MONITORING OPTICAL POWER OF COMMUNICATION SIGNALS USING MANIFESTATIONS OF POLARIZATION-DEPENDENT LOSS AT THE RECEIVER

Abstract

 An optical WDM system configured to set the launch power of individual wavelength channels at the transmitter based on statistical characteristics of certain manifestations of PDL observed at the receiver. In an example embodiment, the pertinent effects of PDL are estimated at the receiver based on SNR or BER measurements performed in a polarization-resolved manner. The SNR (or BER) values for the two orthogonal polarizations measured over a sequence of symbol periods are (i) sorted to form a first set by placing therein the lower (or higher) values from each of the symbol periods and (ii) processed to compute a second set having the differences between the higher and the lower values from each of the symbol periods. The variances of the two sets are then used to determine the sign and magnitude of the launch-power adjustment needed to have a nearly optimal launch power for the corresponding wavelength channel.

Description

BACKGROUND

Field

[0001] The present disclosure relates to optical communication equipment and, more specifically but not exclusively, to methods and apparatus for monitoring and/or setting the optical power of communication signals.

Description of the Related Art

[0002] This section introduces aspects that may help facilitate a better understanding of the disclosure. Accordingly, the statements of this section are to be read in this light and are not to be understood as admissions about what is in the prior art or what is not in the prior art.

[0003] Polarization-dependent loss (PDL) is an optical phenomenon due to which two orthogonal polarizations in a fiber-optic link may be subjected to different rates of loss. The effects of PDL are typically time-dependent (e.g. stochastic). Conventionally, PDL is reported as the difference between the maximum and minimum loss among all states of polarization, expressed in decibels.

[0004] In optical communications, PDL can degrade the signal quality and system performance. Typical sources of PDL include multiplexers, demultiplexers, couplers, isolators, circulators, switches, fiber imperfections, fiber asymmetries, etc. Loss typically accumulates along the link and causes temporal fluctuations of the signal-to-noise ratio (SNR) at the receiver, which can result in the corresponding variation of the instant bit-error rate (BER).

SUMMARY OF SOME SPECIFIC EMBODIMENTS

[0005] Disclosed herein are various embodiments of an optical wavelength-division-multiplexing (WDM) system configured to set the launch power of individual wavelength channels at the transmitter based on statistical characteristics of certain manifestations of PDL observed at the receiver. In an example embodiment, the pertinent effects of PDL are estimated at the receiver based on SNR or BER measurements performed in a polarization-resolved manner. The SNR (or BER) values for the two orthogonal polarizations measured over a sequence of symbol periods are (i) sorted to form a first set by placing therein the lower (or higher) values from each of the symbol periods and (ii) processed to compute a second set having the differences between the higher and the lower values from each of the symbol periods. The variances of the two sets are then used to determine the sign and magnitude of the launch-power adjustment needed to have a nearly optimal launch power for the corresponding wavelength channel.

[0006] According to an example embodiment, provided is an apparatus comprising: a polarization-sensitive coherent optical receiver that includes an optical hybrid and a plurality of light detectors connected to receive light outputted by the optical hybrid in response to an optical data signal applied thereto, the optical data signal having encoded thereon different respective data streams in each of first and second orthogonal polarizations thereof; and a digital signal processor configured to recover said respective data streams by processing digital samples representing electrical output signals generated by the light detectors in response to the received light; and wherein the digital signal processor comprises a power-regime monitor configured to: compute first and second sets of values representing a measure of transmission quality of the optical data signal in a sequence of symbol periods, the first and second sets corresponding to the first and second orthogonal polarizations, respectively; and determine a launch-power adjustment for the optical data signal using statistical characteristics of the values from said first and second sets.

[0007] According to another example embodiment, provided is a machine-implemented optical-power control method comprising the steps of: computing first and second sets of values representing a measure of transmission quality of an optical data signal in a sequence of symbol periods, the optical data signal having encoded thereon different respective data streams in each of first and second orthogonal polarizations thereof, the first and second sets corresponding to the first and second orthogonal polarizations, respectively; converting said first and second sets into third and fourth sets of values, the third set having smaller values from the first and second sets in the symbol periods, and the fourth set having differences of the values from the first and second sets in the symbol periods; and determining a launch-power adjustment for the optical data signal based on variances of the third and fourth sets.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] Other aspects, features, and benefits of various disclosed embodiments will become more fully apparent, by way of example, from the following detailed description and the accompanying drawings, in which:

FIG. 1 shows a block diagram of an optical communication system according to an embodiment;

FIG. 2 shows a block diagram of an individual-channel optical transmitter that can be used in the optical communication system of FIG. 1 according to an embodiment;

FIG. 3 shows a block diagram of an individual-channel optical receiver that can be used in the optical communication system of FIG. 1 according to an embodiment;

FIG. 4 graphically illustrates example performance characteristics of the optical communication system of FIG. 1 according to an embodiment;

FIG. 5 shows a block diagram of a receiver DSP that can be used in the optical communication system of FIG. 1 according to an embodiment;

FIG. 6 shows a flowchart of a signal-processing method that can be used in the receiver DSP of FIG. 5 according to an embodiment;

FIG. 7 graphically illustrates a step of the signal-processing method of FIG. 6 according to an embodiment; and

FIG. 8 graphically illustrates another step of the signal-processing method of FIG. 6 according to an embodiment.

DETAILED DESCRIPTION

[0009] FIG. 1 shows a block diagram of an optical communication system 10 according to an embodiment. System 10 comprises a wavelength-division-multiplexing (WDM) transmitter 12 and a WDM receiver 62 connected using a fiber-optic link 50.

[0010] In some embodiments, system 10 can be used to implement a full duplex (FDX) system that operates to transmit optical signals in one direction on one fiber and in the opposite direction on another fiber. A person of ordinary skill in the art will understand that an FDX system can be implemented, e.g., using two instances (e.g., nominal copies) of system 10.

[0011] In some embodiments, system 10 complies with the ITU-T G.709/Y.1331 Recommendation, which is incorporated herein by reference in its entirety.

