WO2005013519A1

WO2005013519A1 - Dynamic broadband adaptive optical equalizer

Info

Publication number: WO2005013519A1
Application number: PCT/IL2004/000723
Authority: WO
Inventors: Yosi Shani; Dror Meiri; Tomer Segev; Varda Herskowits; Menachem Tipris; Hanna Inbar
Original assignee: Aelis Photonics (Israel) Ltd.
Priority date: 2003-08-04
Filing date: 2004-08-04
Publication date: 2005-02-10

Abstract

An optical equalizer that receives one or more optical transmission channels, possibly distorted by Chromatic Dispersion, CD and Polarization Mode Dispersion, PMD, the equalizer comprising: at least one ring resonator, RR; and at least one Mach-Zehnder interferometer, MZI.

Description

DYNAMIC BROADBAND ADAPTIVE OPTICAL EQUALIZER FIELD OF THE INVENTION The present invention relates to methods and apparatus for moderating distortion in optical pulses transmitted over an optical link of a communication network that are caused by dispersion of energy in the pulses during transmission over the link. BACKGROUND OF THE INVENTION An optical communication network transmits digital data between a transmitter and a receiver in the network in the form of pulses of light, usually representing zeros and ones, that are transmitted between the transmitter and receiver via an optical link comprising optical fibers. At a given transmission rate, pulses in a pulse train transmitted by the transmitter are transmitted during temporally contiguous, sequential periods of time, referred to as repetition periods, having substantially a same duration that is determined by the transmission rate. Each pulse in the pulse train is transmitted during its own pulse repetition period. At transmission, each pulse has a well-defined shape and a pulse width equal to or smaller than the pulse repetition period, as a result of which its optical energy is substantially confined to its repetition period. However, as a pulse propagates through an optical fiber it generally suffers attenuation and dispersion as a result of interaction of the pulse with the material from which the fiber is formed. Attenuation reduces an amount of energy in a light pulse while dispersion redistributes the pulse's energy and generally temporally spreads the pulse. The attenuation and dispersion that a pulse suffers during propagation over a fiber can change the pulse shape and/or amplitude to a degree that makes it difficult to identify which digital symbol the pulse represents. In addition, often, dispersion spreads the energy of a pulse to such an extent that after propagating over a length of fiber, energy from a pulse in a pulse train transmitted by a transmitter in the network appears in repetition periods of other pulses in the pulse train. The mixing of optical energy from different pulses in a same given repetition period increases the difficulty in identifying the symbol that the pulse originally transmitted in the given repetition period is intended to represent. The mixing of energy that interferes with symbol identification is referred to as inter-symbol interference (ISI). Various types of interactions of the energy in an optical pulse with the material of the fiber over which it is transmitted generate dispersion. Dispersion generated by a given type of interaction is generally identified and referred to by the given type of interaction. Among the types of dispersion that can affect optical pulses are, for example chromatic dispersion (CD), self phase modulation (SPM) and polarization mode dispersion (PMD). Variation of the index of refraction of the material in a fiber with wavelength of light results in a variation of the phase velocity of light in the fiber as a function of wavelength and gives rise to chromatic dispersion. A pulse of light having a given pulse width comprises light at different wavelengths in a band of wavelengths having a bandwidth inversely proportional to the given pulse width. As a result, energy in a pulse of light that enters an optic fiber as a well formed pulse having a well defined temporal extent, spreads and is "chromatically dispersed", as it travels along the fiber and light at the different wavelengths in the pulse propagate at different phase velocities. For a given optical link, a major portion of chromatic dispersion is typically compensated or "equalized" using various relatively effective devices known in the art, such as dispersion compensating fibers. Some of the CD compensating devices are broad band devices designed to compensate chromatic dispersion in all channels of a communication network, e.g. all the channels in a WDM network. However, generally, compensation is not perfect for all channels comprised in a communication network bandwidth and a residual amount of chromatic dispersion remains. The residual chromatic dispersion (RCD) is a function of wavelength and varies in time as channels in the network are reconfigured and the ambient environment of the network changes. Residual chromatic dispersion in a communication network may often be as large as 0.5 ps/(km-nm) and changes in the residual chromatic dispersion are typically characterized by time constants of hours. Since the bandwidth of light in a pulse of light increases as pulse width decreases, RCD becomes more disruptive of quality of communication as data transmission rates in a communication network increase and widths of pulses required to support the increased transmission rates decreases. For transmission rates in a communication network equal to or greater than about 40 Gbps, compensation of RCD is generally needed to provide acceptable quality communication over the network. In SPM, intensity of the electric field in a light pulse changes the index of refraction of the material in an optic fiber through which the light pulse propagates. Since the electric field is not constant over the light pulse, different regions of the light pulse "see" different indices of refraction and therefore travel at different phase velocities. The different phase velocities result in SPM dispersion of energy in the pulse, which result in the chromatic dispersion pulse distortion effect. Polarization mode dispersion (PMD) is a dominant source of time dependent dispersion that degrades quality of transmission in optical communication networks that transmit data at transmission rates equal to or in excess of about 10 Gbps. Birefringence of materials from which optical fibers in the network are formed generates PMD. Birefringence in an optical fiber is generally caused by the cross section of the fiber being deformed from a substantially circular shape to an elliptical shape. Various causes can contribute to fiber ellipticity. For example, inherent "ellipticity" of a fiber may be produced in sections of the fiber during its manufacture. A fiber may be subject to bending stresses that deform sections of the fiber during handling and deployment of the fiber. After deployment, a fiber very often is subject to random mechanical and/or thermal stresses that deform the fiber and change ellipticity of different sections of the fiber as the ambient environment of the fiber changes. As a result, ellipticity and concomitant birefringence of a fiber in an optical network is generally time dependent and a function of location along the fiber. A section of fiber that is birefringent may generally be described as having two orthogonal axes, a "fast" axis and a "slow" axis. A component of light in a light pulse having polarization parallel to the fast axis propagates in the section of fiber with a phase velocity that is greater than a phase velocity at which a component of light in the pulse having a polarization parallel to the slow axis propagates. After propagation through the section of fiber a portion of energy in the pulse that travels at the slow phase velocity lags behind a portion of energy in the pulse that travels at the fast phase velocity. A difference in transit time through the section of fiber is conventionally referred to as a "differential group delay" and results in "polarization mode" dispersion of the energy in the pulse. At the interface between two consecutive birefringent sections having birefringent axes which are mutually rotated, a mode coupling effect occurs, whereby "fast" and "slow" traveling pulse portions intermix resulting in intermediate transit times. After propagating through a fiber comprised of a multiplicity of mutually rotated birefringent sections the pulse energy is continuously distributed over a spread of transit times representing the various delays. Time constants that characterize changes in PMD of a fiber in a communication network typically range from a few milliseconds to hours and magnitude of PMD for fibers exhibiting substantial PMD may range from 1-10 ps/km^2. Samples of the differential group delay for a length of fiber acquired over an extended period of time are generally characterized by a Maxwellian probability density function. An article by M. Bo n, et. al. entitled "An Adaptive Optical Equalizer Concept for Single Channel Distortion Compensation" Proceedings of the 27^th European Conference on Optical Communication, September 30, 2001, pp 210-211 describes adaptive combined GVD (Group Velocity Dispersion) and SPM (Self Phase Modulation) compensation for a single optical channel using a single filter. The article also shows compensation for a single channel for PMD (Polarization mode dispersion) and indicates that compensation for all three causes of distortion are possible. The filter concept is based on a lattice structure, which can be implemented as a cascade of symmetrical and asymmetrical Mach-Zehnder interferometers (MZI). The symmetrical MZIs function as directional couplers and the asymmetrical MZI function as delay and phase shift elements. An adaptive algorithm is used to determine tap weights for the filter. There is no indication that the system shown is applicable to multi-channel systems and no methodology for correcting dispersion in multi-channel systems is disclosed. US Patent Application Publication 2001/0055437 Al, the disclosure of which is incorporated herein by reference, describes a compensator that compensates PMD in a plurality of WDM channels in a WDM network. US patent 6,538,787, the disclosure of which is incorporated by reference, describes the us of a Mach Zehnder Interferometer (MZI) to compensate for PMD. A paper by M. Bonn, F. Horst, B.J. Offrein, G.L. Bona, E. Meissner, W. Rosenkranz, titled "Tunable dispersion compensation in a 40 Gb/s system using a compact FIR lattice filter in SiON technology," ECOC 2002, paper 4.2.3, which is incorporated herein by reference, describes the use of MZIs for single channel compensation of CD . An article by Yi Li et. al. entitled "Higher-order Error of Discrete Fiber Model and Asymptotic Bound on Multi staged PMD Compensation", in J. Lightwave Technology, 18(9) pp. 1205, 2000, the disclosure of which is incorporated herein by reference, describes modeling an optic fiber as a concatenation of discrete birefringent sections each of which can be considered an elliptical waveplate. Most of these references relate only to compensation for PMD or CD. They do not claim to correct other forms of distortion. The first mention paper to Bohn, et al., on the other hand, relates to correction of multiple caused of distortion, but neither teaches nor describes a system that would make such corrections for multi-channel systems. WO 03/077449, the disclosure of which is incorporated herein by reference, relates to the compensation of multiple types of distortion including PMD and CD. Fig. 1 corresponds to Fig. 1 of WO 03/077449. As described in the reference Fig. 1 schematically shows a multi-channel optical fiber transmission link 10 feeding a wideband adaptive optical equalizer (WAOE) 12. In the exemplary embodiment shown, WAOE 12 comprises a plurality of tunable optical filter units (TOFUs) 14, each of which comprises a beam splitter 16 and a differential delay element 18. Optionally, the beam splitter splits the beam based on the polarization of the beam. Alternatively, the splitting is not based on polarization. One or more TOFUs may also include a phase shifter (not shown). Differential delay element 18 is designed so that the delay caused by element 18 is different for different polarizations. Since a difference in time delay between the differently polarized waves is equivalent to a differential phase shift between the polarizations, these two concepts may be used interchangeably in the following discussion. The phase shifter and differential delay elements have substantially the same function, except that more controllable differences may be achievable if phase shifters are used. In some embodiments the differential delay element is tunable, i.e., the amount of the differential delay is can be adjusted as required to compensate for distortions along transmission link 10. In some embodiments, the differential delay is provided by a birefringent element, optionally one with a variable difference in path length. It is known in the art to utilize Ring Resonators (RR) in compensating for CD for multiple channels simultaneously. See for example US Patent 6,289,151; Lenz and Madsen, "General optical all-pass filter structures for dispersion control in WDM systems," JLT vol. 17, p.1248, 1999; and G.L. Bona, F. Horst, R. Germann, B.J. Offrein, D. Wiesmann, "Tunable dispersion compensators realized in high-refractive-index-contrast SiON technology," ECOC 2002, paper 4.2.1, the disclosures of all of which are incorporated herein by reference. SUMMARY OF THE INVENTION An aspect of some embodiments of the present invention relates to providing a wideband adaptive optical equalizer (WAOE) for an optic fiber link in a communication network that supports a plurality of different optical channels. The WAOE moderates dispersion of energy in optical pulses transmitted via the optical link in a multiplicity, optionally, in all the optical channels supported by the network. In an embodiment of the invention, the WAOE operates on pulses from all the optical channels without de-multiplexing the pulses and correcting each of the channels separately. Optimally, moderation of dispersion in all channels is substantially transparent to the physical cause of the dispersion and a plurality of types of dispersion, and optionally substantially all types of dispersion, e.g. PMD, RCD, SPM, that affect quality of communication over the link, are adaptively moderated. While RRs have been shown to compensate for CD, there are serious problems in using RRs for such compensation. In particular, RR compensation of CD is, by its nature as a resonant system, sensitive to small drifts in the frequency of the signals in the channels. Furthermore, as the system ages or changes are made or happen upstream (such as temperature changes or changes in components), the compensation becomes imperfect. In addition, since the number of RRs used in a practical system is limited, there is a residual distortion even after applying the RRs. Since the CD compensation of the RRs is very sensitive to small changes in the components, settings and the environment, the present inventors do not consider it desirable (although it is possible) to dynamically compensate for CD using RRs. The inventors have also found that while a series of MZIs do compensate for both PMD and CD, as described in WO 03/077449, the compensation of CD can be less perfect than that of

