Reconstruction and restoration of two polarization components of an optical signal field

A digital version of both amplitude and phase of at least one generic polarization component of a received optical signal is developed using dual-polarization direct differential detection with digital signal processing. The received signal is split into orthogonal polarization components, each of which is split into three copies. For each orthogonal polarization component a) an intensity profile is conventionally obtained using a copy and b) phase information is obtained by supplying each remaining copy to a respective one of a pair of optical delay interferometers having orthogonal phase offsets, followed by respective balanced intensity detectors. The outputs the balanced intensity detectors and the intensity profiles are converted into digital representations and used to develop, via signal processing, the optical field information of at least one generic polarization component of the received optical signal. Compensation of impairments, such as PMD, is realized through further processing.

TECHNICAL FIELD

This invention relates to the reconstruction of the two polarization components of an optical signal field and compensation for polarization-mode dispersion.

BACKGROUND OF THE INVENTION

As is well known, an optical signal may have two orthogonal polarization states, each of which may have different properties. Sometimes such polarization states are intentionally introduced, such as in creating a polarization-multiplexed signal in which the two orthogonal polarization states of the optical carrier are arranged so that each carries different data in order to double the spectral efficiency. Such a polarization-multiplexed signal has two so-called “generic” polarization components, each of which carries a single data modulation. Note that by a generic polarization component it is generally intended the signal at the point at which the modulation of that polarization component is completed. It should be appreciated that each generic polarization component may initially, or otherwise, exist separate from the other generic polarization component with which it is later combined.

The polarization orientations of the generic signal components are generally changed by the birefringence of the fiber, and possibly other fiber properties, during the passage of the signal over the optical path. Such changes may be time varying because at least the fiber birefringence is typically a function of various factors such as ambient temperature, mechanical stress, and so forth, which may vary over time and be different at various points of the transmission path. As a result, the polarization orientation of each of the generic signal components is generally unknown at the receiver.

Sometimes, undesirably, the fiber birefringence is so large that polarization-mode dispersion (PMD) is caused, i.e., a generic optical signal component is decomposed into two orthogonal polarization components along the two principle state of polarization (PSP) axes of the fiber, along one of which the light travels at its fastest speed through the fiber and along the other of which the light travels at its slowest speed through the fiber. In such a case, not only may the phase relationship between the two polarization components be time varying, but also each of the two orthogonal polarization components may arrive at the receiver at different times due to the PMD-induced differential group delay (DGD) between the two PSP axes. Note that, actually, as suggested above, each small section of the fiber behaves as if it is its own mini fiber that introduces its own DGD between the two PSP axes. However, for simplification purposes, one may treat the fiber as a single DGD element that introduces a certain DGD between the two axes, based on a first order approximation of the PMD. Thus, for a particular fiber or optical link, PMD is a stochastic effect, and the PMD-induced DGD may also be time varying.

Other linear effects distort optical signals transmitted over optical fibers. Such effects include chromatic dispersion (CD). Optical compensation methods are typically employed to reduce signal distortion that arises due to CD or PMD.

Electronic chromatic dispersion compensation (EDC) has recently emerged as a technique that can flexibly reduce the distortion induced by CD in a cost effective manner. As explained by M. S. O'Sullivan, K. Roberts, and C. Bontu, in “Electronic dispersion compensation techniques for optical communication systems,” ECOC'05, paper Tu3.2.1, 2005, EDC can be performed at the transmitter. Alternatively, EDC can be performed at the receiver. As described by S. Tsukamoto, K. Katoh, and K. Kikuchi, in “Unrepeated Transmission of 20-Gb/s Optical Quadrature Phase-Shift-Keying Signal Over 200-km Standard Single-Mode Fiber Based on Digital Processing of Homodyne-Detected Signal for Group-Velocity Dispersion Compensation,” IEEE Photonics Technology Letters, Volume 18, Issue 9, 1 May 2006, pp. 1016-1018, EDC is implemented with a coherent-detection receiver. In addition, EDC can be implemented with a special direct differential detection receiver as explained by X. Liu and X. Wei, in U.S. patent application Ser. No. 11/525,786 entitled “Reconstruction and Restoration Of Optical Signal Field”, filed on Sep. 22, 2006 and assigned to Lucent Technologies, which is incorporated by reference as if set forth fully herein and shall be referred to hereinafter as Liu-Wei.

Unlike CD, PMD in a fiber link may change very rapidly and PMD compensation usually has to be done in the receiver. Electronic PMD compensation (EPMDC) has also attracted attention recently for its potential cost effectiveness. As explained by J. Hong, R. Saunders, and S. Colaco, in “SiGe equalizer IC for PMD Mitigation and Signal Optimization of 40 Gbits/s Transmission”, published in Optical Fiber Communication Conference 2005, paper OWO2. However, the capability of the EPMDC with a conventional direct-detection receiver is quite limited in that the improvement in PMD tolerance is usually only about 50%.

SUMMARY OF THE INVENTION

In accordance with the principles of the invention, a digital version of the complex field, i.e., both amplitude and phase, e.g., with respect to a reference point, of each of two orthogonal polarization components of a received optical signal are developed at a receiver by employing a dual-polarization direct differential receiver portion that uses direct differential detection to develop a digital representation of optical signals derived from each of two orthogonal polarization components of a received optical signal which are then processed using digital signal processing (DSP) to develop a digital representation of an intensity and a phase profile representing the polarizations as received at the receiver. The reconstructed digital versions of the complex field of each of the two orthogonal polarization components of the optical signal as received at the receiver are then be further processed jointly to develop at least one so-called “generic” polarization component of the received optical. Note that due to fiber birefringence or PMD, the two orthogonal polarization components of the optical signal as received at the receiver after fiber transmission are generally not the generic polarization components of the signal. In accordance with an aspect of the invention, during the joint processing the relative phase difference between the two reference points used in the two reconstructed optical fields is determined. This may be achieved by employing a searching technique.

