WO2014113275A1 - Optical polarization demultiplexing for a coherent-detection scheme - Google Patents
Optical polarization demultiplexing for a coherent-detection scheme Download PDFInfo
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- WO2014113275A1 WO2014113275A1 PCT/US2014/010950 US2014010950W WO2014113275A1 WO 2014113275 A1 WO2014113275 A1 WO 2014113275A1 US 2014010950 W US2014010950 W US 2014010950W WO 2014113275 A1 WO2014113275 A1 WO 2014113275A1
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04J—MULTIPLEX COMMUNICATION
- H04J14/00—Optical multiplex systems
- H04J14/06—Polarisation multiplex systems
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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
- H04B10/61—Coherent receivers
- H04B10/614—Coherent receivers comprising one or more polarization beam splitters, e.g. polarization multiplexed [PolMux] X-PSK coherent receivers, polarization diversity heterodyne coherent receivers
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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
- H04B10/61—Coherent receivers
- H04B10/616—Details of the electronic signal processing in coherent optical receivers
- H04B10/6166—Polarisation demultiplexing, tracking or alignment of orthogonal polarisation components
Definitions
- the present disclosure relates to optical communication equipment and, more specifically but not exclusively, to coherent detection of modulated optical signals.
- Coherent detection is used in optical transport systems, for example, because it is compatible with advanced modulation formats, such as quadrature phase-shift keying (QPSK) and quadrature- amplitude modulation (QAM), which enable the QPSK and QPSK.
- QPSK quadrature phase-shift keying
- QAM quadrature- amplitude modulation
- an optical receiver having a plurality of optical IQ modulators arranged in a butterfly configuration and configured to operate as an optical polarization de-multiplexer.
- the optical receiver further has (i) an opto-electric circuit configured to apply optical homodyne detection to an optical input signal received by the optical receiver and (ii) a controller configured to generate one or more control signals for driving the IQ modulators based on one or more electrical feedback signals received from the opto-electric circuit.
- an apparatus comprising: a first optical IQ modulator configured to modulate a first portion of a first polarization of an optical polarization-division-multiplexed (PDM) signal to generate a first modulated optical signal; a second optical IQ modulator configured to modulate a second portion of the first polarization of the optical PDM signal to generate a second modulated optical signal; a third optical IQ modulator configured to modulate a first portion of a second polarization of the optical PDM signal to generate a third modulated optical signal; and a fourth optical IQ modulator configured to modulate a second portion of the second polarization of the optical PDM signal to generate a fourth modulated optical signal.
- the apparatus is configured to: combine the first modulated optical signal and the third modulated optical signal to generate a first optical output signal; and combine the second modulated optical signal and the fourth modulated optical signal to generate a second optical output signal.
- the apparatus further comprises a polarization beam splitter configured to split the optical PDM signal into the first and second polarizations, wherein the first polarization is orthogonal to the second polarization.
- the apparatus further comprises a controller configured to generate one or more control signals for driving the first, second, third, and fourth IQ modulators in a manner that causes (i) the first optical output signal to represent a first PDM component of the optical PDM signal and (ii) the second optical output signal to represent a second PDM component of the optical PDM signal.
- the controller is further configured to change at least one of the one or more control signals to track temporal changes in a state of polarization of the optical PDM signal.
- the controller is further configured to update said at least one control signal using an electrical measure of misalignment between the state of polarization of the optical PDM signal and principal polarization axes of the apparatus.
- the apparatus further comprises: a first opto-electric circuit configured to apply optical homodyne detection to the first optical output signal to recover data carried by the first PDM component of the optical PDM signal; and a second opto-electric circuit configured to apply optical homodyne detection to the second optical output signal to recover data carried by the second PDM component of the optical PDM signal.
- the controller is configured to generate the one or more control signals for driving the first, second, third, and fourth IQ modulators based on: one or more electrical feedback signals received from the first and second opto-electric circuits; and a parameter representing an average amplitude in a constellation used to generate the optical PDM signal.
- the constellation is a quadrature- amplitude modulation constellation, in which at least two constellation points have different respective amplitudes.
- the first opto-electric circuit comprises: a first optical hybrid configured to optically mix the first optical output signal and an optical local-oscillator, "OLO," signal to generate a first plurality of mixed optical signals; a first optical-to-electrical converter configured to convert a first subset of the first plurality of mixed optical signals into a first electrical analog signal (e.g., 120i x ); and a second optical-to-electrical converter configured to convert a second subset of the first plurality of mixed optical signals into a second electrical analog signal (e.g., 120 Qx ).