PMD. However, the present inventors have surprisingly discovered that the active compensation of CD by the MZIs is apparently sufficient to make RRs a stable methodology for compensation for the bulk of CD distortion. In addition the MZIs compensate for the varying PMD distortion. In an exemplary embodiment of the present invention, a WAOE comprises a plurality of tunable optical modules including at least one (and preferably a plurality of) tunable ring resonator (RR) and at least one (and preferably a plurality of) tunable Mach Zehnder Interferometer (MZI). In some embodiments of the invention there are a plurality of tunable RRs and tunable MZIs either in parallel or series. As used herein, the term tunable means one or more of frequency, phase, resonance quality and time delay are adjustable and are adjusted during compensation of the system for optical transmission distortions. In exemplary embodiments of the invention, the input beam is split into TE and TM components prior to feeding into the MZIs. In some embodiments, the RRs are all situated prior to splitting the input. In some the RRs are situated in one or both lines after splitting the input signals. In some embodiments the signals are first fed into the RRs and then into the MZIs in others, the order is inverted. In some embodiments, an MZI and RR are incorporated into a same element. In an embodiment of the invention, after splitting, one or both of the TE and TM components are rotated so that they are both either TE or TM. This can greatly simplify the system, since birefringence of components in the system is no longer a factor. The rotation can occur before or after the RRs. Optionally, the WAOE comprises a monitor that samples pulses from a plurality of the channels that are transmitted over the link and through the WAOE to monitor at least one characteristic of the pulses. The control coefficients of the WAOE are determined responsive to the at least one characteristic so as to moderate pulse dispersion over the link for all the channels monitored. In some embodiments, all of the channels are monitored and the dispersion moderated for all of them. In exemplary embodiments of the invention, the dispersion is moderated for all of the channels passing through the WAOE. In some implementations of the invention, the monitored characteristic is a single global parameter, characterizing all the channels passing through the

WAOE. In others it is a vector with elements representing the at least one characteristic for individual elements. In other embodiments of the invention each channel is corrected separately, by separating the channels and passing each channel through a WAOE of the invention. While the WAOEs of the invention are capable of correcting a plurality of channels, in some embodiments it is simpler to use a version of the WAOEs of the invention to correct individual channels. For correction of individual channels, the WAOE may have fewer elements and fewer variables than for multi-channel correction. In an embodiment of the invention, the individual channel characteristic is pulse shape.