In one embodiment of the invention, a polarization beam splitter (PBS) is first used to separate the received optical signal into two arbitrarily orthogonal polarization components, Ex′and Ey′. Each of the orthogonal polarization components is supplied to a special direct differential detection receiver, which employs a special pair of optical delay interferometers (ODIs) with a phase delay difference of about π/2, such as are described in Liu-Wei and which are herein referred to as an I/Q ODI pair. At least the four outputs of each I/Q ODI pair are then detected by two balanced detectors, whose two outputs are sampled by respective analog to digital converters (ADCs), are then processed to obtain a digital representation of the received signal optical field along the corresponding polarization axis, i.e., x′ or y′, according to Liu-Wei.

In a second embodiment of the invention, the received optical signal is supplied directly into a single polarization-independent I/Q ODI pair and the resulting four outputs are each connected to a respective associated one of four PBSs, all of which have the same polarization orientation. Each of the PBS produces two outputs, so that in total there are eight outputs from the four PBSs, consisting of four outputs derived from the first polarization, e.g., x′-polarized outputs, and four outputs derived from the second polarization, e.g., y′-polarized outputs. Each pair of outputs of the PBSs that corresponds to a single optical delay interferometer and a single polarization are supplied to a respective one of four balanced detectors, whose outputs are sampled by a respective one of four corresponding ADCs. Each of the resulting sampled waveforms are then processed to obtain a digital representation of the received signal's optical field along each of the polarization axes x′ and y′ in the manner described in Liu-Wei.

Even though this second embodiment requires the use of three additional PBSs as compared to the first embodiment, due to the relative cost of I/Q ODIs themselves and the control electronics associated therewith, as compared to the cost of PBSs, advantageously, because only one I/Q ODI pair is employed, significant cost savings can be achieved. Furthermore, the second embodiment may be more compactly implemented.

Either embodiment of the invention may be implemented with free space or fiber based optics, or any combination thereof.

Although to save cost it is expected that implementers generally will approximate the intensity profile of one or more of the polarization components of the received signal from the absolute value of their respective complex waveforms, they may instead employ direct intensity detection to obtain a more accurate measurement of the intensity profile.

The techniques of the instant invention are suitable to be employed with various types of optical differential phase-shift keying (DPSK) signals, such as differential binary phase-shift keying (DBPSK) and differential quadrature phase-shift keying (DQPSK) signals. They may also be employed with amplitude-shift keying (ASK), combined DPSK/ASK, and quadrature amplitude modulation (QAM).

DETAILED DESCRIPTION

In the claims hereof any element expressed as a means for performing a specified function is intended to encompass any way of performing that function. This may include, for example, a) a combination of electrical or mechanical elements which performs that function or b) software in any form, including, therefore, firmware, microcode or the like, combined with appropriate circuitry for executing that software to perform the function, as well as mechanical elements coupled to software controlled circuitry, if any. The invention as defined by such claims resides in the fact that the functionalities provided by the various recited means are combined and brought together in the manner which the claims call for. Applicant thus regards any means which can provide those functionalities as equivalent as those shown herein.

Unless otherwise explicitly specified herein, the drawings are not drawn to scale.

In the description, identically numbered components within different ones of the FIGs. refer to the same components.

FIG. 1shows an exemplary apparatus, typically in a receiver, arranged in accordance with the principles of the invention, for developing at least one so-called “generic” polarization component of a received optical signal. This is achieved by developing an electronic version of the entire complex optical field of a received optical signal by employing direct differential detection in conjunction with digital signal processing. It is further possible to compensate for various impairments that were inflicted upon the optical signal as it traveled from its source.FIG. 1shows a) polarization beam splitter (PBS)101; b) 1×3 optical splitters102and103; c) optical delay interferometers (ODIs)105,106,107, and108; d) balanced intensity detectors111,112,113, and114; e) single intensity detectors115and116; f) optional amplifiers121,122,123,124,125, and126; g) optional automatic-gain controllers (AGCs)131,132,133,134,135, and136; h) analog-to-digital converters (ADCs)141,142,143,144,145, and146; and i) digital signal processing (DSP) unit150.

More specifically, polarization beam splitter101separates the received optical signal to produce two orthogonal polarization components, Ex′and Ey′therefrom. Ex′is supplied to 1×3 optical splitter102while Ey′is supplied to 1×3 optical splitter103. However, the orthogonal polarization components, Ex′and Ey′are highly unlikely to correspond to the polarization of the generic components that were originally transmitted as they are currently manifest in the received signal.

1×3 optical splitter102replicates the received optical signal so as to produce three copies. One of the three beams produced by 1×3 optical splitter102is supplied to optical delay interferometer (ODI)105, another of the three beams produced by 1×3 optical splitter102is supplied to ODI106, and the last beam is supplied to photodiode115. The optical power allotted to each of the copies from the originally input optical signal is at the discretion of the implementer. In one embodiment of the invention, the power is divided up so that about between 40 to 45 percent of the input power is supplied as output to each of ODIs105and106and the remaining power, e.g., between 10 and 20 percent, is supplied to photodiode115.

