Clock recovery method for polarization multiplexed coherent optical communications

An apparatus comprising a plurality of receivers each configured to receive a plurality of polarized signals, a voltage control oscillator (VCO) coupled to the receivers and configured to control timing and sampling frequency of the polarized signals, and a signal processing component coupled to the receivers and configured to update a plurality of weighted linear factors, wherein the polarized signals and the weighted linear factors are used to obtain a combined signal, and wherein the weighted linear factors are updated using a real part or an imaginary part of the combined signal. Included is a method comprising using a linear factor to combine a plurality of polarized optical signals to provide time recovery information, and updating the linear factor using a combination of the polarized optical signals.

CROSS-REFERENCE TO RELATED APPLICATIONS

Not applicable.

Not applicable.

REFERENCE TO A MICROFICHE APPENDIX

Not applicable.

BACKGROUND

In optical communications, many modulation schemes have been used to transport data. On-Off keying (OOK) is one modulation scheme that has been used, where data is encoded using signal intensity variation. OOK introduces strong characteristic tones in the signal frequency domain, which can be detected as periodic intensity variations in the signal. To detect the signals, a conventional clock recovery scheme is used to obtain the timing information in the tones, such as by filtering the detected signal intensities using a narrow band pass filter. Phase Shift Keying (PSF) is another modulation scheme that has been used more recently. In PSF, the data is encoded using signal phase variation. Therefore, the conventional clock recovery for OOK, which is based on detecting signal intensities, is not suitable for PSF modulation. Instead, other clock recovery algorithms have been proposed for PSF, such as early-late gate, Mueller-Muller, and Gardner algorithms. These algorithms are based on the assumption that the distortion of the signal passing through an optical fiber, e.g. due to fiber dispersion and polarization mode dispersion (PMD), is relatively small with respect to the signal and thus the rising/falling edge of the signal can be easily detected. However, for higher data transmission rates, such as 100 Gigabit systems, the signal can be highly distorted due to fiber dispersion and PMD and the rising/falling edge of the signal cannot be easily detected. Therefore, other clock recovery algorithms that are based on timing information in the signal are used.

SUMMARY

In a first embodiment, the disclosure includes an apparatus comprising a plurality of receivers each configured to receive a plurality of polarized signals, a voltage control oscillator (VCO) coupled to the receivers and configured to control timing and sampling frequency of the polarized signals, and a signal processing component coupled to the receivers and configured to update a plurality of weighted linear factors, wherein the polarized signals and the weighted linear factors are used to obtain a combined signal, and wherein the weighted linear factors are updated using a real part or an imaginary part of the combined signal.

In a second embodiment, the disclosure includes an apparatus comprising at least one processor configured to implement a method comprising receiving a plurality of polarized multiplexed optical signals multiplying the polarized multiplexed optical signals by a plurality of weighted linear terms combining the polarized multiplexed optical signals multiplied by the weighted linear terms to obtain a combined complex signal using one of the real part or imaginary part of the combined complex signal to update the weighted linear terms, and using the other one of the real part or imaginary part of the combined complex signal to control clock timing.

In a third embodiment, the disclosure includes a method comprising using a linear factor to combine a plurality of polarized optical signals to provide time recovery information, and updating the linear factor using a combination of the polarized optical signals.

DETAILED DESCRIPTION

Disclosed herein is a system and method for improved clock recovery. The clock recovery may be used for optical signals that may be distorted by fiber dispersion and/or PMD. The optical signals may be polarization multiplexed optical signals comprising two differently polarized signal components, which may have different clock timing offsets and PMD caused timing offset. In the frequency domain, the clock recovery may combine the real parts of the two signal components using a first weighted linear term and may combine the imaginary parts of the two signal components using a second weighted linear term. The combined real parts and imaginary parts may then be used to obtain a combined signal. The imaginary part of the combined signal may be used by a VCO to control timing and sampling frequency and the real part of the combined signal may be used to update the first weighted linear term and the second weighted linear term. Alternatively, in the time domain, the clock recovery may combine a first weighted linear term and a second weighted linear term with the first signal component and the second signal component to obtain a combined signal. The real part of the combined signal may then be used by the VCO and the imaginary part of the combined signal may be used to update the first weighted linear term and the second weighted linear term.