Optionally, the characteristic is a power spectrum of the pulses provided by an auto-correlation function determined for the pulses. Optionally, the auto-correlation function is determined using a method and apparatus described in PCT application PCT/IL02/00165, published as WO

03/075490, the disclosure of which is incorporated herein be reference. In an embodiment of the invention, control coefficients of the WAOE are determined responsive to a characteristic defined in terms of a cost function, metric or signal. As used herein the term "cost function" is meant to include any of a cost function, metric or other signal. In some embodiments of the present invention, a cost function is determined responsive to data received from at least one receiver comprised in the network that receives pulses transmitted over the link in a plurality of the channels as to what symbols the received pulses most likely represent. In some embodiments of the present invention, a cost function is determined responsive to a decision made by the at least one receiver as to what symbols the received pulses most likely represent. In some embodiments of the present invention, a cost function is determined responsive to eye opening data for each channel. Alternatively or additionally it is responsive to a Q-factor derived from channel BER data. Optionally, the BER is measured by a component in the receiver, for example an FEC unit. Optionally, the BER signal itself forms the basis for the cost function. Optionally, the cost signal is any measurement, process or calculated number that is monotonic with or correlated with the BER. Optionally, a partial BER (e.g., failure of detection of ones or zeros) is used as the basis for the cost function. For an explanation of the meaning of BER and Q factor, see for example, Agrawal, G.P., Fiber-Optic Communication Systems, 2nd edition, pages 170-175, the disclosure of which is incorporated herein by reference. In some embodiments of the invention, a training sequence is used to determine the settings for the correction network. Since the values received are known for such a sequence, the error measurements are easier to determine. Periodically, training sequences may be sent to allow for a periodic update of the correction network. Alternatively, such sequences may be requested when degeneration of the correction becomes apparent. In some embodiments of the invention, a training sequence is used either initially, periodically or when required to initialize the WAOE and correction is updated using the actual data carrying signals. Alternatively, only the data carrying signals are used. In some embodiments of the present invention, an error signal is a measurement or processed signal that results from a correlation between any of the equalized signals and the unequalized signals. This method of determining error signals is described, for example in "Digital Communications", by J. G. Proakis, Chapter 11, McGraw-Hill, New York, Fourth Edition, 2001. In some embodiments of the present invention, a cost function is a function, such as a sum, weighted sum or other function of error differences or other error indicators as noted above, determined for each of the channels. Optionally, the sum is periodically updated and used to update the control coefficients. As used herein, the term error indicator relates to any of such individual channel metrics. In some embodiments of the present invention each channel is periodically polled and an error indicator determined from at least one received pulse waveform Pk in the channel. The cost function is then updated with respect to the polled error indicator for the channel and the control coefficients updated. In some embodiments of the invention a measure of signal quality is determined by averaging the signal for times related to the period for which the signal is below a given threshold to produce a first metric, which can be used as a measure of signal quality. Optionally a second metric is determined by further averaging the signal for a further reduced period, based on a time for which ISI distortion is prominent, which second metric is predominantly a measure of noise. Optionally, a third metric is determined by averaging signals during the period in which ISI is prominent and optionally reducing the average by a factor related to the second metric. The third metric can be used as a signal quality measure. In some embodiments of the invention, the WAOE settings (or at least some of them) are continuously updated. In others, the settings are only updated when the cost function (or an error indicator for one or more channels) indicates a deterioration of the system below some level. In some embodiments of the invention, only the MZIs are undated continuously or periodically and the RRs are set only at startup or at most very occasionally. In other systems, the RRs are also updated continuously or occasionally together with the MZIs. In some embodiments, the MZIs are updated much more often than the RRs and the RRs are updated when periodic updating of the MZIs does not allow for a sufficiently low BER or sufficiently large eye opening or other quality monitor. In some embodiments of the invention, the settings are initialized by a user or host computer, independent of the cost function. There is thus provided, in accordance with an embodiment of the invention, an optical equalizer that receives one or more optical transmission channels, possibly distorted by Chromatic Dispersion, CD and Polarization Mode Dispersion, PMD, the equalizer comprising: at least one ring resonator, RR; and at least one Mach-Zehnder interferometer, MZI. In an embodiment of the invention, the equalizer comprises a polarizing beam splitter that separates the TM and TE polarization of a signal entering the optical equalizer. In an embodiment of the invention, the equalizer includes a polarization rotator that substantially aligns the polarizations of the split signals. Optionally, at least one of the at least one ring resonators are situated upstream of the beam splitter. Optionally, at least one of the at least one ring resonators are located downstream of the beam splitter. Optionally, at least one of the at least one ring resonators are located between the beam splitter and the polarization rotator. Optionally, at least one of the at least one ring resonators are located downstream of the polarization rotator. Optionally, at least some of the ring resonators are located upstream of all of the MZIs. Optionally, at least one of the at least one ring resonators are located downstream of all the MZIs. Optionally, at least one of the at least one ring resonators are located between MZIs. In an embodiment of the invention, the optical equalizer comprises a polarizing beam combiner, which receives beams along two paths and recombines them into a single path with different polarizations. Optionally, at least one of the at least one ring resonators are located upstream of the polarization beam combiner. Optionally, at least one of the at least one ring resonators are located downstream of the polarization beam combiner. Optionally, the optical equalizer comprises a second polarization rotator in one of the paths feeding the polarization beam combiner that rotates at least one polarization of a signal in one of the paths so that the polarization of the signals in the two paths are substantially orthogonal. Optionally, at least one of the at least one ring resonators are situated between the polarization beam combiner and the second polarization rotator. Optionally, at least one of the at least one ring resonators are situated upstream of the second polarization rotator. Optionally, the optical equalizer comprises at least one adjustable variable differential time delay component that provides a differential delay to signals in two optical paths. Optionally, at least one of the at least one adjustable variable differential time delay components is situated between the MZIs. Optionally, at least one of the at least one RRs is comprised in an adjustable variable differential delay components. Optionally, the optical equalizer includes a plurality of directional couplers that mix waves traveling along different paths in the equalizer. There is further provided, in accordance with an embodiment of the invention, apparatus for correcting distortion on an optical transmission link carrying at least one optical transmission channel, the apparatus comprising: an adjustable optical equalizer according to an embodiment of the invention, through which at least one of said at least one channels pass; a field sampler that samples signals passing through said equalizer; and a controller that receives the samples, determines control parameters for the equalizer therefrom and adjusts the equalizer, responsive to said determined control parameters. Optionally, only a single channel passes through the adjustable optical equalizer and the equalizer is adjusted based only on channel of signals along that channel. In an embodiment of the invention, a first plurality of channels pass through the adjustable optical equalizer; the field sampler separately samples at least a second plurality of channels; and the controller determines control parameters for the equalizer from the second sampled signals and adjusts the equalizer, responsive to said determined control parameters. There is further provided, in accordance with an embodiment of the invention, an optical transmission system that carries a plurality of wavelength spaced optical channels on a optical transmission line, the network comprising an