[0012] In an example embodiment, link 50 can be implemented using two or more spans of optical fiber 40. In addition, link 50 typically has one or more optical amplifiers 30, each connected between two respective spans of fiber 40. Link 50 may also include additional optical elements (not explicitly shown in FIG. 1), such as optical splitters, combiners, couplers, switches, add-drop multiplexers, etc., as known in the pertinent art.

[0013] In an example embodiment, an optical amplifier 30 can be implemented as known in the pertinent art, e.g., using an erbium-doped fiber, a gain-flattening filter, and one or more laser-diode pumps. The number of optical amplifiers 30 used in optical link 50 depends on the particular embodiment and may be in the range, e.g., from 1 to ∼200. A typical length of the fiber span between two adjacent optical amplifiers 30 may range from ∼50 km to ∼100 km.

[0014] In some embodiments, link 50 may not have any optical amplifiers 30 therein.

[0015] In an example embodiment, WDM transmitter 12 and a WDM receiver 62 are configured to use carrier wavelengths λ₁-λ_N arranged on a frequency (wavelength) grid, such as a frequency grid that complies with the ITU-T G.694.1 Recommendation, which is incorporated herein by reference in its entirety. The frequency grid used in system 10 can be defined, e.g., in the frequency range from about 186 THz to about 201 THz, with a 100, 50, 25, or 12.5-GHz spacing of the channels therein. While typically defined in frequency units, the parameters of the grid can equivalently be expressed in wavelength units. For example, in the wavelength range from about 1528.8 nm to about 1563.9 nm, the 100-GHz spacing between the centers of neighboring WDM channels is equivalent to approximately 0.8 nm spacing. In alternative embodiments, other suitable frequency grids (e.g., flexible or having other spacing grids) can also be used.

[0016] In an example embodiment, system 10 can be configured to transport polarization-division-multiplexed (PDM) signals, wherein each of the two orthogonal polarizations of each optical WDM channel can be used to carry a different respective data stream.

[0017] In an example embodiment, WDM transmitter 12 comprises N individual-channel optical transmitters 14₁-14_N, where the number N is an integer greater than one. An example embodiment of transmitter 14 is described in more detail below in reference to FIG. 2. In operation, each of optical transmitters 14₁-14_N generates a respective WDM component of the optical output signal using a different respective carrier wavelength (e.g., one of wavelengths λ₁-λ_N, as indicated in FIG. 1). A multiplexer (MUX) 20 then operates to combine these WDM components, thereby generating the optical output signal that is applied to fiber 40 for transmission to WDM receiver 62. Along the propagation path, the transmitted optical signal is amplified using optical amplifiers 30.

[0018] WDM receiver 62 comprises N individual-channel optical receivers 74₁-74_N. An example embodiment of receiver 74 is described in more detail below in reference to FIG. 3. In operation, each of optical receivers 74₁-74_N detects and decodes a respective WDM component of the optical signal received by way of link 50 from WDM transmitter 12. A demultiplexer (DMUX) 70 operates to separate the WDM components of the received optical signal, thereby generating optical input signals for the optical receivers channel optical receivers 74₁-74_N.

[0019] In an example embodiment, each of MUX 20 and DMUX 70 can be implemented as known in the pertinent art, e.g., using one or more of the following: (i) a wavelength-selective optical filter; (ii) a wavelength-selective switch; (iii) a diffraction grating; (iv) an array of micro-mirrors; (v) a MEMS device; and (vi) an LCoS filter or modulator. Herein, the acronym "MEMS" refers to micro-electro-mechanical systems; and the acronym "LCoS" refers to liquid crystal on silicon.

[0020] System 10 further comprises an optical-power controller 80 configured to generate a control signal 84 and, optionally, one or more control signals 82. Control signal 84 is applied to WDM transmitter 12 and used therein to set and/or change the output optical power of at least some laser sources (not explicitly shown in FIG. 1, see FIG. 2) used in individual-channel optical transmitters 14₁-14_N. Each control signal 82 can be applied to a respective one of optical amplifiers 30 to set and/or change the optical gain thereof. In operation, optical-power controller 80 generates control signals 82 and 84 in response to a feedback signal 78 received from WDM receiver 62. Feedback signal 78 is a multicomponent signal, different components of which are generated by different ones of the individual-channel optical transmitters 14₁-14_N, (e.g., as described in more-detail below in reference to FIG. 6) using the respective PDL statistics observed thereat.

[0021] FIG. 2 shows a block diagram of an individual-channel optical transmitter 14_n that can be used in system 10 (FIG. 1) according to an embodiment, where n=1, 2, ..., N.

[0022] In operation, transmitter 14 receives a digital electrical input stream 102 of payload data and applies it to a digital signal processor (DSP) 112. DSP 112 processes input data stream 102 to generate digital signals 114₁-114₄. In an example embodiment, DSP 112 may perform, inter alia, one or more of the following: (i) de-multiplex input stream 102 into two sub-streams, each intended for optical transmission using a respective one of orthogonal (e.g., X and Y) polarizations of an optical output signal 130; (ii) encode each of the sub-streams using a suitable code, e.g., to prevent error propagation and enable error correction at receiver 190; (iii) convert each of the two resulting sub-streams into a corresponding sequence of constellation symbols; and (iv) perform digital signal pre-distortion, e.g., to mitigate the adverse effects imposed by an electrical-to-optical (E/O) converter (also sometimes referred to as a front-end circuit) 116 of transmitter 110, optical transport link 140, and/or a front-end circuit 172 of receiver 74_n (see FIG. 3). In each signaling interval (also referred to as a symbol period or time slot), signals 114₁ and 114₂ carry digital values that represent the in-phase (I) component and quadrature (Q) component, respectively, of a corresponding (possibly pre-distorted) constellation symbol intended for transmission using a first (e.g., X) polarization of light. Signals 114₃ and 114₄ similarly carry digital values that represent the I and Q components, respectively, of the corresponding (possibly pre-distorted) constellation symbol intended for transmission using a second (e.g., Y) polarization of light.