FIG. 1illustrates one embodiment of a clock recovery system100, which may be used to obtain the timing information of polarization multiplexed optical signals. The clock recovery system100may be based on the Godard clock recovery algorithm for frequency domain signals and may be implemented using hardware, software, or both. The Godard clock recovery algorithm is described by Dominique Godard in “Passband Timing Recovery in an All-Digital Modem Receiver,” which was published in May 1977 by the Institute of Electrical and Electronics Engineers (IEEE) Transactions on Communications and is incorporated herein by reference as if reproduced by its entirety. However, unlike the Godard clock recovery algorithm that was established for optical signals comprising one component or channel, e.g. one polarized component, the clock recovery system100may be used for polarization multiplexed optical signals, which may comprise an X-polarized optical component and a Y-polarized optical component. The two different polarized components may be distorted due to fiber dispersion and PMD effect, e.g. between the transmitter and the receiver, and therefore may have different clock timing offsets and PMD caused timing offset.

Typically, the received multiplexed optical signals may be split into four channels, for instance by an optical demultiplexer or splitter. The first channel may comprise the real part of the X-polarized component (XI), the second channel may comprise the imaginary part of the X-polarized component (XQ), the third channel may comprise the real part of the Y-polarized component (YI), and the fourth channel may comprise the imaginary part of the Y-polarized component (YQ). The four channels may be received at about twice the baud or modulation rate of the transmitted multiplexed optical signals, and may be quantized for digital signal processing (DSP), for instance using an analog to digital converter (ADC). The DSP processing may comprise compensating for relatively large dispersion in the transmitted signal, which may be stable over relatively long time transmission periods.

The quantized signal channels may be processed using the clock recovery system100, which may comprise a plurality of functional blocks. Specifically, the clock recovery system100may comprise a first Fast Fourier Transform (FFT) block110(e.g. for X-polarization), a second FFT block120(e.g. for Y-polarization), a complex conjugate block130, an imaginary component block140, a real component block145, a first and second weighted linear term or factor (h1& h2) update block150, and a VCO block160. Additionally, the clock recovery system100may comprise different mathematical operation blocks, such as addition and multiplication blocks, which are indicated by the circles containing the “+” and “×,” respectively.

In an embodiment, the first FFT block110may be configured to receive X-polarization signals, e.g. the first channel and second channel, in the time domain and convert the signals into corresponding signals in the frequency domain. When the complex signal is in the frequency domain (e.g. after the FFT), it is truncated to obtain an Up-Side-Band (USB) signal XUSBand a Low-Side-Band (LSB) signal XLSB. Similarly, the second FFT block120may receive Y-polarization signals, e.g. the third channel signal YI and fourth channel signal YQ, and convert the signals into the frequency domain, e.g. an USB signal YUSBand LSB signal YLSB, respectively.

Each of XUSBand XLSBmay then be multiplied by a first weighted linear term h1, and each of YUSBand YLSBmay be multiplied by a second weighted linear term h2. The resulting weighted frequency domain signals, e.g. XUSB, XLSB, YUSB, and YLSB, may then be combined to obtain two signal components SUand SL. The complex conjugate block130may be configured to obtain the complex conjugate of SU(S*U), which may then be multiplied by SLto obtain a frequency domain combined signal. For instance, the combined signal may be equal to the sum of the products of S*Uand SLfor a plurality of frequencies, such as

∑k=1N⁢SU*⁡(k)·SL⁡(k),
where k is an integer that enumerates the frequencies and N is the number of considered frequencies in the signal. The step size used to evaluate the combined signal using the equation above may be chosen to track the State of Optical Polarization (SOP) of the signal transmission link without interrupting clock recovery. Tracking the SOP of the link without interrupting clock recovery may be possible since the frequency rate of SOP may be on the order of tens of kilohertz (kHz) and the jitter bandwidth in clock recovery may be faster on the order of megahertz (MHz).