As will be readily recognized by those of ordinary skill in the art, optical delay interferometers (ODIs)105,106,107and108may be any type of interferometer having the required characteristics. For example, the ODIs may be based on the well-known, so-called Mach-Zehnder interferometer. Alternatively, the ODIs may be based on the well-known, so-called Michaelson interferometer. Preferably, ODIs105and106are made in a pair so that their phase orthogonality (or π/2 offset in their differential phases between interfering arms) is automatically guaranteed, e.g., using techniques such as disclosed in U.S. patent application Ser. No. 10/875,016 applied for on Jun. 23, 2004 by Christopher R. Doerr and Douglas M. Gill, entitled “Apparatus and Method for Receiving a Quadrature Differential Phase Shift Key Modulated Optical Pulsetrain” published as 2005/0286911 on Dec. 29, 2005 and U.S. patent application Ser. No. 11/163,190 applied for on Oct. 8, 2005 by Xiang Liu, entitled “Optical Demodulating Apparatus and Method” published as 2007/0081826 on Apr. 12, 2007. Further preferably, the two ODI pairs are either monolithically integrated on a same substrate so that their characteristic polarization orientations are the same. Note that the characteristic polarization orientations of an ODI is analogous to the PSP of a fiber.

Each of the ODIs105and107has a delay of about ΔT in the optical path between its respective two arms and a phase difference, i.e., offset, of φ0, where

Δ⁢⁢T=TSsps,(1)
where TSis the symbol period of the signal, sps is the number of samples per symbol taken by the analog to digital converters and φ0is an arbitrarily selected number, which is preferably set at π/4. If so, the free spectral range (FSR), i.e., 1/ΔT, of the ODIs is related to the signal symbol rate (SR) as FSR=SR·sps. Note that, based on numerical simulations, it has been found that, preferably, sps be set to a value of 4. This is because an sps value of less than 4 tends to not be sufficient to accurately represent the signal waveform sufficiently given the procedures described hereinbelow, while sps greater than 4 provides only negligible improvement.

The delay difference may be achieved, in one embodiment of the invention, by adjusting one arm of the interferometer to have a gross length difference of ΔT*C/n, where C is the speed of light in vacuum and n is the index of refraction of the medium of the arm, and then adjusting the length further to cause a phase shift of φ0. Note that in practice, because a phase shift of φ0corresponds to a very small length difference, the phase shift portion may actually be somewhat longer or shorter, so that the total length is φ0plus or minus a multiple of 2π. That way, even thought the length is not precisely φ0, the phase change is effectively φ0.

The total length change used to achieve the effective length change of φ0may be some percentage of the length ΔT·C/n. While even up to 25 percent can work, preferably, the percentage is less than 10 percent, and of course, the more accurate the length can be made to match the actual desired length the better the performance will be. In other embodiments of the invention, the delay required may be divided between the arms, so long as the required delay and phase difference is achieved. Those of ordinary skill in the art will readily recognize how to develop an appropriate arrangement to implement ODIs105and107.

While any value may be employed as the value of phase offset φ0, for compatibility with conventional receivers, as will be seen hereinbelow, certain values of φ0may be advantageously employed. For example, a good value of φ0is π/4 for DQPSK and 0 for DBPSK.

Each of ODIs106and108are similar to ODIs105and107, in that each has delay of about ΔT in the optical path between their respective two arms, but between their arms they each have a phase offset of φ0−π/2. Thus, the difference between the phase offsets of ODIs105and106is π/2, so ODIs105and106are said to have orthogonal phase offsets. Similarly, the difference between the phase offsets of ODIs107and108is π/2, so ODIs107and108are said to have orthogonal phase offsets.

Together, ODI105and106make up a so-called “I/Q ODI pair”. The four outputs of I/Q ODI pair made up of ODIs105and106are then detected by two balanced detectors111and112, respectively, in the manner shown inFIG. 1. The outputs of balanced detectors111and112are amplified by a respective one of amplifiers121and122, and they may then be normalized by one of optional automatic-gain controllers (AGCs)131and132.

Balanced intensity detectors111and112are conventional. Typically, each of balanced intensity detectors111and112is made up of a pair of well-matched photodiodes. Balanced intensity detectors111and112convert the output of each of the arms of ODIs105and106to an electrical representation. Thus, balanced intensity detectors111and112obtain an electrical version of the real and imaginary parts of the complex waveform that contains the information about the phase differences between two time locations separated by ΔT in the polarization component of the received optical signal supplied from PBS101to 1×3 optical splitter102.

Photodiode115performs conventional direct intensity detection, and thus obtains the intensity profile of Ex′in electronic form.

Amplifiers121,122, and125amplify the signals supplied as outputs by balanced intensity detector111, balanced intensity detector112, and photodiode115, respectively. Typically, amplifiers121,122, and125convert the current which is output by the various photodiodes of balanced intensity detector111, balanced intensity detector112, and photodiode115to respective corresponding voltages. To this end, amplifiers121,122, and125may be trans-impedance amplifiers. Furthermore, amplifiers121and122may be differential amplifiers. After amplification, each of the outputs is typically single ended. Optional automatic-gain controllers (AGCs)131,132, and135may be employed to normalize the electronic waveforms prior to digitization.

Analog-to-digital converters (ADCs)141,142, and143perform “digital sampling” of the amplified signals to develop a digital representation of the amplified signals. ADCs141,142, and145typically have the same resolution, e.g., 8 bits.

1×3 optical splitter103, similar to 1×3 optical splitter102, replicates the received optical signal so as to produce three copies. One of the three beams produced by 1×3 optical splitter103is supplied to optical delay interferometer (ODI)107, another of the three beams produced by 1×3 optical splitter103is supplied to ODI108, and the last beam is supplied to photodiode116.