- the second opto-electric circuit comprises: a second optical hybrid configured to optically mix the second optical output signal and the
- OLO signal to generate a second plurality of mixed optical signals; a third optical-to- electrical converter configured to convert a first subset of the second plurality of mixed optical signals into a third electrical analog signal (e.g., 120 ly ); and a fourth optical-to- electrical converter configured to convert a second subset of the second plurality of mixed optical signals into a fourth electrical analog signal (e.g., 120 ⁇ 3 ⁇ 4,).
- the controller is configured to generate the control signals for driving the first, second, third, and fourth IQ modulators using the first, second, third, and fourth electrical analog signals.
- the controller is configured to generate a first set of the one or more control signals using the following quantity: I 2 +Q 2 -R 2 ; and generate a second set of the one or more control signals using the following quantity: I y 2 +Q y 2 -R 2 , where I x , Q x , I y , and Q y are magnitudes of the first, second, third, and fourth electrical analog signals, respectively; and R is a parameter representing an average amplitude in a constellation used to generate the optical PDM signal.
- the controller is configured to generate a first one of the one or more control signals using the following quantity: I x (I 2 +Q 2 -R 2 ); generate a second one of the one or more control signals using the following quantity: Q x (I 2 +Q 2 -R 2 ); generate a third one of the one or more control signals using the following quantity: I y (I 2 +Q 2 -R 2 ); generate a fourth one of the one or more control signals using the following quantity: Q y (I 2 +Q 2 -R 2 ); generate a fifth one of the one or more control signals using the following quantity: I x (I 2 +Q 2 -R 2 ); generate a sixth one of the one or more control signals using the following quantity: Q x (I 2 +Q 2 -R 2 ); generate a seventh one of the one or more control signals using the following quantity: I y (I 2 +Q 2 -R 2 ); and generate a eighth one of the one or more control signals using the
- the apparatus is configured to drive the first optical IQ modulator using the first and second control signals; drive the second optical IQ modulator using the third and fourth control signals; drive the third optical IQ modulator using the fifth and sixth control signals; and drive the fourth optical IQ modulator using the seventh and eighth control signals.
- the first opto-electric circuit comprises: a first optical hybrid configured to optically mix the first optical output signal and an optical local-oscillator (OLO) signal to generate a first plurality of mixed optical signals; a first optical-to-electrical converter configured to convert a first subset of the first plurality of mixed optical signals into a first electrical analog signal (e.g., 120i y ); a second optical-to-electrical converter configured to convert a second subset of the first plurality of mixed optical signals into a second electrical analog signal (e.g., 120 ⁇ 3 ⁇ 4,); a first slicer circuit configured to slice the first electrical analog signal to generate a first electrical digital signal (e.g., 128 ly ); and a second slicer circuit configured to slice the second electrical analog signal to generate a second electrical digital signal (e.g., 128 ⁇ 3 ⁇ 4,).
- a first optical hybrid configured to optically mix the first optical output signal and an optical local-oscillator (OLO) signal to generate a first plurality of mixed
- the apparatus further comprises a phase detector configured generate an electrical measure of a phase difference between the OLO signal and a carrier wave of the optical PDM signal, with said electrical measure being generated using the first electrical analog signal, the first electrical digital signal, the second electrical analog signal, and the second electrical digital signal.
- the apparatus is configured to use said electrical measure to phase-lock the OLO signal and the carrier wave of the optical PDM signal.
- said electrical measure is proportional to the following quantity: I' y Q y - Q' y I y , where I' y , Q y , Q' y , and I y are magnitudes of the first electrical digital signal, the second electrical analog signal, the second electrical digital signal, and the first electrical analog signal, respectively.
- the phase detector comprises: a first signal multiplier configured to multiply the first electrical analog signal and the first electrical digital signal to generate a first multiplied electrical signal; a second signal multiplier configured to multiply the second electrical analog signal and the second electrical digital signal to generate a second multiplied electrical signal; and a signal adder configured to generate the electrical measure based on a difference between the second multiplied electrical signal and the first multiplied electrical signal.
- the apparatus further comprises an optical local-oscillator (OLO) source configured to generate an OLO signal used in the optical homodyne detections in the first and second opto-electric circuits, wherein the OLO source comprises: a laser configured to generate an optical beam; a fifth optical IQ modulator configured to optically modulate the optical beam to generate a modulated optical beam; and an optical filter configured to filter the modulated optical beam to generate the OLO signal.
- OLO optical local-oscillator
- the fifth optical IQ modulator is configured to optically modulate the optical beam using a feedback signal received from either the first opto-electric circuit or the second opto-electric circuit.
- the fifth optical IQ modulator is configured to optically modulate the optical beam using carrier- suppression modulation; and the optical filter is configured to pass a modulation sideband of the modulated optical beam and to block a residual carrier of the optical beam generated by the laser, wherein the OLO signal comprises light of the modulation sideband that passed the optical filter.