[0023] E/O converter 116 operates to transform digital signals 114₁-114₄ into a corresponding modulated optical output signal 130. More specifically, drive circuits 118₁ and 118₂ transform digital signals 114₁ and 114₂, as known in the art, into electrical analog drive signals I_X and Q_X, respectively. Drive signals I_X and Q_X are then used, in a conventional manner, to drive an I-Q modulator 124_X. In response to drive signals I_X and Q_X, I-Q modulator 124_X operates to modulate an X-polarized beam 122_X of light supplied by a laser source 120 as indicated in FIG. 1, thereby generating a modulated optical signal 126_X.

[0024] The output wavelength of laser source 120 is wavelength λ_n. The optical output power of laser source 120 can be set and/or changed in response to a corresponding component 84_n of control signal 84 (also see FIG. 1).

[0025] Drive circuits 118₃ and 118₄ similarly transform digital signals 114₃ and 114₄ into electrical analog drive signals I_Y and Q_Y, respectively. In response to drive signals I_Y and Q_Y, an I-Q modulator 124_Y operates to modulate a Y-polarized beam 122_Y of light supplied by laser source 120 as indicated in FIG. 1, thereby generating a modulated optical signal 126_Y. A polarization beam combiner 128 operates to combine modulated optical signals 126_X and 126_Y, thereby generating optical output signal 130, this optical output signal being a polarization-division-multiplexed (PDM) signal. Optical output signal 130 is then applied to optical transport link 140.

[0026] FIG. 3 shows a block diagram of an individual-channel optical receiver 74_n that can be used in system 10 (FIG. 1) according to an embodiment, where n=1, 2, ..., N.

[0027] After propagating through link 50, optical signal 130 (see FIG. 2) becomes optical signal 130', which is applied to receiver 74_n. Optical signal 130' may differ from optical signal 130 because optical transport link 50 typically adds noise and imposes various linear and nonlinear signal distortions. The linear distortions may be caused, e.g., by chromatic dispersion (CD), polarization mode dispersion (PMD), and the above-mentioned PDL. The nonlinear distortions may be caused, e.g., by the Kerr effect, including one or more of self-phase modulation (SPM), cross-phase modulation (XPM), and four-wave mixing (FWM).

[0028] Front-end circuit 172 of receiver 190 comprises an optical hybrid 160, light detectors 161₁-161₄, analog-to-digital converters (ADCs) 166₁-166₄, and an optical local-oscillator (OLO) source 156. Optical hybrid 160 has (i) two input ports labeled S and R and (ii) four output ports labeled 1 through 4. Input port S receives optical signal 130' from optical link 50. Input port R receives an OLO signal 158 generated by OLO source 156. OLO signal 158 has an optical-carrier wavelength (frequency) that is sufficiently close to that of signal 130' to enable coherent (e.g., intradyne) detection of the latter signal. OLO signal 158 can be generated, e.g., using a relatively stable tunable laser whose output wavelength (frequency) is approximately the same as the carrier wavelength (frequency) of optical signal 130.

[0029] In an example embodiment, optical hybrid 160 operates to mix input signal 130' and OLO signal 158 to generate different mixed (e.g., by interference) optical signals (not explicitly shown in FIG. 3). Light detectors 161₁-161₄ then convert the mixed optical signals into four electrical signals 162₁-162₄ that are indicative of complex values corresponding to two orthogonal-polarization components of signal 130'. For example, electrical signals 162₁ and 162₂ may be an analog I signal and an analog Q signal, respectively, corresponding to a first (e.g., horizontal, h) polarization component of signal 130'. Electrical signals 162₃ and 162₄ may similarly be an analog I signal and an analog Q signal, respectively, corresponding to a second (e.g., vertical, v) polarization component of signal 130'. Note that the orientation of the h and v polarization axes at receiver 74_n may not coincide with the orientation of the X and Y polarization axes at transmitter 14_n.

[0030] Each of electrical signals 162₁-162₄ is converted into digital form in a corresponding one of ADCs 166₁-166₄. Optionally, each of electrical signals 162₁-162₄ may be amplified in a corresponding electrical amplifier (not explicitly shown) prior to the resulting signal being converted into digital form. Digital signals 168₁-168₄ produced by ADCs 166₁-166₄ are then processed by a DSP 170 to: (i) recover the data of the original input data stream 102 applied to transmitter 14_n, and (ii) generate a corresponding component 78_n of control signal 78 (also see FIG. 1).

[0031] In an example embodiment, DSP 170 may perform, inter alia, one or more of the following: (i) signal processing directed at dispersion compensation; (ii) signal processing directed at compensation of nonlinear distortions; (iii) electronic polarization demultiplexing; and (iv) error correction based on the data encoding applied at DSP 112. Example embodiments of DSP 170 are described in more detail below in reference to FIGs. 5-8.

[0032] In some embodiments, at least some of the signal processing directed at dispersion compensation and/or compensation of nonlinear distortions can be performed at DSP 112 instead of being performed at DSP 170. In this case, this signal processing can be used to pre-distort optical output signal 130 in a manner that causes optical signal 130' to be less distorted than in the absence of this pre-distortion.

[0033] FIG. 4 graphically illustrates example performance characteristics of system 10 according to an embodiment. More specifically, a curve 402 in the graph of FIG. 4 represents an example dependence of the SNR measured at optical receiver 74_n as a function of output optical power of laser 120 of the corresponding transmitter 14_n. Curve 402 has a maximum at the output power P_t. At the optical power levels that are below P_t, the SNR generally increases with an increase of the laser output power. At the optical power levels that are above P_t, the SNR generally decreases with an increase of the laser output power, primarily due to the increasing detrimental contributions of nonlinear optical effects in link 50.

[0034] Target characteristics of a transmitter/receiver pair that communicate with one another over link 50 can be specified using a range of the optical power values around the output power P_t. Typically, the upper and/or lower bounds of this range are the design and/or configuration parameters of system 10 that may be selected and/or specified by the system operator based on the intended use of the system and target performance characteristics.