Together, ODIs107and108make up an I/Q ODI pair. The four outputs of I/Q ODI pair made up of ODIs107and108are then detected by two balanced detectors113and114, respectively, in the manner shown inFIG. 1. The outputs of balanced detectors113and114are amplified by a respective one of amplifiers123and124, and they may then be normalized by one of optional automatic-gain controllers (AGCs)133and134.

Balanced intensity detectors113and114are conventional. Typically, each of balanced intensity detectors113and114is made up of a pair of well-matched photodiodes. Balanced intensity detectors113and114convert the output of each of the arms of ODIs107and108to an electrical representation. Thus, balanced intensity detectors113and114obtain an electrical version of the real and imaginary parts of the complex waveform that contains the information about the phase differences between two time locations separated by ΔT in the polarization component of the received optical signal supplied from PBS101to 1×3 optical splitters103.

Photodiode116performs conventional direct intensity detection, and thus obtains the intensity profile of Ey′in electronic form.

Amplifiers123,124, and126amplify the signals supplied as outputs by balanced intensity detector113, balanced intensity detector114, and photodiode116, respectively. Typically, amplifiers123,124, and126convert the current which is output by the various photodiodes of balanced intensity detector113, balanced intensity detector114, and photodiode116to respective corresponding voltages. To this end, amplifiers123,124, and126may be trans-impedance amplifiers. Furthermore, amplifiers123and124may be differential amplifiers. After amplification, each of the outputs is typically single ended. Optional automatic-gain controllers (AGCs)133,134, and136may be employed to normalize the electronic waveforms prior to digitization.

Analog-to-digital converters (ADCs)143,144, and146perform “digital sampling” of the amplified signals to develop a digital representation of the amplified signals. ADCs143,144, and146typically have the same resolution, e.g., 8 bits.

Digital signal processing unit150receives the digital representation of all of the digitized signals supplied from ADCs141-146and develops at least one so-called “generic” polarization component of the received optical. Note that by a generic polarization component it is generally intended the original signal that corresponds to the received signal at the point at which the modulation of that polarization component for transmission is completed.

In accordance with an aspect of the invention, reconstruction unit151-1receives the digitized signals supplied from ADCs141,142, and145and develops a digital representation of the amplitude and phase profiles of one of the polarizations of the received optical signal, e.g., x′. Similarly, in accordance with an aspect of the invention, reconstruction unit151-2receives the digitized signals supplied from ADCs143,144, and146and develops a digital representation of the received optical signal field, i.e., the amplitude and phase profiles, of one of the other polarization of the received optical signal, e.g., y′. To this end, reconstruction unit151-1treats its inputs as if they were the entirety of the optical signal and processes those inputs according to Liu-Wei, e.g., using m=1, prior to any compensation for distortions, e.g., according to the processing described in Liu-Wei in connection with reconstruction unit151thereof. The resulting output for this reconstruction, referred to in Liu-Wei as ER(ts), is referred to herein as Ex′(t). Similarly, reconstruction unit151-2treats its inputs as if they were the entirety of the optical signal and processes those inputs according to according to Liu-Wei, e.g., using m=1, prior to any compensation for distortions, e.g., according to the processing described in Liu-Wei in connection with reconstruction unit151thereof, to develop received optical signal field, i.e., the amplitude and phase profiles. The resulting output for this reconstruction, referred to in Liu-Wei as ER(ts), is referred to herein as Ey′(t).

Due to fiber birefringence or PMD, the two orthogonal polarization components of the optical signal as received at the receiver after fiber transmission are generally not the generic polarization components of the signal. Therefore, in accordance with an aspect of the invention, the reconstructed digital versions of the complex field of each of the two orthogonal polarization components of the optical signal as received at the receiver Ex′(t) and Ey′(t) need to be further processed jointly to develop at least one “generic” polarization component of the received optical, as to be described below.

FIG. 4shows an exemplary polarization “evolution” as an optical signal passes over a typical fiber transmission link402that causes PMD. The two polarization components of the signal after fiber transmission along the two PSP axes of the fiber, defined hereafter as E∥outand E⊥out—treating the fiber as a single DGD element that introduces a certain DGD between the two axes—can be linked to the two orthogonal components of the received signal field Ex′and Ey′as follows,

[Eout⁡(t)E⊥out⁡(t)]=[cos⁡(θ2)sin⁡(θ2)-sin⁡(θ2)cos⁡(θ2)]·[Ex′⁡(t)Ey′⁡(t)·ⅇj·δϕ2]=[cos⁡(θ2)⁢Ex′⁡(t)+sin⁡(θ2)⁢Ey′⁡(t)·ⅇj·δϕ2-sin⁡(θ2)⁢Ex′⁡(t)+cos⁡(θ2)⁢Ey′⁡(t)·ⅇj·δϕ2],(2)
where θ2is the angle between the two characteristic orientations of PBS101and the two PSP axes of fiber402, and δφ2is the additional phase difference between the two reconstructed signal fields Ex′and Ey′as compared to the phase difference of the two received polarization components right after PBS403. The additional phase difference includes an initially unknown relative phase difference between the two reference points used in the two reconstructed optical fields. In accordance with an aspect of the invention, during the joint processing the phase difference δφ2is determined by employing a searching technique, such as is described hereinbelow. Ex′and Ey′are the reconstructed optical fields of the two orthogonal polarization components of the received optical signal as separated by polarization beam splitter403.