- At least one of the first, second, third, and fourth IQ modulators comprises a nested Mach-Zehnder modulator.
- a method of polarization rotation comprising: modulating a first portion of a first polarization of an optical
- FIG. 1 shows a block diagram of a coherent receiver according to an embodiment of the disclosure
- FIG. 2 shows a block diagram of an optical polarization de-multiplexer that can be used in the coherent receiver shown in FIG. 1 according to an embodiment of the disclosure
- FIGs. 3A-3B show block diagrams of a polarization controller that can be used in the coherent receiver shown in FIG. 1 according to an embodiment of the disclosure
- FIG. 4 shows a block diagram of a phase detector that can be used in the coherent receiver shown in FIG. 1 according to an embodiment of the disclosure
- FIG. 5 shows a block diagram of a light source that can be used in the coherent receiver shown in FIG. 1 according to an embodiment of the disclosure.
- FIG. 6 shows a block diagram of an IQ modulator that can be used in the optical polarization de-multiplexer shown in FIG. 2 according to an embodiment of the disclosure.
- FIG. 1 shows a block diagram of a coherent receiver 100 according to an embodiment of the disclosure.
- Receiver 100 is configured to receive a polarization- division-multiplexed (PDM) optical input signal 102, e.g., from a remote transmitter (not explicitly shown in FIG. 1), and process the received signal, e.g., as further described below, to recover the data carried by that signal.
- Receiver 100 then directs the recovered data, via output data streams 198x and 198y, to the intended data destination(s).
- receiver 100 may include a multiplexer (not explicitly shown in FIG. 1) configured to combine data streams 198x and 198y into a corresponding single output data stream.
- optical input signal 102 is delivered to receiver 100 via an optical fiber (not explicitly shown in FIG. 1).
- Different sections of the optical fiber typically have different principal states of polarization (PSPs) that are not necessarily aligned with the original (e.g., independently modulated) PDM components of signal 102.
- PSPs principal states of polarization
- This misalignment causes each of the PDM components to generally have a nonzero projection on each of the PSPs, which mixes the PDM components and alters the polarization state of the optical signal.
- the PSPs of the optical fiber tend to change over time, e.g., due to varying physical conditions along the length of the fiber. This temporal variation causes the polarization state of optical input signal 102 to vary over time as well.
- An optical polarization de-multiplexer (D-MUX) 106 serves to undo, to a significant extent, the polarization rotation imposed onto optical input signal 102 by the optical fiber. More specifically, polarization D-MUX 106 is configured to demultiplex optical input signal 102 into optical signals 108x and 108y in a manner that causes each of optical signals 108x and 108y to represent a respective one of the original PDM components generated at the remote transmitter with as little crosstalk from the other PDM component as practically possible.
- the configuration of polarization D-MUX 106 is controllable via a multi-component control signal 104 generated by a controller 122.
- controller 122 can configure polarization D-MUX 106 to appropriately track the polarization- rotation changes in the optical fiber, thereby enabling receiver 100 to detect and decode optical signals 108x and 108y in a relatively straightforward manner, e.g., without the need for relatively complicated polarization-de-multiplexing processing in the electrical digital domain.
- controller 122 generates control signal 104 using a blind polarization-equalization scheme, e.g., as described in more detail below in reference to FIGs. 2 and 3.
- a blind polarization-equalization scheme e.g., as described in more detail below in reference to FIGs. 2 and 3.
- One beneficial feature of this scheme is that it is independent of and does not rely on the phase and frequency locking implemented in the receiver's optical phase-lock loop comprising a phase detector 140, a loop filter 144, and an optical local-oscillator (OLO) source 148.
- polarization D-MUX 106 can be implemented as a single-tap optical equalizer (see, e.g., FIG. 2) or a multi-tap optical equalizer.
- the additional taps in polarization D-MUX 106 can be used, e.g., to implement some optical dispersion-compensation processing (e.g., polarization-mode- dispersion compensation) in addition to optical polarization de-multiplexing.
- Each of optical signals 108x and 108y is applied to a corresponding one of optical hybrids llOx and HOy.
- An optical hybrid 110 has (i) two input ports labeled S and R and (ii) four output ports labeled 1 through 4.
- Input port S is configured to receive a corresponding one of optical signals 108x/108y.
- Input port R is configured to receive a respective portion (e.g., an attenuated copy) of an optical reference (e.g., OLO) signal 150 generated by OLO source 148.
- OLO signal 150 is phase- and frequency-locked to the carrier wave of optical input signal 102, e.g., as described in more detail below in reference to FIGs. 4 and 5, to enable homodyne detection of signals 108x/108y in receiver 100.