adjustable optical equalizer according to an embodiment of the invention or apparatus for correcting distortion according to the invention In an embodiment of the invention, the apparatus includes a method of calibrating an apparatus for correcting distortion according to an embodiment of the invention, the method including: initially setting the MZIs to a initial MZI state; adjusting the RRs such that distortion of the optical signals at the output of the adjustable optical equalizer is reduced; and then adjusting the MZIs such that the distortion of the optical signals at the output of the adjustable optical equalizer is further reduced. In an embodiment of the invention, initially setting comprises setting the MZIs to a transparent condition. Optionally, after adjusting the MZIs, the RRs are adjusted a further time, with the setting of the MZIs at the adjusted values. Optionally, after further adjusting the RRs the MZIs are further adjusted with the settings of the RRs at the further adjusted values. There is further provided, in accordance with an embodiment of the invention, a method of calibrating an apparatus for correcting distortion according to an embodiment of the invention, the method including: initially setting the MZIs to a initial MZI state; initially setting the RRs to an initial RR state; adjusting the MZIs and the RRs together such that distortion of the optical signals at the output of the adjustable optical equalizer is reduced. There is further provided, in accordance with an embodiment of the invention, a method of calibrating an apparatus for correcting distortion according to an embodiment of the invention, wherein only a small or no amount of CD is present, the method including: initially setting the RRs to a transparent state; and adjusting the MZIs such that distortion of the optical signals at the output of the adjustable optical equalizer is reduced. There is further provided, in accordance with an embodiment of the invention, a method of calibrating an apparatus for correcting distortion according to an embodiment of the invention, wherein a substantially known amount of CD is present, the method comprising: initially setting the RRs to a predetermined state; and adjusting the MZIs such that distortion of optical signals at the output of the adjustable optical equalizer is reduced. There is further provided, in accordance with an embodiment of the invention, a method of adjusting an apparatus for correcting distortion according to an embodiment of the invention, the method comprising: performing a calibration according to any a method of the invention; determining the quality of equalization on an ongoing basis; adjusting only the MZIs in response to said determination or periodically, to reduce signal distortions. Optionally, when adjusting only the MZIs does not result in a satisfactorily low level of signal distortions, adjusting the RRs to further reduce the signal distortion. Optionally, when adjusting only the MZIs does not result in a satisfactorily low level of signal distortions, recalibrating the apparatus using the method of any of claims 25-31. Optionally, when there are frequency changes of the channels such that the RRs no longer satisfactorily correct for CD, said adjusting only of the MZIs is effective to correct said frequency shift caused CD. Optionally, the method maximizes or minimizes a cost function. Optionally, the cost function is derived from signals passed on the individual channels. Alternatively or additionally, the cost function is optionally responsive to a quality of match between an actual pulse shape and an ideal pulse shape. Alternatively or additionally, the cost function is optionally responsive to a quality of match between an actual pulse shape and an undistorted pulse shape. Alternatively or additionally, the cost function is optionally responsive to a peak of pulses in the channels. Alternatively or additionally, the cost function is optionally responsive to a BER in the respective channels. Alternatively or additionally, the cost function is optionally responsive to a Q factor for the respective channels. Alternatively or additionally, the cost function is optionally responsive to an eye opening for the respective channels. Alternatively or additionally, the cost function is optionally responsive to a measured metric of the ISI for the respective channels. Optionally, the cost function gives a higher weight to those channels that are further from desired values than to those that are closer to desired values. Optionally, the controller determines initial control parameters based on measurements on a training sequence of pulses. Optionally, the controller sets initial control parameters based on known or assumed distortions in the transmission link. Optionally, the controller updates the control parameters based on periodic sets of training pulses. Optionally, the controller updates the control parameters based on actual data transmitted on the transmission link. BRIEF DESCRIPTION OF FIGURES Non-limiting examples of embodiments of the present invention are described below with reference to figures attached hereto and listed below. In the figures, identical structures, elements or parts that appear in more than one figure are generally labeled with a same numeral in all the figures in which they appear. Dimensions of components and features shown in the figures are chosen for convenience and clarity of presentation and are not necessarily shown to scale. Fig. 1 schematically shows an optical fiber transmission link comprising a WAOE, in accordance with WO 03/077449; Fig. 2 shows a schematic representation of a first embodiment of a WAOE, in accordance with an exemplary embodiment of the present invention; Fig. 3 shows a schematic representation of an exemplary polarization beam splitter and polarization rotator useful for explaining the operation thereof; Fig. 4 shows a schematic representation of an exemplary ring resonator useful for explaining the operation thereof; Fig. 5 shows a schematic representation of an exemplary tunable coupler useful for explaining the operation thereof; Fig. 6 shows a schematic representation of an exemplary variable differential delay useful for explaining the operation thereof; Fig. 7 shows a schematic representation of an exemplary polarization beam combiner useful for explaining the operation thereof; Fig. 8 schematically shows an alternative feedback mechanism, in accordance with an exemplary embodiment of the invention; Fig. 9 shows a comparison of using only RRs with using an exemplary embodiment of the present invention using RRs and MZIs to correct for CD only; Fig. 10 shows a simplified flow chart for updating a WAOE, in accordance with an exemplary embodiment of the invention; Fig. 11 schematically shows an optical fiber transmission link having a separate path for testing corrections, in accordance with an exemplary embodiment of the invention; Fig. 12 schematically shows an optical fiber transmission link in which multiple sub-bands of channels are corrected, in accordance with an exemplary embodiment of the invention; Fig. 13 shows a schematic representation of a second embodiment of a WAOE, in accordance with an exemplary embodiment of the invention; Fig. 14 shows a schematic representation of a third embodiment of a WAOE, in accordance with an exemplary embodiment of the invention; and Fig. 15 shows a schematic representation of a fourth embodiment of a WAOE, in accordance with an exemplary embodiment of the invention. DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS As indicated above, while RRs have been shown to compensate for CD, there are serious problems in using RRs for such compensation. In particular, RR compensation of CD is, by its nature as a resonant system, sensitive to small drifts in the frequency of the signals in the channels. Furthermore, as the system ages or changes are made or happen upstream (such as temperature changes or changes in components), the compensation becomes imperfect. In addition, since the number of RRs used in a practical system is limited, there is a residual distortion even after applying the RRs. Since the CD compensation of the RRs is very sensitive to small changes in the components, settings and the environment, the present inventors do not consider it desirable (although it is possible) to dynamically compensate for CD using RRs. The inventors have also found that while a series of MZIs do compensate for both PMD and CD, as described in WO 03/077449, the compensation of CD can be less perfect than that of PMD. However, the present inventors have surprisingly discovered that the active compensation of CD by the MZIs is apparently sufficient to make RRs a stable methodology for compensation for the bulk of CD distortion. In addition the MZIs compensate for the varying PMD distortion. Fig. 2 schematically shows a wideband adaptive optical equalizer (WAOE) 412, in accordance with an embodiment of the invention. In the following discussion, WAOE 412 replaces WAOE 12 of Fig. 1, so that in the following discussion reference will be made to other parts of Fig. 1, which when WAOE 12 is replaced by a WAOE of the present invention, is no longer prior art. It should be noted that WAOE 412 utilizes both RRs and MZIs to provide compensation. The special utility of this combination is described below with respect to Fig. 9. The optical signal enterin, g WAOE 412 has TE and TM field components, and is TE,„ described using the input vector . The WAOE functions as a matrix multiplying this TM. vector, to provide an output field output vector:

As shown in Fig. 2, WAOE 412 comprises a polarization beam splitter (PBS) 414, a polarization rotator (PR) 416, a plurality of ring resonators (RR) 418j, a plurality of directional couplers 420j, a plurality of Mach-Zehnder interferometers (MZI) 422k (each including a delay element and pre-and post directional couplers 420] and 420j+ι (Fig. 5) variable differential delay circuits 460m (each including a variable delay element 424m and optionally a fixed delay element), an output polarization rotator 480, a polarization beam combiner (PBC) 426, and an optionally variable optical attenuator 427, where i, j, k, and m are indicies. The following sections describe the matrix representations of the elements included in

WAOE 412 and in other embodiments of the invention described supra. Polarization Beam Splitter A representation of polarization beam splitter 414 is shown in Fig. 3. Polarization beam splitter 414 has an input and two outputs, namely an upper arm 430 and a lower arm 432. Throughout the following discussion, for simplicity, reference 430 shall be used for the upper arm and reference 432 shall be used for the lower arm. For the WAOE, assuming that the upper arm 430 is the PBS TE port, while the lower arm

432 is the PBS TM port, the scattering matrix of the ideal PBS is [PBS] ^■■ Ifthe PBS is

switched such that the upper arm is the PBS TM port while the lower arm is the PBS TE port, the

scattering matrix of the PBS is The following analysis is based on the upper

arm being connected to the TE port; a similar analysis occurs if the upper arm is the TM port. Polarization Rotator - in 7t For a standard PR, shown as PR 415 in Fig. 3, and assuming Θ_{PR in} = — and that upper .__. arm 430 is the PBS TE port, while lower arm 432 is the PBS TM port, the scattering matrix for ^"l : the PR is [PR_in] = Note that outputs 430' and 432' are both TE. Placing a rotator in the 0 1 upper arm is also possible in which case both would be TM. It should be understood that it is also possible to use a different angle of rotation and also to have no rotation at all, but this leads to a much more complex system and adds a possible problem of birefringence of the components used. Ring Resonator Fig.4 shows a representation of a ring resonator 418. RR 418 is made up of a coupler 440 that couples a proportion of the energy input from the left into the ring, a fixed delay 442 and an optional variable delay 443 which provides a phase difference between a signal entering the ring and a signal returning to coupler 440. Coupler 440 couples a portion of the energy that enters it from the ring to the right. For a ring resonator, as shown in Fig. 4, the scattering matrix is

line of Fig. 2,

[RR. seπes ]^■■ where ΗRR [ denotes the transfer

function of the i-th resonator, K_RR is the cross power-coupling-ratio and Θ_RR [rad] is the phase acquired along the ring circumference. While it is presently contemplated that the correction in the upper and lower arms be the same, based on an assumption that CD is the same for both polarizations of the input signal, it is possible, and may be desirable, to vary correction between the arms (i.e., between the input TE and TM polarizations). In some embodiments of the invention, the cross-coupling power ratio may be different for different elements on a same arm and may also be adjustable. The phase acquired along the ring circumference is controlled, for example, by using thermo-optic or electro-optic effects (i.e., by changing the refractive index by heating or by the application of an electric field). Other methods of adjusting the phase may be used. MZI Based Tunable Coupler (MZI) A directional coupler, a following differential delay and a second directional coupler together constitute an MZI based tunable coupler 450. One such tunable coupler is shown in Fig. 5. A typical tunable coupler 450 is made up of a first directional coupler 420j, a variable delay 454 in one of the arms and a second directional coupler 420j+ι . Directional Couplers 420 For directional coupler (DC) 420] (and 420j+ι), the scattering matrix is given by

[DC) = where K _DC is the cross power-coupling-ratio. While the

cross-coupling ratio is shown as being invariant (for example 1/2), the ratio may be variable and, even if constant, may be different for the different couplers. Tunable Coupler-MZI Portion The scattering matrix representing the MZI arms is given by for variable delay (phase) in arm 430 where the sum of π and

ΦMZI [rad] is the phase deviation of MZI arm 430 relative to other arm (effects such as the thermo-optic effect (e.g., using a heater) or the electro-optic effect can be used (e.g., using a variable electric field)), where the effect of both the fixed and variable delays (phase changes) are included. An initial phase shift of π is assumed.

Complete Tunable Coupler (MZI) Accordingly, the matrix for a typical Tunable Coupler (MZI) is [r_ .₁]= [E>C₂]- [ Z/_E^₁]- [E⁾C₁].

Delay A typical variable differential delay circuit 424 is shown schematically in Fig. 6. It includes a variable delay 460 in one arm and a fixed delay 461 in the other arm. The scattering matrix representing differential variable delay circuit 424, assuming for example, delay as

shown, is _-JΘ_D , where Θ_D [rad] is the phase acquired along the arm

with the fixed delay and Φ_H__D [rad] is the phase deviation acquired along the arm with the variable delay. The time delay in the variable delay line (the phase) is controlled for example by a thermo-optic effect, but other mechanisms known in the art can also be used. While a fixed delay is shown in the lower line and a variable delay is shown in the upper line, this is merely a convenience. If a large enough variable delay is available, the fixed delay may be omitted. Alternatively, the fixed and variable delays may be in a same arm and/or both arms may have variable delays. These are all functionally the same, since they give a variable differential delay between the arms. Polarization Rotator - out Assuming that the upper arm 430 in the rotator is the PBS TE port, while the lower arm 432 is the PBS TM port, it follows that TE enters output PR 480 (Fig. 2). The scattering matrix is:

Polarization Beam Combiner A schematic of a typical polarization beam combiner 426 is shown in Fig. 7. Assuming that the upper arm 430 is connected to the PBS TE port, while the lower arm 432 is connected to the PBS TM port, it follows that TE enters the PBC upper arm while TM enters the PBC lower

arm. The scattering matrix is

Variable Optical Attenuator In the VOA 427 in Fig. 2 (which follows the polarization combiner in Fig. 1) there are once again both TE and TM field components. The scattering matrix of the VOA is

[VOA] = [/] • 10 ²⁰ , where IL _VOA [dB] is the VOA insertion loss Complete PLC representation For complete PLC representation the scattering matrices should be multiplied. The PLC matrix is thus given by: [PLC] = [VOA] * [PBC] * [PR_0Ut ] * [T _ Cps + Delays] * [RR] * [PR _ta ] * [PBS] , where:

[T _ Cps + Delays] = [Delay₄ ] * [T _ Cp ₄ ] * [Delay₃ ] * [T_Cp₃]* [Delay₂]* [T_Cp₂]* [Delay₁]* [T_Cp₁] ^' The WOAE of Figs. 2, and 13-15 (described below) can be implemented in integrated planar lightwave circuit (PLC) technology although other technologies, including discreet technologies can also be used. Different parts of the WAOE can be implemented in different technologies and/or separate modules. Referring again to Figs. 1 and 2, WAOE 412 receives a multi-channel signal from transmission link 10 (Fig. 1). Polarization Beam Splitter (PBS) separates the two input polarizations so that they travel down the upper and lower arms of WAOE 412. PR 416 rotates the TM component, so that the wave on both upper and lower lines is TE. (It is also possible to rotate the TE into TM.) As indicated above, this simplifies the system and makes it more robust. In an exemplary implementation of the invention, the following prameters are variable: the resonant frequencies of the RRs, and the variable time delays of the adaptive variable phase shifters and tunable couplers. Resonance quality (set by coupling factors to the rings) of the RRs can also be a variable parameter. As indicated above, since the CD compensation of the RRs is very sensitive to small changes in the components, settings and the environment, RRs parameters may or may not be dynamically compensated. In some embodiments of the invention the differential delay elements and RRs are aligned in such a way that if all the control parameters are at their nominal settings, the equalizer can be considered as transparent. An optical feedback monitor 20, including a multi-channel coupler 22, provides inputs to a controller 24, which adjusts the various parameters of the WAOE to compensate for errors in the transmission along link 10. Alternatively, the feedback may be supplied by a receiver or a stand alone electrical converter which samples the output of the WAOE in parallel to the receiver. This embodiment is shown in Fig. 8. Detector 20' of Fig. 8 may, for example, be a monitor such as that taught in PCT patent application PCT/IL03/00646, the disclosure of which is incorporated by reference. In this application a measure of signal quality is determined by averaging the signal for times related to the period for which the signal is below a given threshold to produce a first metric, which can be used as a measure of signal quality. Optionally a second metric is determined by further averaging the signal for a further reduced period, based on a time for which ISI distortion is prominent, which second metric is predominantly a measure of noise. Optionally, a third metric is determined by averaging signals during the period in which ISI is prominent and optionally reducing the average by a factor related to the second metric. The third metric can be used as a signal quality measure. In the prior art, as shown in Fig. 1, TOFU 14 can be represented by a concatenation of a rotation matrix,

and a differential delay for the two polarizations represented by:

and the transmission matrix for the equalizer for an arbitrary number N of TOFUs in the equalizer may be expressed as: r= W_NR_N* W_N._JR_N._J... W₂R₂* WIRJ . Alternatively, the TOFU can be represented by a concatenation of a rotation matrix and a differential phase shifter. As is well known, a differential phase shifter and a differential delay are equivalent, at least over a range of frequencies. In the present invention, the structure is more complex and includes a variety of correction elements including RRs and MZIs, with additional delay elements. This allows for improved compensation. It should be understood that while increase numbers of variables make for a better theoretical level of correction, there is a balance between theoretical correction accuracy and the ability to converge to a solution when many variables are present. In an embodiment of the invention, the inventors have found that six RRs in each arm and four MZIs give good results. However, in some cases a greater or lesser number can be used. Fig. 9 shows a comparison of the results of compensation for CD by a system with only six RRs, which are fixed and the same system of fixed RRs followed by a series of four MZIs as shown in Fig. 2. In the simulation shown in Fig. 9, the OSNR is 12 dB, there is no PMD and CD is at a level of 7000 picosec/nanometer. The center frequency is 195 THz. As can be seen, the RR only solution is very sensitive to small variations, while the addition of the MZIs gives a much more robust solution. Improvement of stability of CD correction is also expected for other changes in system CD. In some embodiments of the invention the CD is known or can be estimated. For example, for single channel compensation as described herein, knowledge of the CD for the channel allows for pre-setting of the RRs rather than setting them in the field. For multiple channels, if the amount of CD is known for all (or even some) of the channels, the RRs may also be preset to substantially reduce the CD. Adjusting the MZIs will then further improve the CD, as well as reducing the other distortion components. In exemplary embodiments of the invention, individual error indicators for a plurality of channels are defined. Exemplary definitions of such error indicators are described below. The individual error indicators can be expressed as a vector q = [q_j . q_k. . q_κ]; k = 1....K; where K is the number of channels used in the equalization method. Ideally, all of the channels are used in the correction. However, for some systems, cost, convergence and/or response time may mandate using fewer than all of the channels. It is expected that the results using fewer than all the channels may provide results that are close to those using the full number of channels. Since the number of available elements is limited, inter alia for cost reasons and to insure convergence, perfect correction is not generally possible. In some embodiments of the invention, all of the channels transmitted on the transmission system are equalized simultaneously. However, for some systems it may be preferably to simultaneously equalize only a portion of the whole transmitted band comprised in a sub-band. Typically, such a sub-band may comprise 4, 8, 16 or some other number of channels. In such a system, the band is split into such sub-bands which are then equalized, in accordance with apparatus and techniques of the present invention. In some embodiments of the invention, a cost function, based on error indicators for individual channels is used to determine the "goodness" of the equalization. It should be noted that for systems in which multiple sources of distortion are present, the error indicators should be sensitive to all of the distortions that are to be corrected. However, a lesser correction can be achieved with error indicators that are sensitive to only some of the distortions. In some embodiments of the invention, the cost function is a sum (or weighted sum) of the error indicators. In some embodiments of the invention, the cost function is a sum of the squares (or higher order function) of the error indicators. Such a higher order cost function gives a greater weight to those channels in which the error is larger and de-emphasizes (or ignores) the channels for which the error is smaller. This forces the system to search for a solution in which all of the channels are equally corrected. This can be emphasized further by utilizing only error indicators that are above some threshold. Optionally, only the difference from the threshold is used as the indicator. Other ways to form the cost function from error indicators that provide the similar results, will occur to persons of skill in the art. In an embodiment of the invention, the signal at each of the channels (or at least a plurality of the channels) is compared to an idealized input signal. When the actual input signal is known, it can be used for the comparison. Alternatively, an appropriate signal shape is assumed or comparison is made with a δ function. Furthermore, since each of the signals is also attenuated (in addition to the phase distortion being corrected), the output signals may be normalized, in some embodiments of the invention, to correct for attenuation. The form of the signals produced by the multi-channel coupler can (or at least their power spectrum), for example, be determined utilizing the methodology described in WO 03/075490, referenced above. This signal (or spectrum) is then compared with the ideal signal (or spectrum) and a value representing the error is determined. Alternatively, the actual signal is cross-correlated with the ideal signal and a cross-correlation value is determined. An appropriate individual channel error indicator αj is, for example the mean square difference between the actual pulse shape and the ideal pulse shape in one polarization. Alternatively, the couplers couple energy from both polarizations and the shape used for the comparison is the shape of the combined, coupled signal. One useful individual channel error indicator is the peak value of the pulses in the channel. This factor is sensitive to all types of distortion. The peak value is maximized, with qjς being 1 -Vp, where Vp is the peak voltage normalized to 1 as described above. One can then form a scalar cost function as: q= ∑qi. • 1