The imaginary component block140may be configured to obtain the imaginary part of the combined signal,

e.g.⁢⁢Im⁢{∑k=1N⁢SU*⁡(k)·SL⁡(k)},
and discard the real part of the combined signal. Similarly, the real component block145may be configured to obtain the real part of the combined signal,

e.g.⁢⁢Re⁢{∑k=1N⁢SU*⁡(k)·SL⁡(k)},
and discard the imaginary part of the combined signal. Additionally, the VCO block160may be configured to control clock timing and sampling frequency based on the imaginary part of the combined signal. The first and second weighted linear term or factor update block150may be configured to update the first and second weighted linear terms h1and h2based on the real part of the combined signal. Specifically, the first weighted linear term h1may be updated based on the last updated value and the real part of the combined signal, according to

h1⁡(n+1)=h1⁡(n)+μ⁢∂∂h1⁢Δɛ2⁡(h1,h2),
where μ is a constant that may be determined empirically and n enumerates the sequence of updated h1values. Similarly, the second weighted linear term h2may be updated according to

h2⁡(n+1)=h2⁡(n)+μ⁢∂∂h2⁢Δɛ2⁡(h1,h2).
The value of Δε2(h1,h2) may be equal to

Re⁢{∑k=1N⁢⁢SU*⁡(k)·SL⁡(k)},
and h1and h2may satisfy the constraint |h1|2+|h2|2=1. The value of

∂∂h1⁢Δɛ2⁡(h1,h2)
may be estimated according to:

∂∂h1⁢Δɛ2⁡(h1,h2)=12·Re⁢∑k⁢⁢[h2⁢XUSB⁡(k)·YLSB*⁡(k)+h2⁢XLSB⁡(k)·YUSB*⁡(k)],
and the value of

∂∂h2⁢Δɛ2⁡(h1,h2)
may be estimated according to:

FIG. 2illustrates another embodiment of a clock recovery system200, which may be used for polarization multiplexed optical signals in the time domain. The clock recovery system200may comprise a plurality of functional blocks including a first Time Domain-Bandpass Filter (TD-BPF) block210(e.g. for X-polarization) and a second TD-BPF block220(e.g. for Y-polarization). The clock recovery system200may also comprise a complex conjugate block230, an imaginary component block240, a real component block245, a first and second weighted linear term or factor (h1& h2) update block250, and a VCO block260, which may be configured similar to the corresponding blocks of the clock recovery system100. Additionally, the clock recovery system200may comprise different mathematical operation blocks, such as addition and multiplication blocks.

The first TD-BPF block210may be configured to receive an X-polarization signal comprising a real part XI and an imaginary part XQ, and transform the signal into a USB signal component XUSBand a LSB signal component XLSB. To imaginary part XQ may be represented by the value of XQ multiplied by the imaginary number j, as shown inFIG. 2. The XUSBand XLSBsignals may be frequency domain signals similar to the corresponding signals provided by the first FFT block110and the second FFT block120, respectively. Similarly, the second TD-BPF bock220may receive a Y-polarization signal comprising a real part YI and an imaginary part YQ (indicated by the preceding imaginary number j) and convert the signal into a USB signal component YUSBand a LSB signal component YLSB. Similar to the clock recovery system100, the clock recovery system200may multiply the XUSBand XLSBsignals by a first weighted linear term h1, and the YUSBand YLSBsignals by a second weighted linear term h2. The resulting weighted signals may then be combined to obtain two signal components SUand SL. The complex conjugate block230may obtain the complex conjugate of SU, S*U, and multiply S*Uby SLto obtain a time domain combined signal,

e.g.∑k=1N⁢⁢SU*⁡(k)·SL⁡(k).
The real component block245may obtain the real part of the combined signal, which may be used to update the first and second weighted linear terms by the first and second weighted linear term or factor update block250. The imaginary component block240may obtain the imaginary part of the combined signal, which may be used by the VCO260to control clock timing and sampling frequency.