- optical hybrid 110 splits each of the signals received at ports S and R into two (attenuated) copies, e.g., using a conventional 3-dB power splitter (not explicitly shown in FIG. 1). A relative phase shift of about 90 degrees ( ⁇ /2 radian) is then applied to one copy of the signal received at port S and one copy of the signal received at port R, e.g., using phase shifters (not explicitly shown in FIG. 1). The various signal copies are then optically mixed with each other, and the resulting four mixed signals are directed to output ports 1-4, respectively.
- Exemplary optical hybrids that can be used to implement optical hybrids 110 in receiver 100 are described, e.g., in U.S. Patent Application Publication Nos. 2007/0297806 and 2011/0038631, both of which are incorporated herein by reference in their entirety.
- each O/E converter 116 comprises a pair of photo-detectors (e.g., photodiodes, not explicitly shown in FIG. 1) serially connected in a differential configuration, with each photo-detector configured to receive a corresponding one of the mixed signals received from the corresponding output port of the corresponding optical hybrid 110.
- the electrical output signal generated by each pair of photo- detectors is optionally amplified to generate a corresponding one of electrical signals 120i x , 120 Qx , 120i y , and 120 Qy .
- the sampling rate of O/E converters II6 1 -II6 4 is controlled by a clock signal (CLK) generated by a clock-recovery circuit 130.
- clock-recovery circuit 130 generates clock signal CLK in a manner that causes the clock signal to have a frequency that is equal to the per-polarization symbol rate of optical input signal 102.
- Representative examples of clock-recovery circuits that can be used as clock-recovery circuit 130 in receiver 100 are disclosed, e.g., in U.S. Patent Nos. 8,290, 107, 8, 190,032, 7,805,083, and 7,209,666, all of which are incorporated herein by reference in their entirety.
- electrical signals 120 lx and 120 Qx represent the in-phase (I) and quadrature (Q) components, respectively, of optical signal 108x.
- electrical signals 120 ly and 120( 3 ⁇ 4 , represent the I and Q components, respectively, of optical signal 108y.
- Copies of electrical signals 120 lx , 120 Qx , 120 ly , and 120Q y are supplied to controller 122, which uses them to generate control signal 104.
- Copies of electrical signals 120 ly and 120( 3 ⁇ 4 are also supplied to phase detector 140, which uses them to enable phase and frequency locking of OLO signal 150 to the carrier wave of optical input signal 102.
- A/D analog-to-digital
- Digital signals 128 lx , 128 Qx , 128 ly , and 128Q y produced by A/D slicer circuits 124i-
- decoders 136i and 130 2 are then decoded in a conventional manner by decoders 136i and 130 2 to generate data streams 198x and 198y.
- decoders 136i and 130 2 may be configured to coordinate their decoding operations, e.g., using a communication path 138 between them.
- FEC forward-error- correction
- clock-recovery circuit 130 is configured to generate clock signal CLK based on digital signal 128 lx .
- clock-recovery circuit 130 can be configured to generate clock signal CLK based on any of digital signals 128 Qx , 128 ly , and 128 Qy , or based on any suitable combination of digital signals 128 lx , 128Q x , 128 ly , and 128Q y .
- phase detector 140 which uses them, together with electrical signals 120 ly and 120 Qy , to enable phase and frequency locking of OLO signal 150 to the carrier wave of optical input signal 102.
- an electrical phase-error signal 142 generated by phase detector 140 has a voltage proportional to the phase difference between OLO signal 150 and the carrier wave of optical input signal 102.
- Loop filter 144 integrates and low-pass filters, in continuous time, phase-error signal 142 to generate a drive signal 146 for OLO source 148.
- frequency characteristics of loop filter 144 determine the overall dynamic time-response characteristics of the receiver's optical phase-lock loop. For example, a narrower bandwidth of loop filter 144 generally enables a more accurate phase lock, e.g., a smaller phase error. The higher accuracy however comes at the expense of a slower time response and a narrower capture range.
- phase detector 140 can be configured to use copies of signals 128 lx , 128Q x , 120 1x , and 120Q x instead of signals 128 ly , 128Q y , 120 ly , and 120 Qy , respectively.
- FIG. 2 shows a block diagram of an optical polarization D-MUX 200 that can be used as optical polarization D-MUX 106 (FIG. 1) according to an embodiment of the disclosure.
- polarization D-MUX 200 is shown in FIG. 2 as being configured to: (i) receive optical input signal 102; (ii) generate optical signals 108x and 108y; and (iii) receive control signal 104 having signal components labeled 104i-104 4 .
- each of signal components labeled 104i-104 4 can itself be a multi-component signal.
- Optical input signal 102 is applied to a polarization beam splitter 202
- each of optical signals 204x and 204y generally has contributions from both PDM components of optical input signal 102.