If n is odd, the absolute value of qj_j should be used. Optionally, n may be more than 1 to emphasize the channels for which the correction is poor. Use of higher order functions also is believed to minimize the effect of noise. As indicated above, alternatively to using all of the measured individual error indicators in the computation of the cost function, only those error indicators greater than some threshold value may be used. This threshold value could be lower than the threshold value for accepting a result as being good. In some embodiments of the present invention, an error indicator is determined responsive to eye opening data for each channel. Alternatively or additionally it is responsive to a Q-factor derived from channel BER data. Optionally, the BER is measured by a component in the receiver, for example an FEC unit. Optionally, the BER signal itself forms the basis for the error indicator. Optionally, the error indicator is any measurement, process or calculated number that is monotonic with or correlated with the BER. Optionally, a partial BER (e.g., failure of detection of ones or zeros) is used as the basis for the error function. Alternatively, the method described in PCT patent application PCT/IL03/00646, can be used. In some embodiments of the present invention, an error signal is a measurement or processed signal that results from a correlation between any of the equalized signals and the unequalized signals. This method of determining error signals is described, for example in "Digital Communications", by J. G. Proakis, Chapter 11, McGraw-Hill, New York, Fourth Edition, 2001. In each of these embodiments multiple feedback signals are provided, one for each channel used in the correction. Alternatively, in some embodiments of the invention, the WAOEs of the invention, can be used to correct only a single channel. In this case, the channels are first separated and then each channel is corrected using a WAOE of the present invention for example as shown in Fig. 8. It may be useful to use more than one comparison method in correcting the transmission. For example, when distortion is high, it may be difficult to use the peak voltage or eye opening methods, since these methods works best when the 0s and Is can be differentiated. Thus, a method that is sensitive only to PMD such as maximizing the power in one of the polarizations or a cross-correlation method may be used first and then one of the more shape specific methods may be used in order to keep the distortion low. Alternatively to utilizing a scalar cost function aggregating the combined individual error indicators , a vector cost function can be used. The control parameters of the system can be defined as a vector: P=Pl - - Pm - - PMl> where m is the index of the control parameter which varies from 1 to M the number of controlled parameters. These parameters include PMZI ^and PRR parameters, which control the MZI and RR elements, respectively. Fig. 10 shows a general flow-chart 200 for a correction scheme according to an embodiment of the invention. The coefficients of the correction vectors E ZI ^and ?RR ^{are se}t to some initial condition at 202. If there is some knowledge of the distortion of the transmission system, the initial condition is set as a first guess for correction, responsive to the known distortion. Otherwise, an arbitrary initial condition or one which adds as little distortion as possible is used. Without loss of generality, the P ZI parameters are set so that the MZIs are transparent. In an embodiment of the invention, optimization of parameters is first performed for the RRs and then for the MZIs. Fig. 10 is a general flow chart for each of these optimizations. The starting point for the MZI optimization is the optimized RR parameters. In an embodiment of the invention when a re-calibration of the system is to be performed, the starting point can be the previous settings of the parameters of the WAOE. A number of iterations (currently 100) is optionally randomly performed over the entire range of variable parameters and the cost function for each is calculated at 204, 206. Then, the best result (e.g. the one with the lowest cost function) is chosen as the new starting point at 208 and the range of parameter searched is reduced by one-half at 210. A number of iterations is then performed at 210, 212 (again, presently 100) using the new, more limited range of parameter variations. This process is repeated a plurality of times, for example until the range is small (214), for example, it is near or under the noise limit of the system. After adjusting the RRs are indicated, then the MZIs are corrected in a similar manner. The inventors have found that this search method is sufficiently robust so that it leads to good results. However, other search methods are also applicable as are other methods for determining convergence. For example, search methods such as those taught in WO 03/077449 may be used for each of the RRs and MZIs. Optionally, after optimizing the RRs and MZIs once, a further optimization of the RRs and optionally the MZIs is performed one or more times. Other optimization methods can be used. For example, in some embodiments of the invention, the coefficients are determined using neural network techniques. In some embodiments of the invention, the coefficients are updated according to LMS, RLS or LS type algorithms or other search algorithims known in the art. In Fig. 1 , a system is shown in which the feedback monitor 20 is in the "main line" of the system. Fig. 11 shows an alternative system in which the control parameters are first tested against a side line and the main line equalizer is controlled only when a sufficiently better result is obtained. Other forms of such dual path filters will occur to persons of skill in the art. Another such implementation is shown in Fig. 8. Fig. 12 shows a system in which the total band of input channels is split into multiple sub-bands, each containing one or more (e.g., 1, 4, 8, 16 or more) channels. While this requires more hardware than the structure of Fig. 1, the iterative algorithms described above can be expected to converge more quickly for the apparatus shown in Fig. 12 than for the apparatus of Figs. 1 or 11. When multiple channels are corrected together, the saving in hardware is still significant over that shown in the prior art. As indicated above, the general structure of Fig. 12 can be used where the division is into individual channels. In that case, the system would be different from the prior art, inter alia because the WAOE is new. Figs. 13-15 are schematic diagrams of various additional embodiments of WAOEs according to the present invention. The embodiment shown in Fig. 13 differs from that shown in Fig. 2 in that the RRs are situated prior to the beam splitter. The embodiment shown in Fig. 14 differs from that shown in Fig. 2 in that the RRs are situated between MZIs. The embodiment shown in Fig. 15 differs from that shown in Fig. 2 in that the RRs are integrated within the MZIs. While the implementations of Figs. 13-15 are believed to give results similar to that of Fig. 2, only the embodiment of Fig. 2 has been implemented and studied extensively. In the description and claims of the present application, each of the verbs, "comprise"

"include" and "have", and conjugates thereof, are used to indicate that the object or objects of the verb are not necessarily a complete listing of members, components, elements or parts of the subject or subjects of the verb. The present invention has been described using detailed descriptions of embodiments thereof that are provided by way of example and are not intended to limit the scope of the invention. The described embodiments comprise different features, not all of which are required in all embodiments of the invention. Some embodiments of the present invention utilize only some of the features or possible combinations of the features. Variations of embodiments of the present invention that are described and embodiments of the present invention comprising different combinations of features noted in the described embodiments will occur to persons of the art. The scope of the invention is limited only by the following claims.

Claims

1. An optical equalizer that receives one or more optical transmission channels, possibly distorted by Chromatic Dispersion, CD and Polarization Mode Dispersion, PMD, the equalizer comprising: at least one ring resonator, RR; and at least one Mach-Zehnder interferometer, MZI.

2. An optical equalizer according to claim 1 wherein the equalizer comprises a polarizing beam splitter that separates the TM and TE polarization of a signal entering the optical equalizer.

3. An optical equalizer according to claim 2 and also including a polarization rotator that substantially aligns the polarizations of the split signals.