The two polarization components of the signal along the two PSP axes of fiber402at the input of fiber402, defined hereafter as E∥inand E⊥in, can be related to E∥outand E⊥outas
E∥in(t)=E∥out(t−τDGD)·ej·δφ,
E⊥in(t)=E⊥out(t),  (3)
where τDGDis the PMD-induced DGD, and δφ is the PMD-induced or birefringence-induced phase difference between the two PSPs, which may be time varying, e.g., due to environmental, e.g., mechanical or temperature changes. Conventionally, the ∥ and ⊥ axes are called the fast PMD axis and the slow PMD axis, respectively. In the case that PMD is sufficiently small, τDGDcan be approximated as 0 in Eq. (3), but the PMD-induced or birefringence-induced phase difference δφ cannot be neglected.

When the original signal emitted from transmitter401is polarization multiplexed to carry two generic polarization components, Exand Ey, the two generic components can be linked to E∥inand E⊥inas

Combing equations (2), (3), and (4), two generic polarization components, Exand Ey, can then be expressed in terms of the received polarization components as

[Ex⁡(t)Ey⁡(t)]=⁢[cos⁡(θ1)-sin⁡(θ1)sin⁡(θ1)cos⁡(θ1)]·[Ei⁢⁢n⁡(t)Ei⁢⁢n⁡(t)]=⁢[cos⁡(θ1)-sin⁡(θ1)sin⁡(θ1)cos⁡(θ1)]·[Eout⁡(t-τDGD)·ⅇj·δϕE⊥out⁡(t)]=⁢[cos⁡(θ1)⁢Eout⁡(t-τDGD)·ⅇj·δϕ-sin⁡(θ1)⁢E⊥out⁡(t)sin⁡(θ1)⁢Eout⁡(t-τDGD)·ⅇj·δϕ+cos⁡(θ1)⁢E⊥out⁡(t)]=⁢[cos⁡(θ1)⁡[cos⁡(θ2)⁢Ex′⁡(t-τDGD)+sin⁡(θ2)⁢Ey′⁡(t-τDGD)·ⅇj·δϕ2]⁢ⅇj·δϕ-sin⁡(θ1)⁡[-sin⁡(θ2)⁢Ex′⁡(t)+cos⁡(θ2)⁢Ey′⁡(t)⁢ⅇj·δϕ2]sin⁡(θ1)⁡[cos⁡(θ2)⁢Ex′⁡(t-τDGD)+sin⁡(θ2)⁢Ey′⁡(t-τDGD)·ⅇj·δϕ2]⁢ⅇj·δϕ+cos⁢(θ1)⁡[-sin⁡(θ2)⁢Ex′⁡(t)+cos⁡(θ2)⁢Ey′⁡(t)⁢ⅇj·δϕ2]].(5)
In the case that the original signal is singly polarized, i.e., it has only one generic polarization component at the transmitter, e.g., Ex, only half of the computation in Eq. (5) is needed.

As shown in Eq. (5), five parameters, θ1, θ2, δφ, δφ2, and τDGD, are generally needed to recover the original optical signal field, which can be either single polarized or polarization multiplexed. When PMD is sufficiently small, e.g., the PMD induced DGD is much smaller than the signal symbol period, τDGDmay be safely set to zero in deriving the original signal field, leaving four parameters, θ1, θ2, δφ, and δφ2, to be determined. Since these parameters are generally time varying, it is needed to find the values of these parameters dynamically.

The digital signal processing needed to recover the generic polarization components from the reconstructed optical fields of the two orthogonal polarization components of the received optical signal can be performed on a block by block basis, with each block having multiple samples.FIG. 5shows an exemplary arrangement to perform the digital signal processing needed to recover the generic polarization components from the reconstructed optical fields of the two orthogonal polarization components of the received optical signal, in accordance with an aspect of the invention. This circuit consists of demultiplexers501and502, M processing units (PUs)505, and multiplexers503and504.

The inputs to the arrangement ofFIG. 5are Ex′(t) and Ey′(t) and the outputs therefrom are Ex(t) and Ey(t). Each of demultiplexers501and502divides the samples it receives over M parallel paths, thereby reducing the processing speed requirement of Pus505. Eventually multiplexers503and504multiplex the processed samples to construct Ex(t) and Ey(t). Note that at any given time, the blocks of samples supplied to one of PUs505may have samples overlapping with those of its adjacent PUs. Note also that the multiplexers and demultiplexers can be shared with the field reconstruction process, e.g., as described in Liu-Wei.

FIG. 6shows an exemplary high level block diagram of arrangement600which is suitable to be used to make up each of PUs505. Arrangement600, when employed as a PU505, receives at one time, for each time period, two corresponding blocks of samples, each block being of length N. N is typically greater than 4 and less than 40, with a suitable value being about 10. The value selected for N represents a tradeoff between accuracy achievable and the speed of computation needed to process the samples. The received samples are supplied to feed-forward subprocessor601and real-time subprocessor602. Feed-forward subprocessor601finds the best guesses of the parameters needed to recover Ex(t) and Ey(t), and feeds these parameters, except for δφ2, to real-time subprocessor602. Real-time subprocessor602receives N pairs of Ex′(t) and Ey′(t) samples, as well as the best guesses of parameters θ1, θ2, δφ, and τDGD, which were determined by feed-forward path601, and supplies as an output the N pairs of Ex(t) and Ey(t) samples after processing the received inputs as described hereinbelow.