- optical signal processing implemented in polarization D- MUX 200 causes (i) optical signal 108x to primarily carry hght corresponding to the first PDM component of optical input signal 102 with relatively small (e.g., negligible, for practical purposes) amounts of hght corresponding to the second PDM component of the optical input signal and (ii) optical signal 108y to primarily carry hght corresponding to the second PDM component of the optical input signal with relatively small amounts of hght corresponding to the first PDM component of the optical input signal.
- These properties of optical signal 108x and 108y are beneficial, e.g., because they enable the subsequent decoding of these signals to be relatively straightforward and the corresponding signal-processing circuitry to be relatively inexpensive.
- the optical signal processing in polarization D-MUX 200 is implemented using four optical IQ modulators 220n, 22O21, 220 12 , and 22 ⁇ 22 arranged in a butterfly configuration, as indicated in FIG. 2. More specifically, each of IQ modulators 220n and 22O21 is configured to receive an attenuated copy of optical signal 204x generated by a polarization-maintaining coupler (PMC) 210i. Similarly, each of IQ modulators 220i 2 and 22 ⁇ 22 is configured to receive an attenuated copy of optical signal 204y generated by a polarization-maintaining coupler 2IO2.
- PMC polarization-maintaining coupler
- the output signals generated by IQ modulators 220n and 220i 2 are combined in a polarization-maintaining coupler 2IO 3 to generate optical signal 108x.
- the output signals generated by IQ modulators 22O21 and 22 ⁇ 22 are similarly combined in a polarization-maintaining coupler 210 4 to generate optical signal 108y.
- each of optical IQ modulators 220n, 22O21, 220 12 , and 22 ⁇ 22 can be implemented using a Mach-Zehnder modulator that is commercially available from Fujitsu Optical Components Limited of Japan as part number
- each of optical IQ modulators 220n, 22O21, 220 12 , and 22 ⁇ 22 can be implemented using the IQ modulator shown in FIG. 6.
- Each polarization-maintaining coupler 210 is a three-port device that can route an optical signal between the corresponding two optically coupled ports substantially without altering the polarization state of the signal. Depending on the hght propagation direction, polarization-maintaining coupler 210 can operate as an optical splitter or as an optical combiner. For example, each of polarization-maintaining couplers 210i and 2IO 2 operates as an optical splitter. In contrast, each of
- polarization-maintaining couplers 2IO 3 and 210 4 operates as an optical combiner.
- Representative polarization-maintaining couplers that can be used to implement polarization-maintaining couplers 210 in optical polarization D-MUX 200 are disclosed, e.g., in U.S. Patent Nos. 6,463, 195, 6,504,961, 7,203,397, and 7,373,028, all of which are incorporated herein by reference in their entirety.
- Each of optical IQ modulators 220n, 22O 21 , 220 12 , and 22 ⁇ 22 is configured, using the corresponding one of control signals 104i-104 4 , to modulate the respective optical input signal 212 so that the resulting optical output signal 222 generated by the IQ modulator is described by Eq. (1):
- controller 122 generates control signals 104i-104 4 to cause IQ modulators 220n, 22O 21 , 220 12 , and 22 ⁇ 22 to have transfer functions described by Eqs. (2a)-(2i):
- ⁇ 21 ⁇ 0 2l [k-l] + ⁇ QM ((I y [k]) 2 +(Q y [k]) 2 -R 2 ) (2g)
- r 22 [k] : r 22 [k-l] + fi l y [k] ((I y [k]) 2 +(Q y [k]) 2 -R 2 ) (2h)
- ⁇ 22 ⁇ 0 22 [k-l] + ⁇ Q y [k] ((I y [k]) 2 +(Q y [k]) 2 -R 2 ) (2i)
- Jj m and ⁇ P tm are the real and imaginary parts, respectively, of transfer function Hi m (as defined in Eq. (2a))
- k is a time-slot index
- ⁇ is an error-weighting coefficient
- I x , Q x , l y , and Q y are the magnitudes of electrical signals 120 lx , 120Q X , 120 ly , and 120(3 ⁇ 4, (see FIG.
- each of Eqs. (2b)-(2i) is a recursive formula that enables controller 122 to determine the value of transfer function H te for the k-t time slot based on the magnitudes of electrical signals 120 lx , 120 QX , 120 ly , and 120( 3 ⁇ 4 , and the value of transfer function H te in the previous (i.e, (fc-l)-th) time slot.
- the second term in the right part of each of Eqs. (2b)-(2i) provides a measure of the misalignment between the current state of polarization of signal 102 and the orientation of the principal polarization axes of polarization D-MUX 200.
- the operative constellation is a phase-shift keying (PSK) constellation
- PSK phase-shift keying
- all constellation points are located on a circle centered at the origin of the constellation (e.g., the origin of the corresponding complex plane).
- parameter R represents the radius of this circle.