4. An optical equalizer according to claim 2 or claim 3 wherein at least one of the at least one ring resonators are situated upstream of the beam splitter.

5. An optical equalizer according to any of claims 2-4 wherein at least one of the at least one ring resonators are located downstream of the beam splitter.

6. An optical equalizer according to claim 3 wherein at least one of the at least one ring resonators are located between the beam splitter and the polarization rotator.

7. An optical equalizer according to claim 3 wherein at least one of the at least one ring resonators are located downstream of the polarization rotator.

8. An optical equalizer according to any of the preceding claims wherein at least some of the ring resonators are located upstream of all of the MZIs.

9. An optical equalizer according to any of claims 1-7 wherein at least one of the at least one ring resonators are located downstream of all the MZIs.

10. An optical equalizer according to any of claims 1 -7 wherein at least one of the at least one ring resonators are located between MZIs.

11. An optical equalizer according to any of the preceding claims and comprising a polarizing beam combiner, which receives beams along two paths and recombines them into a single path with different polarizations.

12. An optical equalizer according to claim 11 wherein at least one of the at least one ring resonators are located upstream of the polarization beam combiner.

13. An optical equalizer according to claim 11 wherein at least one of the at least one ring resonators are located downstream of the polarization beam combiner.

14. An optical equalizer according to claim 11 and comprising a second polarization rotator in one of the paths feeding the polarization beam combiner that rotates at least one polarization of a signal in one of the paths so that the polarization of the signals in the two paths are substantially orthogonal.

15. An optical equalizer according to claim 14 wherein at least one of the at least one ring resonators are situated between the polarization beam combiner and the second polarization rotator.

16. An optical equalizer according to claim 14 wherein at least one of the at least one ring resonators are situated upstream of the second polarization rotator.

17. An optical equalizer according to any of the preceding claims also including at least one adjustable variable differential time delay component that provides a differential delay to signals in two optical paths.

18. An optical equalizer according to claim 17 wherein at least one of the at least one adjustable variable differential time delay components is situated between the MZIs.

19. An optical equalizer in accordance with claim 17 in which at least one of the at least one RRs is comprised in an adjustable variable differential delay components.

20. An optical equalizer according to any of the preceding claims and including a plurality of directional couplers that mix waves traveling along different paths in the equalizer.

21. Apparatus for correcting distortion on an optical transmission link carrying at least one optical transmission channel, the apparatus comprising: an adjustable optical equalizer according to any of the preceding claims, through which at least one of said at least one channels pass; a field sampler that samples signals passing through said equalizer; and a controller that receives the samples, determines control parameters for the equalizer therefrom and adjusts the equalizer, responsive to said determined control parameters.

22. Apparatus according to claim 21 wherein only a single channel passes through the adjustable optical equalizer and the equalizer is adjusted based only on channel of signals along that channel.

23. Apparatus according to claim 21 wherein a first plurality of channels pass through the adjustable optical equalizer; the field sampler separately samples at least a second plurality of channels; and the controller determines control parameters for the equalizer from the second sampled signals and adjusts the equalizer, responsive to said determined control parameters.

24. Optical transmission system apparatus that carries a plurality of wavelength spaced optical channels on an optical transmission line, the network comprising an adjustable optical equalizer according to any of claims 1-20 or apparatus according to any of claims 21-23.

25. A method of calibrating an apparatus according to any of claims 21-24, the method including: initially setting the MZIs to a initial MZI state; adjusting the RRs such that distortion of the optical signals at the output of the adjustable optical equalizer is reduced; and then adjusting the MZIs such that the distortion of the optical signals at the output of the adjustable optical equalizer is further reduced.

26. A method according to claim 25 wherein initially setting comprises setting the MZIs to a transparent condition.

27. A method according to claim 25 or claim 26 wherein after adjusting the MZIs, the RRs are adjusted a further time, with the setting of the MZIs at the adjusted values.

28. A method according to claim 27 wherein after further adjusting the RRs the MZIs are further adjusted with the settings of the RRs at the further adjusted values.

29. A method of calibrating an apparatus according to any of claims 21-24, the method including: initially setting the MZIs to a initial MZI state; initially setting the RRs to an initial RR state; adjusting the MZIs and the RRs together such that distortion of the optical signals at the output of the adjustable optical equalizer is reduced.

30. A method of calibrating an apparatus according to any of claims 21-24, wherein only a small or no amount of CD is present, the method including: initially setting the RRs to a transparent state; and adjusting the MZIs such that distortion of the optical signals at the output of the adjustable optical equalizer is reduced.

31. A method of adjusting an apparatus according to any of claims 21-24, wherein a substantially known amount of CD is present, the method comprising: initially setting the RRs to a predetermined state; and adjusting the MZIs such that distortion of optical signals at the output of the adjustable optical equalizer is reduced.

32. A method of dynamically adjusting an apparatus according to any of claims 21-24, the method comprising: performing a calibration according to any of claims 24-30; determining the quality of equalization on an ongoing basis; adjusting only the MZIs in response to said determination or periodically, to reduce signal distortions.

33. A method according to claim 32, wherein when adjusting only the MZIs does not result in a satisfactorily low level of signal distortions, adjusting the RRs to further reduce the signal distortion.

34. A method according to claim 32, wherein when adjusting only the MZIs does not result in a satisfactorily low level of signal distortions, recalibrating the apparatus using the method of any of claims 25-31.

35. A method according to any of claims 32-34 wherein, when there are frequency changes of the channels such that the RRs no longer satisfactorily correct for CD, said adjusting only of the MZIs is effective to correct said frequency shift caused CD.

36. A method according to any of claims 25-35 wherein said method maximizes or minimizes a cost function.

37. A method according to claim 35 wherein the cost function is derived from signals passed on the individual channels.

38. A method according to claim 36 or claim 37 wherein the cost function is responsive to a quality of match between an actual pulse shape and an ideal pulse shape.

39. A method according to any of claims 36-38 wherein the cost function is responsive to a quality of match between an actual pulse shape and an undistorted pulse shape.

40. A method accordmg to any of claims 36-39 wherein the cost function is responsive to a peak of pulses in the channels.

41. A method according to any of claims 36-40 wherein the cost function is responsive to a BER in the respective channels.

42. A method according to any of claims 36-41 wherein the cost function is responsive to a Q factor for the respective channels.

43. A method according to any of claims 36-42 wherein the cost function is responsive to an eye opening for the respective channels.

44. A method according to any of claims 34-41 wherein the cost function is responsive to a measured metric of the ISI for the respective channels.

45. A method according to any of claims 36-44 wherein the cost function gives a higher weight to those channels that are further from desired values than to those that are closer to desired values.

46. A method according to any of claims 36-45 wherein the controller determines initial control parameters based on measurements on a training sequence of pulses.

47. A method according to any of claims 36-46 wherein the controller sets initial control parameters based on known or assumed distortions in the transmission link.

48. A method according to any of claims 36-47 wherein the controller updates the control parameters based on periodic sets of training pulses.

49. A method according to any of claims 36-47, wherein the controller updates the control parameters based on actual data transmitted on the transmission link.