications, additions, or omissions may be made to the embodiments shown with reference to
FIGS. 5A through 5D without departing from the scope of the invention. For example, although a Bias-T module 72 is shown to include a capacitor C and an inductor L, any other, or additional components suitable for splitting a signal into an RF signal and DC current may be used without departing from the scope of the invention. As another example, with reference toFIG. 5A , DC monitor 76 may be modified to include a forward error correction (FEC) unit to monitor a bit error rate corresponding to the detectedsignal 29 that may be used byprocessor 38 to adjust the relative delay atoptical delay interferometer 32. Additionally, functions may be performed using any suitable logic comprising software, hardware, other logic, or any suitable combination of the preceding. -
FIG. 6 is a flowchart of a method for automatic feedback control for fine tuning a delay interferometer. It will be understood that the method may be used in connection with any number ofoptical receivers 30 and any number of optical information signals 24. - The method begins at
step 100 whereoptical delay interferometer 32 receives a modulated signal. According to the illustrated embodiment, the modulated signal comprises an intensity modulated DPSK signal (IMDPSK). The method proceeds to step 102, whereoptical delay interferometer 32 generates one or more interfered signals. As was described with reference toFIGS. 4A and 4B ,optical delay interferometer 32 may split a signal into a plurality of signals, delay one of the signals relative to the other, and interfere the plurality of signals. For example, according to the embodiment described with reference toFIG. 4A ,MZI 60 splits the signal into anupper arm signal 62 and alower arm signal 64 and introduces a relative delay atupper arm signal 62 with reference tolower arm signal 62. - The method proceeds to step 104, where signal monitor 34 monitors signal quality criteria of at least one interfered signal. According to one embodiment, signal monitor 34 monitors a detected signal using a bit error rate (BER), an extinction ratio, and an optical power. At
step 106,processor 38 determines the signal adjustment based on the monitored quality criteria. For example, if signal monitor 38 monitors the BER of a detected signal,processor 38 determinesfeedback signal 42 comprising forward error correction information based on the monitored BER. - The method then proceeds to step 108, where
processor 38 adjusts the phase shift of the interfered signals based on the monitored quality criteria. For example,processor 38 may use afeedback signal 42 comprising forward error correction information to adjust the relative delay between theupper arm 62 andlower arm 64 ofoptical delay interferometer 32. After adjusting the phase shift atstep 108, the method terminates. The method may be repeated continuously, or periodically, for example, once a second, a few times a minute, once per bit period T, or otherwise. - Steps may be added, omitted, modified, or performed in any suitable order without departing from the scope of the invention. For example, a step of detecting at a photodetector an optical signal may be added between the steps of generating interferometer paths at
step 102 and monitoring signal properties atstep 104. As another example, a step may be added after generating interferometer paths according to a phase shift atstep 102, where the interferometer paths are optically interfered to yield an interfered signal. - Certain embodiments may provide one or more technical advantages. A technical advantage of one embodiment may include improving signal quality at the receiver. More particularly, signal degradation may be reduced or eliminated at the optical receiver by providing for automatic feedback control of a delay interferometer. Another technical advantage of one or more embodiments may include accurate and efficient fine-tuning of a delay interferometer by monitoring quality criteria of optical signals. Yet another advantage of one or more embodiments may include improving DPSK/IMDPSK systems.
- Another technical advantage of one or more embodiments includes using quality criteria to automatically adjust the optical signal at the optical delay interferometer. Therefore, an optical communications system may be more cost effective since the optical delay interferometer may not be adjusted manually and drift tolerances associated with transmitter lasers may be lessened. Yet another technical advantage of one or more embodiments may include the use of DPSK/IMDPSK technology with Ultra Long Haul (ULH) systems due to improved tolerance to non-linear effects, optical signal-to-noise ratio (OSNR) and dispersion.
- Although an embodiment of the disclosure and its advantages are described in detail, a person skilled in the art could make various alterations, additions, and omissions without departing from the spirit and scope of the invention as defined by the appended claims.
Claims (11)
1. A method for receiving an optical signal, comprising: splitting an ingress signal into a first portion and second portion; inducing a relative delay between the first portion and the second portion; optically interfering the first and second portions to generate at least one interfered signal; monitoring a Bit Error Rate (BER) estimate of a monitored signal at least based on the at least one interfered signal; and adjusting the relative delay based on the BER estimate.
2. The method of claim 1 , wherein the BER estimate is obtained from a number of errors corrected per frame detected by a Forward Error Correction (FEC) unit.
3-24. (canceled)
25. A method for transmitting and receiving an optical signal, comprising:
transmitting the optical signal using a transmitter;
receiving the optical signal using a receiver, the receiver including an interferometer, the interferometer: splitting the optical signal into a first portion and second portion; inducing a phase shift between the first portion and the second portion; and optically interfering the first and second portions to generate at least one interfered signal;
monitoring a Bit Error Rate (BER) estimate of a monitored signal at least based on the at least one interfered signal; and
adjusting the phase shift based on the BER estimate.
26. The method as recited in claim 25 wherein the transmitter and receiver are part of a WDM system.
27. The method as recited in claim 25 wherein the phase shift is adjusted by adjusting a relative delay in the interferometer.
28. The method as recited in claim 25 wherein the receiver includes a device for adjusting the phase shift between the first and second portions.
29. A method for transmitting and receiving an optical signal, comprising:
transmitting the optical signal using a transmitter;
receiving the optical signal using a receiver, the receiver including an interferometer, the interferometer: splitting the optical signal into a first portion and second portion; inducing a phase shift between the first portion and the second portion; and optically interfering the first and second portions to generate at least one interfered signal;
monitoring a Bit Error Rate (BER) estimate of a monitored signal at least based on the at least one interfered signal; and
canceling phase shifts in the interferometer by adjusting a phase shift between the first portion and the second portion.
30. The method as recited in claim 29 wherein the receiver includes a device for adjusting the phase shift between the first and second portions.
31. The method as recited in claim 29 further comprising feeding back the BER estimate.
32. The method as recited in claim 29 wherein the canceled phase shifts are caused by the transmitter.
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2009
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Also Published As
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EP1703651A1 (en) | 2006-09-20 |
US7869719B2 (en) | 2011-01-11 |
US20090214203A1 (en) | 2009-08-27 |
DE602004025742D1 (en) | 2010-04-08 |
US7603045B2 (en) | 2009-10-13 |
JP2005080304A (en) | 2005-03-24 |
EP1513273A3 (en) | 2006-03-29 |
EP1513273A2 (en) | 2005-03-09 |
JP4583840B2 (en) | 2010-11-17 |
EP1703651B1 (en) | 2010-02-24 |
US20050047780A1 (en) | 2005-03-03 |
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