FIG. 7shows an exemplary process expressed in flow-chart form for finding the best guesses of the parameters needed to recover Ex(t) and Ey(t), in accordance with an aspect of the invention. This process may be performed in feed-forward subprocessor601. The process begins in step702, when N pairs of Ex′(t) and Ey′(t) samples are received. Next, in step703, using Eq. (5), candidate values of Ex(t) and Ey(t) are calculated for each of the received N pairs of Ex′(t) and Ey′(t) samples. To do so, for each Ex′(t) and Ey′(t) pair, a candidate value is calculated for each possible combination of values for each of the five parameters over their respective physically allowable ranges. For example, the physically allowable ranges may be θ1ε [0,π), θ2ε [0,π), δφε [0,2π), and δφ2ε [0,2π). For τDGDthe range employed may be from 0 to the symbol period of the signal, although it is recognized that τDGDmay actually be larger.

Preferably, this is performed by selecting a combination of values for each of the five parameters and using them to compute N candidate values of Ex(t) and Ey(t). In one embodiment of the invention, the guess values for each of the parameters are uniformly distributed over within its allowed range. Typically, 10 to 20 guess values for each parameter should be sufficient.

One way to perform the calculation is to implement double loop, where the outer loop is the parameter values and the inner loop is the N sample pairs. A loop so arranged facilitates the computation of step704, in which the particular values of the five parameters that minimizes a variance-type quantity of the N candidate Ex(t) and Ey(t) is selected. For example, the values of the five parameters that minimizes the variance of a candidate set of

Ex⁡(t),i.e.,∑t=1N⁢[Ex⁡(t)2-Ex⁡(t)2_]2
is selected as the best guess. Alternatively, the values of the five parameters that minimizes the variance of a candidate set of

Ey⁡(t),i.e.,∑t=1N⁢[Ey⁡(t)2-Ey⁡(t)2_]2
is selected as the best guess. Alternatively, some combination of the two variances may be specified as variance-type quantity to be minimized.

The forgoing selection of the generic polarization components of the signal, i.e., Ex(t) and Ey(t,) assumes that as originally transmitted the generic polarization components intrinsically had a constant intensity, i.e., amplitude, which is generally the case for DPSK-type formats, which include at least DBPSK and DQPSK. Alternatively, the best guesses of these parameters may be found using approaches similar to or based on the constant modulus algorithm (CMA).

In step705, the best guesses for the four parameters θ1, θ2, δφ, and τDGDare supplied as an output, and control passes back to step702to process the next N pairs of Ex′(t) and Ey′(t) samples. Note that the values of the four parameters θ1, θ2, δφ, and τDGDtypically tend to change at a rate that is much slower than the signal symbol rate. Thus, feed-forward subprocessor601need not process all of the blocks of N pairs of Ex′(t) and Ey′(t) samples that it receives, since doing so will yield essentially the same values for those periods of time over which the parameters remain substantially unchanged. For example, the rate of fiber PMD change is usually slower than 10 KHz, which is 106times slower than the symbol rate of a 10-Gbaud signal. Advantageously, this significantly relaxes the computation speed required feed-forward subprocessor601. Of course, should there be a situation in which the rate of change of the values of four parameters θ1, θ2, δφ, and τDGDis more rapid, they may be computed more often, or even for every block.

In another embodiment of the invention, rather than use 10 to 20 guesses for each of the slow varying parameters, θ1, θ2, δφ, and τDGDonly three guess values are employed for each parameter, one being the previous best guess value and the other two being its nearest neighboring guess values. For the angular parameters, the nearest neighbor guess values should be the cyclic neighbors, by which it is generally meant taking modulus with respect to the appropriate value, e.g., 2π for δφ and π for θ1and θ2. The cyclic spacing between the two nearest neighboring guess values should be much smaller than the allowable range of the parameter. Preferably, the spacing is at least 10 times smaller than the allowable range of the parameter. For τDGDthe nearest neighbors are those values that are one minimum step up and one minimum step down, each step being substantially smaller than the symbol period. Preferably, the spacing is at least 5 times smaller than the allowable range of the parameter. Doing so advantageously reduces the amount of computation that is required.

In yet another embodiment of the invention, the guess value for τDGDdoes not need to be searched. Rather, the guess value for τDGDcan be fixed to a fraction of the symbol period, e.g., 0.4 TS, and useful PMD compensation still results.

FIG. 8shows an exemplary process, expressed in flow-chart form, which is performed by real-time subprocessor602in one embodiment of the invention. In step802, N pairs of Ex′(t) and Ey′(t) samples, as well as the best guesses of parameters θ1, θ2, δφ, and τDGDobtained by feed-forward subprocessor601, are received. Next, step803computes Ex(t) and Ey(t) for a set of guess values of δφ2ε [0, 2π) using Eq. (5).

Thereafter, in step804, the best guess of δφ2is found. In one embodiment of the invention, the best guess is the guess by which at least one of the variance of the N quantities that represents

Ex⁡(t),i.e.,∑t=1N⁢[Ex⁡(t)2-Ex⁡(t)2_]2,
and that represents Ey(t), i.e.,

Ey⁡(t),i.e.,∑t=1N⁢[Ey⁡(t)2-Ey⁡(t)2_]2,
is minimized. Note that typically minimizing one of variance of the N quantities that represents Ex(t) and variance of the N quantities that represents Ey(t) results in the other also being minimized. However, this is not always so, e.g., in the presence of noise, and the implementer may instead choose to minimize the difference between the variance of the N quantities that represents Ex(t) and the variance of the N quantities that represents Ey(t).

In step805the N pairs of Ex(t) and Ey(t) samples that correspond to the best guess of δφ2are supplied as outputs and control passes back to step802to process the next N pairs of Ex′(t) and Ey′(t) samples.