[0035] A person of ordinary skill in the art will understand that the use of the SNR represents only one of many possible ways of attaining a desired (e.g., optimal) configuration of system 10. For example, in an alternative embodiment, the quality factor Q² or the bit error rate (BER) can similarly be used. In some other embodiments, other suitable measures of transmission quality can alternatively be used.

[0036] The BER is the most-direct indicator of the transmission quality. For example, due to the adverse effects of noise, nonlinearities, and dispersion, the waveforms of optical signals coupled into fibers typically become distorted when those optical signals arrive at the remote end of the fiber-optic link, such as link 50. As a result, bit errors are typically present when the receiver converts the optical signals into the corresponding electrical signals and then decodes the latter. A greater number of pre-FEC bit errors is therefore an indication of the poorer transmission quality, and vice versa.

[0037] For example, for PDM-QPSK modulation, the BER can be computed from the SNR as follows:

[0038] The quality factor Q² and BER have a one-to-one correspondence that can be expressed, e.g., as follows:

where Q(dB) = 10log₁₀(Q²).

[0039] In an example embodiment, Eqs. (1)-(2) can be used to program controller 80 and/or DSP 170 to interconvert various possible quantitative measures of the end-to-end transmission performance of a wavelength channel in system 10.

[0040] As used herein below, the term "measure of transmission quality" should be construed to cover each and any of: (i) the quality factor Q²; (ii) the Q value expressed in decibel; (iii) the BER, e.g., expressed as the number of FEC-code-corrected errors per data frame; and (iv) the SNR. These quantities can be inter-converted, e.g., in the above-explained manner. Furthermore, the term "measure of transmission quality" should also be construed to cover any other value or quantity that can be unambiguously mapped onto any one of those parameters. One or more of such "measures of transmission quality" (e.g., one per carrier wavelength, per transmission direction) can be used to generate control signals 78, 82, and 84 in system 10, e.g., as further explained below.

[0041] FIG. 5 shows a block diagram of DSP 170 (FIG. 3) according to an embodiment. Digital signals 168₁-168₄, output data stream 102, and control signal 78_n are also shown in FIG. 5 to better illustrate the relationship between the circuits shown in FIGs. 3 and 5.

[0042] Ideally, digital signals 168₁ and 168₂ represent the I and Q components, respectively, of the horizontal polarization component of optical signal 130', and digital signals 168₃ and 168₄ represent the I and Q components, respectively, of the vertical polarization component of that optical signal. However, various transmission impairments, front-end implementation imperfections, and configuration inaccuracies generally cause each of digital signals 168₁-168₄ to be a convoluted signal that has various linear and nonlinear distortions and/or contributions from both of the original PDM components 126_X and 126_Y (see FIG. 2). The signal-processing chain of DSP 170 is generally directed at reducing the adverse effects of some signal distortions and de-convolving digital signals 168₁-168₄ so that the transmitted data can be properly recovered to generate the output data stream 102 with an acceptably low BER.

[0043] In an example embodiment, DSP 170 comprises a signal-pre-processing module 310 configured to receive digital signals 168₁-168₄. One of the functions of module 310 may be to adapt the signal samples received via digital signals 168₁-168₄ to a form that is more suitable for the signal-processing algorithms implemented in the downstream modules of DSP 170. For example, module 310 may be configured to (i) resample digital signals 252₁-252₄ such that each of these signals carries two samples per symbol period and/or (ii) convert real-valued signal samples into the corresponding complex-valued signal samples. The resulting complex-valued digital signals generated by signal-pre-processing module 310 are labeled 312₁-312₂.

[0044] Complex-valued digital signals 312₁ and 312₂ are applied to a dispersion-compensation module 320 for dispersion-compensation processing therein, and the resulting dispersion-compensated signals are complex-valued digital signals 322₁-322₂. For example, the total amount of chromatic dispersion, CD_t, compensated by dispersion-compensation module 320 can be expressed as follows:

where D_e is the effective dispersion coefficient; and L₀ is the total length of optical fiber 40 used in link 50. Example circuits that can be used to implement dispersion-compensation module 320 are disclosed, e.g., in U.S. Patent Nos. 8,260,154, 7,636,525, 7,266,310, all of which are incorporated herein by reference in their entirety.

[0045] Digital signals 322₁ and 322₂ are applied to a 2×2 MIMO (multiple-input/multiple-output) equalizer 330 for MIMO-equalization processing therein, and the resulting equalized signals are complex-valued digital signals 332_X and 332_Y. In an example embodiment, equalizer 330 can be a butterfly equalizer configured to perform electronic polarization demultiplexing. Example 2×2 MIMO equalizers that can be used to implement equalizer 330 are disclosed, e.g., in U.S. Patent No. 9,020,364 and U.S. Patent Application Publication No. 2015/0372764, both of which are incorporated herein by reference in their entirety.

[0046] Digital signals 332_X and 332_Y generated by equalizer 330 are applied to a carrier-recovery module 340 that is configured to perform signal processing generally directed at (i) compensating the frequency mismatch between the carrier frequencies of optical LO signal 158 and input signal 130' and/or (ii) reducing the effects of phase noise. Various signal-processing techniques that can be used to implement the frequency-mismatch-compensation processing in carrier-recovery module 340 are disclosed, e.g., in U.S. Patent Nos. 7,747,177 and 8,073,345, both of which are incorporated herein by reference in their entirety. Example signal-processing techniques that can be used to implement phase-error-correction processing in carrier-recovery module 340 are disclosed, e.g., in U.S. Patent No. 9,112,614, which is incorporated herein by reference in its entirety.

[0047] Digital signals 342_X and 342_Y generated by carrier-recovery module 340 are applied to a symbol decoder 350 that converts these digital signals into data streams 352_X and 352_Y, respectively. In an example embodiment, symbol decoder 350 is configured to use the complex values conveyed by digital signals 342_X and 342_Y to appropriately map each complex value onto the operative constellation to determine the corresponding received symbol and, based on said mapping, determine the corresponding bit-word encoded by the symbol. Symbol decoder 350 then appropriately multiplexes and concatenates the determined bit-words to generate data streams 352_X and 352_Y.