FIG. 3illustrates an embodiment of a TD-BPF block300, which may be configured similar to the first TD-BPF block210and the second TD-BPF block220. The TD-BPF block300may be configured to transform a polarized complex signal, such as X or Y polarized signal comprising a real part and an imaginary part, into a USB signal component and a LSB signal component. The TD-BPF block300may comprise a plurality of functional sub-blocks including a series formulation sub-block310, a first summation sub-block320, a second summation sub-block330, a third summation sub-block340, and a fourth summation sub-block350. Additionally, the TD-BPF block300may comprise different mathematical operation sub-blocks, such as addition and multiplication blocks. It will be appreciated thatFIG. 3illustrates only one example of how to calculate the USB an LSB signal, e.g. using a block of 16 consequently signals. In other embodiments, the method can be expanded for longer signal sequences or simplified for shorted signal sequences.

The series formulation sub-block310may be configured to express the polarized signal into a series comprising a plurality of terms, e.g. about 16 terms from S0to S15. The first summation sub-block320may add together the first series term S0and every about fourth consecutive term, e.g. S4, S8, and S12, to provide a first term S0′, e.g. S0′=Σk=04S4k+0. The second summation sub-block330may add together the second series term S1and every about fourth consecutive term, e.g. S5, S9, and S13, to provide a second term S1′, e.g. S1′=Σk=04S4k+1. The third summation sub-block340may add together the third series term S2and every about fourth consecutive term, e.g. S6, S10, and S14, to provide a third term S2′, e.g. S2′=Σk=04S4k+2. The fourth summation sub-block350may add together the fourth series term S3and every about fourth consecutive term, e.g. S7, S11, and S15, to provide a fourth term S3′, e.g. S3′=Σk=04S4k+3. The third term may be subtracted from the first term to obtain a first combined term, e.g. S0′−S2′. The fourth term may also be subtracted from the second term and the result may be multiplied by the imaginary number j to obtain a second combined term, e.g. j·(S0′−S2′). Hence, the first combined term and the second combined term may be added to provide the USB signal component (e.g. XUSBor YUSB), and the first combined term may be subtracted from the second combined term to provide the LSB signal component (e.g. XLSBor YLSB).

FIG. 4illustrates another embodiment of a clock recovery system400, which may be used for clock recovery of polarization multiplexed optical signals in the time domain. As such, the clock recovery system400may receive an X-polarized signal component SXand a Y-polarized signal component SY, for instance at about twice the baud or modulation rate of the polarization multiplexed optical signals, and combine the two signals to obtain a time domain combined signal comprising a real part and an imaginary part. The clock recovery system400may comprise a plurality of functional blocks, including a demultiplexer (DEMUX) block410, time delay block420, a complex conjugate block430, an imaginary component block440, a real component block445, a first and second weighted linear term (h1& h2) update block450, and a VCO block460. Additionally, the clock recovery system400may comprise different mathematical operation blocks, such as addition and multiplication blocks.

As shown inFIG. 4, the X-polarized signal component SXand Y-polarized signal component SYmay be multiplied by a first weighted linear term h1and a second weighted linear term h2, respectively, and then added together into an intermediate signal. The DEMUX block410may be configured to split the intermediate signal into a first copy signal and a second copy signal. The time delay block420may be configured to receive the second copy signal and insert a time delay T, which may be predetermined, into the second copy signal to obtain a first delay signal. The first delay signal may be subtracted from the second copy signal to obtain a second delay signal at about half the time delay T (e.g. T/2). The complex conjugate block430may then obtain the complex conjugate of the second delay signal, which may be multiplied by the first copy signal to provide a time domain combined signal S. The imaginary component block440may obtain the imaginary part of the combined signal, which may be used to update the first and second weighted linear terms by the first and second weighted linear term update block450. Additionally, the real component block445may obtain the real part of the combined signal, which may be used by the VCO460to control clock timing and sampling frequency.