- the operative constellation is a quadrature- amplitude modulation (QAM) constellation
- QAM quadrature- amplitude modulation
- different constellation points may have different amplitudes, which is manifested by their respective different distances from the origin of the constellation.
- parameter R can be calculated, e.g., using
- M is the number of constellation points in the QAM constellation
- a is a scaling coefficient that reflects signal amplification/attenuation between the transmitter and receiver
- r is the normalized amplitude of the i-th constellation point in the QAM constellation. Note that Eq. (3) is also valid for a PSK constellation, wherein all r, values are the same.
- IQ modulators 220n, 22O21, 220 12 , and 22 ⁇ 22 are configured so that the polarization rotation imposed on input signal 102 in the optical transport link is fully reversed, and signals 108x and 108y represent the "pure" first and second PDM components, respectively, of the input signal.
- R is defined (see, e.g., Eq. (3))
- the time average of each of the second terms in Eqs. (2b)-(2i) is zero, and the configurations of IQ modulators 220n, 22O21, 220 12 , and 22 ⁇ 22 manifested by their respective transfer functions (H m ) remain substantially unchanged over time.
- Error- weighting coefficient ⁇ is an empirical parameter whose value is set to be relatively small to ensure smooth polarization-rotation tracking by polarization D- MUX 200.
- the relatively small value of error- weighting coefficient ⁇ enables sufficient time averaging to occur for proper polarization-rotation tracking when input signal 102 is an M-QAM signal.
- the relatively small value of error- weighting coefficient ⁇ also enables polarization D-MUX 200 not to overreact to burst errors, which ensures relatively slow and gradual adjustment of the overall polarization D-MUX configuration.
- FIGs. 3A-3B show block diagrams of a controller 300 that can be used as controller 122 (FIG. 1) according to an embodiment of the disclosure. More specifically, FIG. 3A shows an overall block diagram of controller 300. FIG. 3B shows a block diagram of a filter 390, eight instances (copies) of which are used in the circuit shown in FIG. 3A. Controller 300 is an electrical controller that implemented using various analog circuits as its building blocks. When controller 300 is used in receiver 100 configured to operate under a relatively low-bit QAM format, such as 16- QAM or 32-QAM, the linearity requirements to the analog circuits used in the controller are not very stringent. For example, controller 300 can be implemented using analog circuits that have a relatively small linear dynamic range.
- Controller 300 comprises a signal generator 306, five squaring circuits 310i- 310s, two three-input signal adders 320i-320 2 , eight analog signal multipliers 330, and eight filters 390i-390 8 , all interconnected as indicated in FIG. 3A.
- Each filter 390 comprises a signal generator 326, an analog signal multiplier 330, a delay element (Z 1 ) 350, and a two-input signal adder 370, all interconnected as indicated in FIG. 3B.
- Signal generator 306 has a register (not explicitly shown in FIG. 3A) that stores the value of parameter R (see Eq. (3)).
- An output signal 308 generated by signal generator 306 is an electrical analog signal having a magnitude proportional to the parameter value stored in the register.
- the input signals received by squaring circuits 310i- 310s are electrical signals 120 lx , 120 Qx , 308, 120 ly , and 120 Qy , respectively.
- Signal adder 320i is configured to add signals 312i and 312 2 , and subtract from the sum signal 312 3 .
- the resulting signal generated by signal adder 320i is signal 322i.
- Signal adder 320 2 is similarly configured to add signals 312 4 and 312 5 , and subtract from the sum signal 312 3 .
- the resulting signal generated by signal adder 320 2 is signal 322 2 .
- the eight analog signal multipliers 330 are configured to variously scale signals 322i and 322 2 , using signals 120 lx , 120 Qx , 120 ly , and 120(3 ⁇ 4, as scaling factors.
- the resulting scaled signals generated by the eight analog signal multipliers 330 are signals 334i-334 8 .
- the magnitudes of signals 334i-334 8 are proportional to the following quantities:
- signal 334 7 I y [k] i(I y [k]) 2 +(Q y [k]) 2 - -R 2 ); and signal 334 8 : Q y [k] i(I y [k]) 2 +(Q y [k]) 2 -R 2 ).
- Filters 390i-390 8 are configured to implement signal processing corresponding to Eqs. (2b)-(2i), respectively. More specifically, signal generator 326 and the analog signal multiplier 330 in filter 390j are configured to scale signal 334i using error- weighting coefficient ⁇ as a scaling factor. A resulting scaled signal 368 is then used for the recursive update of an output signal 392 i; with delay element 350 providing an appropriate time delay for said recursive update.
- output signals 390i-390 8 generated by filters 390i-390 8 are proportional to r n [k], ⁇ Pn[k], ⁇ ⁇ ⁇ , ⁇ Pn[k], r 2 i[k], ⁇ 21 ⁇ , r 2 2[k], and 22 ⁇ , respectively (also see Eqs. (2b)-(2i)).