[0048] Symbol decoder 350 is further configured to provide each of the determined received symbols to a power-regime monitor 500 by way of digital signals 354_X and 354_Y. Each of such determined received symbols is typically represented by a respective complex value corresponding to the location of the corresponding constellation point on the I-Q plane. Digital signals 354_X and 354_Y are generated to carry the complex values of the constellation points onto which the complex values supplied by digital signals 342_X and 342_Y, respectively, are mapped by symbol decoder 350.

[0049] An FEC decoder 360 is configured to perform forward-error-correction (FEC) processing using data redundancies (if any) in the data carried by the corresponding optical signal 130. The error-corrected data stream generated by FEC decoder 360 is the output data stream 102.

[0050] Power-regime monitor 500 operates to process digital signals 342_X, 342_Y, 354_X and 354_Y to generate control signal 78_n, e.g., as described in more detail below. For example, power-regime monitor 500 may be configured to implement the signal-processing method shown and described in reference to FIG. 6.

[0051] Typically, DSP 170 also includes a clock-recovery circuit (not explicitly shown in FIG. 5). In some embodiments, the clock-recovery circuit may be directly inserted into the data-recovery chain of DSP 170, e.g., between equalizer 330 and carrier-recovery module 340. In some other embodiments, the clock-recovery circuit may be connected in a feedback configuration outside the direct data-recovery chain of DSP 170, with the feedback being provided to signal-pre-processing module 310, a signal interpolator (not explicitly shown in FIG. 5), or ADCs 166. Example clock-recovery circuits that can be used for this purpose are disclosed, e.g., in U.S. Patent No. 9,762,379, which is incorporated herein by reference in its entirety.

[0052] FIG. 6 shows a flowchart of a signal-processing method 600 that can be used in power-regime monitor 500 according to an embodiment. Method 600 enables power-regime monitor 500 to generate control signal 78_n in response to digital signals 342_X, 342_Y, 354_X and 354_Y (also see FIG. 5).

[0053] At step 602, power-regime monitor 500 operates to compute two sets of measures of transmission quality. The first set, denoted as Sx, corresponds to the X-polarization and contains K values of the measure of transmission quality Ψ^(X), i.e., Sx = {Ψ_k^(X)}_{k=1, ..., K}, where the index k denotes different symbol periods. The second set, denoted as Sy, corresponds to the Y-polarization and contains K values of the measure of transmission quality Ψ^(Y), i.e., S_Y = {Ψ_k^(Y)}_{k=1,..., K}. The number K can be, e.g., in the range between 10 and 1000. As already indicated above, in different embodiments, the measure of transmission quality Ψ can be any of: (i) the quality factor Q²; (ii) the Q value expressed in decibel; (iii) the BER; and (iv) the SNR.

[0054] In each of the K symbol periods, Ψ_k^(X) is computed using the complex values supplied by digital signals 342_X and 354_X, and Ψ_k^(Y) is similarly computed using the complex values supplied by digital signals 342_Y and 354_Y.

[0055] For example, in an embodiment in which the measure of transmission quality Ψ is the SNR, the computations of step 602 can be performed in accordance with Eq. (4a):

where Z_c is the complex value supplied in the corresponding symbol period by digital signal 354_X (or 354_Y); and Z_s is the complex value supplied in the corresponding symbol period by digital signal 342_X (or 342_Y). In alternative embodiments, Eq. (4) can be appropriately modified, e.g., using Eqs. (1) and (2).

[0056] In another embodiment in which the measure of transmission quality Ψ is the SNR, the computations of step 602 can be performed in accordance with Eq. (4b):

where P_s+n is the measured received power of the signal corrupted by noise; and P_n is the noise power extracted from the scatter of the signal points around the respective decision points. Said scatter can be determined, e.g., by processing digital signals 342_X and 354_X and digital signals 342_Y and 354_Y over a sequence of time slots as known in the pertinent art.

[0057] A person of ordinary skill in the art will understand that, in other embodiments, other suitable implementations of step 602 are also possible.

[0058] At step 604, power-regime monitor 500 operates to convert the sets S_X and Sy into the corresponding sets Sp and S_D. More specifically, the set Sp is generated by sorting the pair of values Ψ_k^(X) and Ψ_k^(Y) for each k into the better one and the poorer one, with the poorer one then being placed into the set Sp. The set S_D is generated by computing the difference between the better and poorer values for each k.

[0059] The meaning of the terms "better" and "poorer" depends on the embodiment. For example, in the embodiment in which the measure of transmission quality Ψ is the SNR, the better SNR is the larger one of the two values, and the poorer SNR is the smaller one of the two values. In contrast, in the embodiment in which the measure of transmission quality Ψ is the BER, the better BER is the smaller one of the two values, and the poorer BER is the larger one of the two values.

[0060] Under typical operating conditions, the manifestations of PDL observed at receiver 74_n cause the set S_P to contain values from both of the sets S_X and S_Y.

[0061] At step 606, power-regime monitor 500 operates to evaluate the statistical properties of the sets Sp and S_D generated at step 604.

[0062] In an example embodiment, the evaluation may be based on the variance values corresponding to the sets S_P and S_D. These variance values can be used, e.g., to compute a power-regime indicator, R_σ, that can then be used to determine the launch-power change (if any) that can place the corresponding WDM channel into better (e.g., more-optimal) operating conditions.

[0063] As used herein, the term "variance" refers to the expectation of the squared deviation of a random variable from its mean. The variance value provides a measure of how far the (random) numbers of a set are spread out from their average value. Mathematically, the variance can be defined as the square of the standard deviation determined over a statistically sufficient set of data.

[0064] In an example embodiment, power-regime monitor 500 can implement step 606 by computing the power-regime indicator R_σ in accordance with Eq. (5):

where "var" denotes the mathematical function of variance.