As mentioned above, the first and second weighted linear term update block450may be configured to update the first and second weighted linear terms h1and h2based on the imaginary part of the combined signal. Specifically, the first weighted linear term h1may be updated based on the last updated value and the imaginary part of the combined signal, according to

h1⁡(n+1)=h1⁡(n)+μ⁢∂∂h1⁢Δɛ2⁡(h1,h2),
where μ is a constant that may be determined empirically and n enumerates the sequence of updated h1values. Similarly, the second weighted linear term h2may be updated according to

In alternative embodiments, a clock recovery system based on other architectures may be used to retrieve timing information in the frequency domain or the time domain from a first polarized signal component and a second polarized signal component. Accordingly, each of the first polarized signal component and second polarized signal component may be multiplied by a first weighted linear term, a second weighted linear term, or both. The resulting weighted signal components may then be used to obtain a combined signal, which may be partitioned into a real part and an imaginary part. The first and second weighted linear terms may then be updated using one of the real part and imaginary part of the combined signal, and the timing information may be obtained from the other one of the real part and imaginary part. In some embodiments, any quantity of weighted linear terms, which may be greater than about two, may be multiplied with any quantity of polarized signal components to obtain the combined signal and hence may be updated based on the combined signal.

FIG. 5is a flowchart of one embodiment of a clock recovery method500, which may be used to retrieve time and/or frequency information for a polarization multiplexed optical signal in the frequency domain. The method may begin at block510, where a plurality of polarized signal components may be received. The polarized signal components may comprise fiber dispersion and/or PMD due to the signal traveling through the transmission link, e.g. the optical fiber. As such, the dispersion may need to be compensated for and the signal may need clock recovery. For example, a clock recovery system, such as the clock recovery system100, may receive an X-polarized signal and/or Y-polarized signal in the time domain, which may comprise a real part component and an imaginary part component. At block520, the polarized signal components may be converted from the time domain to the frequency domain, for example using FFT or TD-BPF. Next at block530, the polarized signal components may be multiplied by a plurality of corresponding weighted linear terms or factors. At block540, the weighted polarized signal components may be processed and combined, for example by a plurality of functional blocks as shown in the clock recovery system100, to obtain a combined complex signal. The combined complex signal may comprise a real part and an imaginary part. At block550, the real part of the combined complex signal may be used to update the weighted linear terms, which may then be multiplied by the next received polarized signal components at the next time block. At block560, the imaginary part of the combined complex signal may be used to control clock timing and sampling frequency. The clock recovery method500may then end.

FIG. 6is a flowchart of one embodiment of a clock recovery method600, which may be used to retrieve time information for a polarization multiplexed optical signal in the time domain. The method may begin at block610, where a plurality of polarized signal components may be received. For example, a clock recovery system, such as the clock recovery system200, may receive an X-polarized signal and/or Y-polarized signal in the time domain. Next at block620, the polarized signal components may be multiplied by a plurality of corresponding weighted linear terms or factors. At block630, the weighted polarized signal components may be processed and combined, for example by a plurality of functional blocks as shown in the clock recovery system200, to obtain a combined complex signal in the time domain. At block640, the imaginary part of the combined complex signal may be used to update the weighted linear terms, which may then be multiplied by the next received polarized signal components at the next time block. At block650, the real part of the combined complex signal may be used to control clock timing. The clock recovery method600may then end.

The network components described above may be implemented on any general-purpose network component, such as a computer or network component with sufficient processing power, memory resources, and network throughput capability to handle the necessary workload placed upon it.FIG. 7illustrates a typical, general-purpose network component700suitable for implementing one or more embodiments of the components disclosed herein. The network component700includes a processor702(which may be referred to as a central processor unit or CPU) that is in communication with memory devices including secondary storage704, read only memory (ROM)706, random access memory (RAM)708, input/output (I/O) devices710, and network connectivity devices712. The processor702may be implemented as one or more CPU chips, or may be part of one or more application specific integrated circuits (ASICs).

The secondary storage704is typically comprised of one or more disk drives or tape drives and is used for non-volatile storage of data and as an overflow data storage device if RAM708is not large enough to hold all working data. Secondary storage704may be used to store programs that are loaded into RAM708when such programs are selected for execution. The ROM706is used to store instructions and perhaps data that are read during program execution. ROM706is a non-volatile memory device that typically has a small memory capacity relative to the larger memory capacity of secondary storage704. The RAM708is used to store volatile data and perhaps to store instructions. Access to both ROM706and RAM708is typically faster than to secondary storage704.