- Signals 390i and 390 2 provide the I and Q components, respectively, for control signal 104i (FIG. 2).
- Signals 390 3 and 390 4 provide the I and Q components, respectively, for control signal 104 2 (FIG. 2).
- Signals 390s and 390 ⁇ provide the I and Q components, respectively, for control signal 104 3 (FIG. 2).
- Signals 390 7 and 390 8 provide the I and Q components, respectively, for control signal 104 4 (FIG. 2).
- FIG. 4 shows a block diagram of a phase detector 400 that can be used as phase detector 140 (FIG. 1) according to an embodiment of the disclosure.
- phase detector 400 is shown in FIG. 4 as being configured to (i) receive electrical signals 120 LY and 120( 3 ⁇ 4 , and digital signals 128 LY and 128( 3 ⁇ 4 , and (ii) generate phase-error signal 142.
- Other signal configurations for phase detector 400 are also possible.
- phase detector 400 can alternatively be configured to use signals 128 LX , 128Q X , 120 1X , and 120Q x instead of signals 128 LY , 128Q Y , 120 LY , and 120Q Y , respectively.
- Phase detector 400 comprises multipliers 402 and 404 and an adder 410 configured to implement the signal-processing operations expressed by Eq. (4):
- Eq. (4) is based on the following property of complex values: when first and second complex values have the same argument, the product of the first complex value and the complex conjugate of the second complex value is a real number. The right side of Eq.
- phase-error signal 142 generated by phase detector 400 as indicated in FIG. 4 provides a measure of the phase-lock error between OLO signal 150 and the carrier wave of optical input signal 102 (also see FIG. 1).
- receiver 100 can then drive OLO source 148, using phase-error signal 142 generated by phase detector 400, in a manner that tends to minimize the absolute value of ⁇ .
- each of multipliers 402 and 404 can be implemented using a Gilbert cell.
- each of multipliers 402 and 404 can be implemented using an XOR gate.
- FIG. 5 shows a block diagram of a light source 500 that can be used as OLO source 148 (FIG. 1) according to one embodiment of the disclosure.
- light source 500 is shown in FIG. 5 as being configured to receive filtered phase-error signal 146 and to generate OLO signal 150.
- Light source 500 comprises a laser (e.g., a laser diode) 502 optically coupled to an optical IQ modulator 506.
- laser 502 e.g., a laser diode
- optical IQ modulator 506 On optical beam 504 generated by laser 502 nominally has the same frequency (wavelength) as the carrier wave of optical input signal 102.
- the phase noise e.g., due to the finite linewidth in optical beam 504 generally causes a non-zero frequency and phase offset between optical beam 504 and the carrier wave of optical input signal 102.
- Optical IQ modulator 506 is configured to modulate optical beam 504 so as to remove the frequency and phase offset and achieve a requisite phase lock.
- a modulated optical signal 508 generated by IQ modulator 506 is then filtered using an optical filter 510 to generate OLO signal 150.
- optical IQ modulator 506 can be implemented using a Mach-Zehnder modulator, such as the above-mentioned model FTM7967EQA manufactured by Fujitsu Optical Components Limited of Japan.
- optical IQ modulator 506 can be implemented using the IQ modulator shown in FIG. 6.
- Electrical drive signals 512i and 512 2 for optical IQ modulator 506 are generated using a voltage-controlled oscillator (VCO) 520 and amplifiers 516i and 5I6 2 .
- Voltage-controlled oscillator 520 is an electronic circuit configured to generate electrical RF signals 518i and 5I8 2 so that (i) these two RF signals have the same frequency proportional to the magnitude (e.g., quasi-dc voltage) of filtered phase-error signal 146 generated by loop filter 144, e.g., by filtering phase-error signal 142 generated by phase detector 400, and (ii) there is a fixed phase offset of approximately 90 degrees between these two RF signals.
- Amplifiers 510i and 5IO 2 are configured to appropriately amplify and bias electrical RF signals 516i and 5I6 2 , thereby generating electrical drive signals 508i and 5 ⁇ 8 2 in a manner that causes optical IQ modulator 506 to apply sideband- and carrier-suppression modulation to optical beam 504.
- Optical filter 510 passes this relatively strong modulation sideband while rejecting or blocking the residual carrier and the other relatively weak modulation sideband. The output of optical filter 510 can then be used as OLO signal 150 in receiver 100 (FIG. 1).
- optical filter 510 is optional and can be removed from light source 500.
- FIG. 6 shows a block diagram of an IQ modulator 600 that can be used as modulator 220 (FIG. 2) according to an embodiment of the disclosure.