[0065] FIG. 7 graphically illustrates an example dependence of the power-regime indicator R_σ (computed at step 606, FIG. 6) on the signal launch power at transmitter 14_n according to an embodiment. More specifically, the results shown in FIG. 7 correspond to an embodiment in which the measure of transmission quality Ψ is the SNR. The corresponding link 50 includes 24 spans of standard single-mode fiber and 20 PDL-causing elements of 0.4 dB each. The number N of WDM channels is N=13.

[0066] The results shown in FIG. 7 indicate that the power-regime indicator R_σ increases when the operating conditions move from the linear regime (where the launch power is smaller than P_t) to a nonlinear regime (where the launch power is greater than P_t). However, in the deep nonlinear regime (where the launch power is greater than about P_t+2dBm), the power-regime indicator R_σ decreases with the increase of the launch power. At the launch power of P=P_t, the power-regime indicator R_σ has the value of R_σ^(Pt) = 0.72. It is also worth noting here that the latter value tends to increase with the number of PDL-causing devices in the link and with the amount of PDL per device.

[0067] Referring back to FIG. 6, at step 608, power-regime monitor 500 operates to compare the value of R_σ computed at step 606 with a reference value R₀. If R_σ is smaller than the reference value R₀, then the processing of method 600 is directed to step 610. Otherwise, the processing of method 600 is directed to step 612.

[0068] In an example embodiment, the reference value R₀ can be selected to be approximately the same as the value of R_σ^(Pt) (also see FIG. 7). However, as already indicated above, R₀ is a parameter of method 600 that can be configuration-specific and/or system-specific. Through experimentation and simulation, it has been determined that, for many practical systems, the value of R₀ = 0.75 can be used to obtain good results. From the provided description, a person of ordinary skill in the art will understand how to select the reference value R₀ for any specific system.

[0069] At step 610, power-regime monitor 500 generates control signal 78_n that requests that the launch power be increased. The processing of method 600 is then directed back to step 602.

[0070] At step 612, power-regime monitor 500 generates control signal 78_n that requests that the launch power be decreased. The processing of method 600 is then directed back to step 602.

[0071] In an example embodiment, the power adjustments ΔP requested at steps 610 and 612 can be read from a preloaded look-up table (LUT). In some embodiments, the values stored in said LUT can be calculated, e.g., using the following approximations.

[0072] If R_σ<0.75, then the corresponding increase ΔP of the launch power requested at step 610 can be computed in accordance with Eq. (6):

[0073] If R_σ≥0.75, then the corresponding decrease ΔP of the launch power requested at step 612 can be computed in accordance with Eq. (7):

[0074] FIG. 8 graphically illustrates SNR gains that can be achieved upon launch-power adjustments of steps 610 and 612 (performed from the launch power indicated on the abscissa) according to an embodiment. More specifically, the results were computed based on the results of FIG. 7, assuming that the launch-power adjustments of steps 610 and 612 are performed in accordance with Eqs. (6) and (7), respectively. The results of FIG. 8 indicate that significant SNR gains can be achieved in this manner. Moreover, after several incremental power-launch adjustments performed using method 600 the resulting launch power tends to end up advantageously close to P_t.

[0075] According to an example embodiment disclosed above, e.g., in the summary section and/or in reference to any one or any combination of some or all of FIGs. 1-8, provided is an apparatus comprising: a polarization-sensitive coherent optical receiver (e.g., 74_n, FIGs. 1, 3) that includes an optical hybrid (e.g., 160, FIG. 1) and a plurality of light detectors (e.g., 161₁-161₄, FIG. 3) connected to receive light outputted by the optical hybrid in response to an optical data signal (e.g., 130', FIGs. 1, 3) applied thereto, the optical data signal having encoded thereon different respective data streams in each of first and second orthogonal polarizations thereof; and a digital signal processor (e.g., 170, FIGs. 1, 5) configured to recover said respective data streams by processing digital samples (e.g., 168₁-168₄, FIG. 3) representing electrical output signals (e.g., 162₁-162₄, FIG. 3) generated by the light detectors in response to the received light; and wherein the digital signal processor comprises a power-regime monitor (e.g., 500, FIG. 5) configured to: compute (e.g., at 602, FIG. 6) first and second sets of values representing a measure of transmission quality of the optical data signal in a sequence of symbol periods, the first and second sets corresponding to the first and second orthogonal polarizations, respectively; and determine (e.g., at 610 or 612, FIG. 6) a launch-power adjustment for the optical data signal using statistical characteristics of the values from said first and second sets (e.g., as determined at 604-606, FIG. 6).

[0076] In some embodiments of the above apparatus, the polarization-sensitive coherent optical receiver is connected to receive the optical data signal through a fiber-optic link (e.g., 50, FIG. 1) characterized by non-zero polarization-dependent loss; and wherein the digital signal processor is configured to compute the statistical characteristics in a manner sensitive to effects of said non-zero polarization-dependent loss.

[0077] In some embodiments of any of the above apparatus, the measure of transmission quality is one of the following: a quality factor Q²; a Q value expressed in decibel; a bit-error rate; and a signal-to-noise ratio.

[0078] In some embodiments of any of the above apparatus, the power-regime monitor is configured to determine said statistical characteristics using third and fourth sets of values, the third set having smaller values from the first and second sets in the symbol periods, and the fourth set having differences of the values from the first and second sets in the symbol periods (e.g., if the measure of transmission quality = SNR).

[0079] In some embodiments of any of the above apparatus, the power-regime monitor is configured to determine an absolute value (e.g., ΔP, Eqs. (6)-(7)) of the launch-power adjustment based on variances of the third and fourth sets (e.g., in accordance with Eqs. (5)-(7)).

[0080] In some embodiments of any of the above apparatus, the power-regime monitor is configured to determine (e.g., at 608, FIG. 6) a sign of the launch-power adjustment by computing an indicator value (e.g., R_σ, Eq. (5)) based on variances of the third and fourth sets and comparing the indicator value with a fixed reference value.