FIG. 9shows an exemplary high level block diagram arrangement900that is suitable to be used to make up each of PUs505but which is arranged to speed up the processing as compared to the arrangement shown inFIG. 6. The arrangement ofFIG. 9takes into consideration the fact that parameters θ1, θ2, and τDGDtypically change much more slowly than δφ changes, and therefore they may to be computed at a slower rate. To this end feed-forward subprocessor601ofFIG. 6is further split into first feed-forward subprocessor901, which computes at a slower rate as compared to feed-forward subprocessor601ofFIG. 6, and second feed-forward subprocessor903, which computes at the same rate as did feed-forward subprocessor601ofFIG. 6.

First feed-forward subprocessor901receives as input N pairs of Ex′(t) and Ey′(t) samples and supplies as outputs the best guesses of parameters θ1, θ2, and τDGD. Second feed-forward subprocessor903likewise receives as input N pairs of Ex′(t) and Ey′(t) samples and it also receives as input the best guesses of θ1, θ2, and τDGDwhich are supplied as outputs by first feed-forward subprocessor901. Second feed-forward subprocessor903supplies the best guess value of δφ as an output to real-time subprocessor602and it also passes on the best guesses of θ1, θ2, and τDGD. Note that, alternatively, the best guesses of θ1, θ2, and τDGDcould be supplied directly from first feed-forward subprocessor901to real-time subprocessor602. Advantageously, since the update rate of first feed-forward subprocessor901can be much slower than that of second feed-forward subprocessor903, the computational speed requirement of the feed-forward path is reduced overall.

Real-time subprocessor602receives the same N pairs of Ex′(t) and Ey′(t) samples and the best guesses of parameters θ1, θ2, δφ, and τDGDas it did inFIG. 6, albeit from second feed-forward subprocessor903rather than feed-forward subprocessor601, and outputs the N pairs of Ex(t) and Ey(t) samples based on the signal processing as described hereinabove.

The effectiveness of the electronic PMD compensation (PMDC) described hereinabove can be further improved by treating the fiber as if it was made up of multiple segments, each of which has its own “virtual” DGD parameters. More specifically, instead of the three parameters θ1, δφ, and τDGDpreviously used to describe the fiber-induced PMD, one can treat the fiber link as a concatenation of M PMD segments, i.e., mini fibers, each described by three parameters, θ1i, δφi, and τDGDiwhere i=1, 2, . . . M is the index of the virtual PMD segment. The two received polarization components can be generally linked to the generic polarization components, Exand Ey, as

[Ex′Ey′]=T·[ExEy]=P·R2·∏i=1M⁢PMDM-i+1·[ExEy],(6)
where matrix T represents the polarization transformation of the fiber link, R2is the rotation matrix associated with the projection of the signal components along the fiber PMD PSP axes at the fiber output on the polarization axes of the PBS used in the receiver, PMDiis the matrix describing the PMD effect of the i-th segment, and P is a phase-delay matrix representing the additional phase delay between the two reconstructed fields after the polarization beam splitting at the receiver.

Using the notations shown inFIG. 4, the rotation matrix R2can be written as

R2=[cos⁡(θ2)-sin⁡(θ2)sin⁡(θ2)cos⁡(θ2)].(7)
The phase-delay matrix P can be written as

P=[100ⅇj·δϕ2].(8)
The PMD matrix of the i-th segment, PMDi, can be written in the frequency domain, e.g., after a Fourier transform from the time domain, as

[ExEy]=T-1·[Ex′⁡(t)Ey⁢′⁡(t)]=∏i=1M⁢(PMDi)-1·R2-1·P-1⁡[Ex′Ey′],(11)
where −1 indicates the standard matrix inverse operation, i.e., the product of a matrix and its inverse is the identity matrix I.

FIG. 10shows exemplary high level block diagram of arrangement1000suitable to be used to make up each of PUs505but which is arranged to treat the fiber as if it was made up of multiple segments, so as to achieve better compensation for PMD, including higher order PMD, than can be achieved than using the arrangements ofFIG. 6or9. The process for performing such electronic PMDC assumes that the fiber was made up of M “virtual” PMD elements. Feed-forward subprocessor1001receives as inputs N pairs of Ex′(t) and Ey′(t) samples and supplies as its output the best guesses of parameters θ11, δφ1, and τDGD1. . . θ1M, δφM, and τDGDM. Real-time subprocessor1002receives as inputs the N pairs of Ex′(t) and Ey′(t) samples as well as the best guesses of the parameters supplied by feed-forward subprocessor1001, and supplies as outputs the N pairs of Ex(t) and Ey(t) samples using the methods described hereinabove.

As M increases, the PMDC capability also increases. However, the needed computation power to perform the PMDC calculations increases as well. Thus, there is a tradeoff between required computation power and the PMDC that is performed. Note that oftentimes setting M=2 is sufficient to provide better than first order PMD compensation without requiring a severe increase in processing power. For further simplification, the guess values for τDGD1. . . τDGDMcan be fixed to a fraction of the symbol period, e.g., 0.4 TS, as noted hereinabove.

FIG. 11shows high level block diagram of arrangement1100suitable to be used to make up each of PUs505but which is arranged to treat the fiber as if it was made up of 2 segments each having DGD values each fixed to 0.4 TS.

Note that, in accordance with an aspect of the invention, the digital PMD compensation schemes described above may also be employed with so-called “dual-polarization coherent-detection” receivers, where digital representations of two orthogonal polarization components of the received optical signal are obtained. A typical dual-polarization coherent-detection receiver with DSP is shown in “Uncompensated Transmission of 86 Gbit/s Polarization Multiplexed RZ-QPSK over 100 km of NDSF Employing Coherent Equalization” by Fludger et al., which was published as ECOC'06 post-deadline paper Th4.3.3, which is incorporated by reference as if fully set forth herein.FIG. 12shows an exemplary high level block diagram of arrangement1200which is suitable to be used to make up each of PUs505but which is arranged for use with a coherent-detection receiver, in accordance with an aspect of the invention.