- IQ modulator 600 is configured to: (i) receive, as an optical input signal, optical signal 212i; (ii) generate optical output signal 222i; and (iii) be driven using control signal 104i.
- IQ modulator 600 comprises a nested Mach-Zehnder modulator (MZM) 610.
- MZM Mach-Zehnder modulator
- the term "nested” means that each of MZM arms 612i and 612 2 of Mach-Zehnder modulator 610 itself includes, as a constituent part, a respective conventional Mach-Zehnder modulator. More specifically, MZM arm 612i includes a conventional Mach-Zehnder modulator 616i, and MZM arm 612 2 includes a conventional Mach-Zehnder modulator 6I6 2 .
- Each of Mach-Zehnder modulators 6I6 1 and 6I6 2 is shown in FIG. 6 as a rectangle, with the arms of the modulator not being explicitly shown to make FIG. 6 easier to read.
- Mach-Zehnder modulator 6I6 1 is driven using the I-component of control signal 104i.
- each of signals 390i, 390 3 , 390s, and 390 7 generated by controller 300 (FIG. 3) is an I-component of the corresponding control signal 104i.
- Mach-Zehnder modulator 6I6 2 is similarly driven using the Q-component of control signal 104i.
- each of signals 390 2 , 390 4 , 390 ⁇ , and 390 8 generated by controller 300 (FIG. 3) is a Q-component of the corresponding control signal 104i.
- Amplifiers 604i and 604 2 are configured to appropriately amplify and apply a dc bias to the I-component and the Q-component, respectively, of control signal 104i to generate electrical drive signals 6O6 1 and 6 ⁇ 6 2 for Mach-Zehnder modulators 6I6 1 and 6I6 2 , respectively.
- MZM arm 612 2 further includes a fixed phase shifter 620 configured to apply a phase shift of about 90 degrees to the optical signal propagating through that MZM arm. This phase shift enables MZM arms 612i and 612 2 to operate as an I-modulation arm and a Q-modulation arm, respectively, of IQ modulator 600.
- Each of MZM arms 612i and 612 2 is illustratively shown as further including a respective one of optional configurable phase shifters 618i and 6I8 2 .
- These phase shifters can be used, e.g., to compensate for possible unwanted phase imbalances between MZM arms 612i and 612 2 . If no such unbalances are present in IQ modulator 600, then each of phase shifters 6I8 1 and 6I8 2 can be configured to receive a zero (e.g., ground) bias potential.
- Coupled refers to any manner known in the art or later developed in which energy is allowed to be transferred between two or more elements, and the interposition of one or more additional elements is
- processors may be provided through the use of dedicated hardware as well as hardware capable of executing software in association with appropriate software.
- the functions may be provided by a single dedicated processor, by a single shared processor, or by a plurality of individual processors, some of which may be shared.
- explicit use of the term "processor” or “controller” should not be construed to refer exclusively to hardware capable of executing software, and may implicitly include, without limitation, digital signal processor (DSP) hardware, network processor, application specific integrated circuit (ASIC), field programmable gate array (FPGA), read only memory (ROM) for storing software, random access memory (RAM), and non volatile storage.
- DSP digital signal processor
- ASIC application specific integrated circuit
- FPGA field programmable gate array
- ROM read only memory
- RAM random access memory
- any switches shown in the figures are conceptual only. Their function may be carried out through the operation of program logic, through dedicated logic, through the interaction of program control and dedicated logic, or even manually, the particular technique being selectable by the implementer as more specifically understood from the context.
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Abstract
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KR1020157019297A KR101747576B1 (en) | 2013-01-17 | 2014-01-10 | Optical polarization demultiplexing for a coherent-detection scheme |
JP2015553761A JP6053961B2 (en) | 2013-01-17 | 2014-01-10 | Optical polarization demultiplexing for coherent detection schemes |
EP14701871.7A EP2946493B1 (en) | 2013-01-17 | 2014-01-10 | Optical polarization demultiplexing for a coherent-detection scheme |
CN201480005054.5A CN104937864B (en) | 2013-01-17 | 2014-01-10 | Optical polarization demultiplexing for coherent detection scheme |
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US13/743,677 US9281915B2 (en) | 2013-01-17 | 2013-01-17 | Optical polarization demultiplexing for a coherent-detection scheme |
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JP6053961B2 (en) | 2016-12-27 |
TW201434281A (en) | 2014-09-01 |
EP2946493A1 (en) | 2015-11-25 |
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TWI506971B (en) | 2015-11-01 |
CN104937864A (en) | 2015-09-23 |
US20140199068A1 (en) | 2014-07-17 |
KR20150095917A (en) | 2015-08-21 |
KR101747576B1 (en) | 2017-06-14 |
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US9281915B2 (en) | 2016-03-08 |
JP2016506209A (en) | 2016-02-25 |
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