[0081] In some embodiments of any of the above apparatus, the power-regime monitor is configured to determine said statistical characteristics using third and fourth sets of values, the third set having higher values from the first and second sets in the symbol periods, and the fourth set having differences of the values from the first and second sets in the symbol periods (e.g., if the measure of transmission quality = BER).

[0082] In some embodiments of any of the above apparatus, the digital signal processor further comprises a symbol decoder (e.g., 350, FIG. 5); and wherein the power-regime monitor is configured to compute the first and second sets based on decoding decisions of the symbol decoder (e.g., in accordance with Eq. (4)).

[0083] In some embodiments of any of the above apparatus, the digital signal processor further comprises a dispersion-compensation module (e.g., 320, FIG. 5) and an electronic polarization demultiplexer (e.g., 320, FIG. 5) connected upstream from the symbol decoder.

[0084] In some embodiments of any of the above apparatus, the apparatus further comprises: an optical transmitter (e.g., 14_n, FIGs. 1, 2) connected to a distal end of a fiber-optic link (e.g., 50, FIG. 1) to launch the optical data signal therefrom toward the polarization-sensitive coherent optical receiver; and an electronic optical-power controller (e.g., 80, FIG. 1) configured to cause the optical transmitter to change a launch-power of the optical data signal in accordance with the launch-power adjustment determined by the power-regime monitor (e.g., using 78 and 84, FIG. 1).

[0085] In some embodiments of any of the above apparatus, the electronic optical-power controller is further configured to set an optical gain of one or more optical amplifiers (e.g., 30, FIG. 1) in the fiber-optic link (e.g., using 82, FIG. 1).

[0086] In some embodiments of any of the above apparatus, the apparatus further comprises a WDM transmitter (e.g., 12, FIG. 1) that includes the optical transmitter.

[0087] In some embodiments of any of the above apparatus, the apparatus further comprises a WDM receiver (e.g., 62, FIG. 1) that includes the polarization-sensitive coherent optical receiver.

[0088] According to another example embodiment disclosed above, e.g., in the summary section and/or in reference to any one or any combination of some or all of FIGs. 1-8, provided is a machine-implemented optical-power control method comprising the steps of: computing (e.g., at 602, FIG. 6) first and second sets of values representing a measure of transmission quality of an optical data signal in a sequence of symbol periods, the optical data signal having encoded thereon different respective data streams in each of first and second orthogonal polarizations thereof, the first and second sets corresponding to the first and second orthogonal polarizations, respectively; converting (e.g., at 604, FIG. 6) said first and second sets into third and fourth sets of values, the third set having smaller values from the first and second sets in the symbol periods, and the fourth set having differences of the values from the first and second sets in the symbol periods; and determining (e.g., at 610 or 612, FIG. 6) a launch-power adjustment for the optical data signal based on variances of the third and fourth sets.

[0089] In some embodiments of the above method, the measure of transmission quality is one of the following: a quality factor Q²; a Q value expressed in decibel; a bit-error rate; and a signal-to-noise ratio.

[0090] In some embodiments of any of the above methods, the step of determining comprises: computing an indicator value (e.g., R_σ, Eq. (5)) using the variances of the third and fourth sets; and determining (e.g., at 608, FIG. 6) a sign of the launch-power adjustment based on a comparison of the indicator value with a fixed reference value.

[0091] In some embodiments of any of the above methods, the method further comprises the step of computing the first and second sets (e.g., in accordance with Eq. (4)) based on signal-decoding decisions directed at recovering said respective data streams from digital samples representing the two orthogonal polarizations of the optical data signal.

[0092] While this disclosure includes references to illustrative embodiments, this specification is not intended to be construed in a limiting sense. Various modifications of the described embodiments, as well as other embodiments within the scope of the disclosure, which are apparent to persons skilled in the art to which the disclosure pertains are deemed to lie within the principle and scope of the disclosure, e.g., as expressed in the following claims.

[0093] Unless explicitly stated otherwise, each numerical value and range should be interpreted as being approximate as if the word "about" or "approximately" preceded the value or range.

[0094] It will be further understood that various changes in the details, materials, and arrangements of the parts which have been described and illustrated in order to explain the nature of this disclosure may be made by those skilled in the art without departing from the scope of the disclosure, e.g., as expressed in the following claims.

[0095] The use of figure numbers and/or figure reference labels in the claims is intended to identify one or more possible embodiments of the claimed subject matter in order to facilitate the interpretation of the claims. Such use is not to be construed as necessarily limiting the scope of those claims to the embodiments shown in the corresponding figures.

[0096] Although the elements in the following method claims, if any, are recited in a particular sequence with corresponding labeling, unless the claim recitations otherwise imply a particular sequence for implementing some or all of those elements, those elements are not necessarily intended to be limited to being implemented in that particular sequence.

[0097] Reference herein to "one embodiment" or "an embodiment" means that a particular feature, structure, or characteristic described in connection with the embodiment can be included in at least one embodiment of the disclosure. The appearances of the phrase "in one embodiment" in various places in the specification are not necessarily all referring to the same embodiment, nor are separate or alternative embodiments necessarily mutually exclusive of other embodiments. The same applies to the term "implementation."

[0098] Unless otherwise specified herein, the use of the ordinal adjectives "first," "second," "third," etc., to refer to an object of a plurality of like objects merely indicates that different instances of such like objects are being referred to, and is not intended to imply that the like objects so referred-to have to be in a corresponding order or sequence, either temporally, spatially, in ranking, or in any other manner.

[0099] Also for purposes of this description, the terms "couple," "coupling," "coupled," "connect," "connecting," or "connected" refer 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 contemplated, although not required. Conversely, the terms "directly coupled," "directly connected," etc., imply the absence of such additional elements.

[0100] As used herein in reference to an element and a standard, the term compatible means that the element communicates with other elements in a manner wholly or partially specified by the standard, and would be recognized by other elements as sufficiently capable of communicating with the other elements in the manner specified by the standard. The compatible element does not need to operate internally in a manner specified by the standard.