As is well known, such coherent detection receivers employ, an optical local oscillator (OLO) that provides an absolute phase reference for both received polarization components. As a result, there is no uncertainty in the additional phase difference δφ2due to the field reconstruction process. Consequently, δφ2does not need to be estimated in real-time subprocessor1202, and instead can be estimated in feed-forward subprocessor1201so that the computational effort in the real-time subprocessor is much reduced. Typically, δφ2changes at a very low speed, e.g., <1 KHz, so feed-forward subprocessor1201only needs to update δφ2at a much lower speed than the signal date rate.

FIG. 13, similar toFIG. 8, shows an exemplary process, expressed in flow-chart form, that is performed in real-time subprocessor1202, in one embodiment of the invention. In step1302N pairs of Ex′(t) and Ey′(t) samples, as well as the best guesses of parameters θ1, θ2, δφ, δφ2, and τDGDfor each of M virtual segments obtained by feed-forward subprocessor1201, are received. Next, step1303computes Ex(t) and Ey(t) using Eq. (11). In step1305the N pairs of Ex(t) and Ey(t) samples are supplied as outputs and control passes back to step1302to process the next N pairs of Ex′(t) and Ey′(t) samples.

As mentioned hereinabove, to simplify, the DGD values can be fixed, e.g., each equal to about 0.4 Ts. Also, M can be chosen to be 2.

Once the original signal field is obtained, further compensation for other impairments, e.g., chromatic dispersion and/or self-phase modulation, and data recovery following suitable demodulation, can be performed to extract the data content from at least on generic polarization, using compensation for other impairments unit152, demodulation unit154, and data recovery unit155.

A practical issue with the use of differential detection is that it employs ODIs, which typically exhibit polarization-dependent phase shift (PDPS). In other words, generally, the phase offset between the two arms of an ODI, φ, is dependent on the polarization state of the optical signal. When the signal polarization is aligned with one of the two characteristic polarization orientations of the ODI, the phase offset φ reaches its maximum or its minimum. The PDPS is the difference between the maximum and the minimum phase offsets.

There are three common types of ODI: 1) fiber-based, 2) planar lightwave circuit (PLC)-based, and 3) free-space optics-based. The PDPS of a fiber-based ODI is typically due to the birefringence of the fiber resulting from mechanical stress. The PDPS of a PLC-based ODI is typically due to the birefringence of the waveguide structure of the PLC. The PDPS of a free-space optics based ODI is typically due to the polarization-dependent phase delay of the beam splitter used in forming the two optical interference paths. The PDPS can range from about 2 degrees (0.035 rad.) to about 20 degrees (0.35 rad.), the particular value for any ODI depending on its design.

The field reconstruction process as described herein relies on the phase difference estimation at multiple sampling locations. If the signal polarization is not aligned with one of the two characteristic polarization orientations of the ODI, the PDPS will accumulate as the number of sampling points increases and prevent accurate phase estimation. Thus, it is preferred to align PBS101in such a way that the polarization of each of the two split signals, i.e., Ex′and Ey′is aligned with one of the two characteristic polarization orientations of the ODIs. This can be achieved by using, e.g., 1) polarization-maintaining fibers with suitable orientations to connect the two outputs of PBS101with the two inputs of ODI pairs105-106and107-108, or 2) polarization-maintaining free space optical connections between PBS101and the two inputs of ODI pairs105-106and107-108when the ODIs are free-space optics based.

FIG. 2shows an embodiment of the invention similar to that shown inFIG. 1but in which the intensity detection branches are omitted. In accordance with an aspect of the invention, the intensity profile for each polarization component is approximated from the absolute value of its respective one of the complex waveforms rather than directly recovered from the received optical signal.

FIG. 3shows another embodiment of the invention similar to that shown inFIG. 1but which employs only a single I/Q ODI pair. This is achieved by eliminating PBS101ofFIG. 1and employing in lieu thereof four PBSs301at the four outputs of the first I/Q ODI. Furthermore, as inFIG. 2, the intensity detection branches are omitted and the intensity profile for each polarization component is approximated rather than directly recovered from the received optical signal. Advantageously, the cost of the overall arrangement, as with respect to the arrangement ofFIG. 1, is significantly reduced.

In the embodiment ofFIG. 3the received signal is supplied directly into a single polarization-independent I/Q ODI pair made up of ODIs305and306, whose four outputs are each connected to a respective one of PBSs301with the same polarization orientation. The eight outputs from PBSs301, consisting of four x′-polarized outputs and four y′-polarized outputs, are then treated in the same manner as the outputs of the I/Q ODI pairs ofFIG. 1, i.e., detected by balanced detectors whose outputs are sampled, after optional amplification and gain control, by a respective ADC. The sampled waveforms are then processed to obtain a digital representation of the signal optical field as described hereinabove.

As will be readily understood by those of ordinary skill in the art, the instant invention may be applied to optical differential phase-shift keying (DPSK) signals, such as differential binary phase-shift keying (DBPSK) and differential quadrature phase-shift keying (DQPSK) signals, since ODI(s) and balanced detection are commonly used for DPSK detection. Furthermore, this invention may also be applied to amplitude-shift keying (ASK), combined DPSK/ASK, and differential QAM.