[0101] The described embodiments are to be considered in all respects as only illustrative and not restrictive. In particular, the scope of the disclosure is indicated by the appended claims rather than by the description and figures herein. All changes that come within the meaning and range of equivalency of the claims are to be embraced within their scope.

[0102] The description and drawings merely illustrate the principles of the disclosure. It will thus be appreciated that those of ordinary skill in the art will be able to devise various arrangements that, although not explicitly described or shown herein, embody the principles of the disclosure and are included within its spirit and scope. Furthermore, all examples recited herein are principally intended expressly to be only for pedagogical purposes to aid the reader in understanding the principles of the disclosure and the concepts contributed by the inventor(s) to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions. Moreover, all statements herein reciting principles, aspects, and embodiments of the disclosure, as well as specific examples thereof, are intended to encompass equivalents thereof.

[0103] The functions of the various elements shown in the figures, including any functional blocks labeled as "processors" and/or "controllers," may be provided through the use of dedicated hardware as well as hardware capable of executing software in association with appropriate software. When provided by a processor, 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. Moreover, 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. Other hardware, conventional and/or custom, may also be included. Similarly, 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.

[0104] It should be appreciated by those of ordinary skill in the art that any block diagrams herein represent conceptual views of illustrative circuitry embodying the principles of the disclosure. Similarly, it will be appreciated that any flow charts, flow diagrams, state transition diagrams, pseudo code, and the like represent various processes which may be substantially represented in computer readable medium and so executed by a computer or processor, whether or not such computer or processor is explicitly shown.

Claims

1. An apparatus comprising:

a polarization-sensitive coherent optical receiver (74_n) that includes an optical hybrid (160) and a plurality of light detectors (161₁-161₄) connected to receive light outputted by the optical hybrid in response to an optical data signal (130') applied thereto, the optical data signal having encoded thereon different respective data streams in each of first and second orthogonal polarizations thereof; and

a digital signal processor (170) configured to recover said respective data streams by processing digital samples (168₁-168₄) representing electrical output signals (162₁-162₄) generated by the light detectors in response to the received light; and

wherein the digital signal processor comprises a power-regime monitor (500) configured to:

compute (602) first and second sets of values representing a measure of transmission quality of the optical data signal in a sequence of symbol periods, the first and second sets corresponding to the first and second orthogonal polarizations, respectively; and

determine (610/612) a launch-power adjustment for the optical data signal using statistical characteristics of the values from said first and second sets.

2. The apparatus of claim 1,
wherein the polarization-sensitive coherent optical receiver is connected to receive the optical data signal through a fiber-optic link (50) characterized by non-zero polarization-dependent loss; and
wherein the digital signal processor is configured to compute the statistical characteristics in a manner sensitive to effects of said non-zero polarization-dependent loss.

3. The apparatus of claim 1, wherein the measure of transmission quality is one of the following:

a quality factor Q²;

a Q value expressed in decibel;

a bit-error rate; and

a signal-to-noise ratio.

4. The apparatus of claim 1, wherein the power-regime monitor is configured to determine said statistical characteristics using third and fourth sets of values, the third set having smaller values from the first and second sets in the symbol periods, and the fourth set having differences of the values from the first and second sets in the symbol periods.

5. The apparatus of claim 4, wherein the power-regime monitor is configured to determine an absolute value of the launch-power adjustment based on variances of the third and fourth sets.

6. The apparatus of claim 4, wherein the power-regime monitor is configured to determine a sign of the launch-power adjustment by computing an indicator value based on variances of the third and fourth sets and comparing the indicator value with a fixed reference value.

7. The apparatus of claim 1, wherein the power-regime monitor is configured to determine said statistical characteristics using third and fourth sets of values, the third set having higher values from the first and second sets in the symbol periods, and the fourth set having differences of the values from the first and second sets in the symbol periods.

8. The apparatus of claim 1,
wherein the digital signal processor further comprises a symbol decoder (350); and
wherein the power-regime monitor is configured to compute the first and second sets based on decoding decisions of the symbol decoder.

9. The apparatus of claim 8, wherein the digital signal processor further comprises a dispersion-compensation module (320) and an electronic polarization demultiplexer (320) connected upstream from the symbol decoder.

10. The apparatus of claim 1, further comprising:

an optical transmitter (14_n) connected to a distal end of a fiber-optic link to launch the optical data signal therefrom toward the polarization-sensitive coherent optical receiver; and

an electronic optical-power controller (80) configured to cause the optical transmitter to change a launch-power of the optical data signal in accordance with the launch-power adjustment determined by the power-regime monitor.

11. The apparatus of claim 10, wherein the electronic optical-power controller is further configured to set an optical gain of one or more optical amplifiers (30) in the fiber-optic link.

12. The apparatus of claim 10, further comprising one or both of:

a WDM transmitter (12) that includes the optical transmitter; and

a WDM receiver (62) that includes the polarization-sensitive coherent optical receiver.

13. A machine-implemented optical-power control method comprising:

computing (602) first and second sets of values representing a measure of transmission quality of an optical data signal in a sequence of symbol periods, the optical data signal having encoded thereon different respective data streams in each of first and second orthogonal polarizations thereof, the first and second sets corresponding to the first and second orthogonal polarizations, respectively;

converting (604) said first and second sets into third and fourth sets of values, the third set having smaller values from the first and second sets in the symbol periods, and the fourth set having differences of the values from the first and second sets in the symbol periods; and

determining (610/612) a launch-power adjustment for the optical data signal based on variances of the third and fourth sets.

14. The method of claim 13, wherein the determining comprises:

computing an indicator value using the variances of the third and fourth sets; and

determining a sign of the launch-power adjustment based on a comparison of the indicator value with a fixed reference value.

15. The method of claim 13, further comprising computing the first and second sets based on signal-decoding decisions directed at recovering said respective data streams from digital samples representing the two orthogonal polarizations of the optical data signal.

Drawing