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  Adaptive optical equalization for chromatic and/or polarization mode dispersion compensationRelated Patent Categories: Optical Communications, Transmitter, Including Compensation, Including Feedback, For Modulator ControlThe Patent Description & Claims data below is from USPTO Patent Application 20060034618. Brief Patent Description - Full Patent Description - Patent Application Claims
TECHNICAL FIELD [0001] This invention relates to optical transmission systems and, more particularly, to optical equalization. BACKGROUND OF THE INVENTION [0002] Intersymbol interference (ISI) is a problem commonly encountered in high-speed fiber-optic communication systems. This ISI problem can introduce bit errors and thus degrade the system performance and reliability. It is typically caused by two major impairment sources: chromatic dispersion (sometimes called group velocity dispersion or GVD) and polarization mode dispersion (PMD). Another source of transmission impairments is optical noise. [0003] In a fiber-optic link, a number of optical amplifiers are employed to strengthen the optical signal, but at the same time add in incoherent amplified spontaneous emission (ASE) noise (commonly called optical noise). [0004] Because of the frequency-dependent propagation constant in optical fibers, different spectral components of a pulse travel at slightly different velocities, resulting in pulse broadening in the optical domain. Two parameters are commonly used to characterize first-order and second-order chromatic dispersion (GVD) of a fiber: a dispersion parameter, in ps/km/nm, and a dispersion slope parameter, in ps/km/nm.sup.2. GVD of any order is linear in the optical domain but becomes nonlinear after square-law photo-detection. Usually chromatic dispersion is static and can be effectively compensated by a dispersion compensation module (DCM) comprised of specialty fibers and other passive components. However, a DCM is usually expensive and may add unwanted latency in the optical link that causes a drop in the network quality of service (QoS). It is also possible that residual chromatic dispersion remains even after employing a DCM in the optical ink, and is desirably compensated for by an equalizer. Therefore, for the purpose of evaluating the performance of an adaptive equalizer, the first-order chromatic dispersion is specified in terms of ps/nm without explicitly specifying the fiber type and transmission distance. [0005] Polarization mode dispersion (PMD) is caused by different travelling speeds of two orthogonal polarization modes due to fiber birefringence. Fiber birefringence originates from non-circularity of the fiber core and can also be induced by stress, bending, vibration, and so on. Thus, PMD is dynamic in nature and drifts slowly over time. PMD can be modeled as dispersion along randomly concatenated birefringent fiber segments through mode coupling between neighboring sections. Differential group delay (DGD) is the parameter used to characterize the PMD-induced pulse broadening and follows a Maxwellian distribution. As a result of this variability, the PMD of a fiber is usually characterized by the mean DGD parameter in terms of ps/sqrt(km). In addition, PMD is frequency-dependent. First-order PMD is the frequency-independent component of this frequency-dependent PMD. Second-order (or higher-order) PMD is frequency-dependent and has an effect similar to chromatic dispersion on pulse broadening. [0006] To evaluate the performance of an equalizer, the instantaneous DGD is used instead to describe the delay between the fast and slow orthogonal polarization modes (in particular, the principal states of polarization or PSPs of a fiber). In the worst-case scenario, the input power is split equally between these two orthogonal polarization modes, i.e., the power-splitting ratio=0.5. The performance against the first-order instantaneous DGD (frequency-independent dispersion component) in ps is essential in evaluating the effectiveness of a dispersion compensator. Since these two polarization modes are orthogonal to each other, the photo-current I(t) at the photo-detector is proportional to the summation of the optical power in each polarization. Thus, first-order PMD creates linear ISI at the output of the photo-detector. [0007] Optical equalizers have been used in attempts at compensating for these impairments. The most common form of these equalizers is a cascaded structure, which tends to have less flexibility in control of filter parameters. [0008] In controlling these optical equalizers, usually non-adaptive equalization approaches-are used, but it has been shown that adaptive control algorithms provide good performance improvement. One such adaptive scheme is to monitor the frequency component(s) of the electronic signal. Other adaptive approaches involve nonlinear least squares optimization of criteria such as minimum mean-square-error (MSE), minimization of ISI, or maximization of eye openings. This requires the use of the modified Gauss-Newton or Levenberg-Marquardt methods, which are iterative and incapable of tracking fast change of channel condition. SUMMARY OF THE INVENTION [0009] These and other problems and limitations of prior known optical equalization arrangements are overcome in applicants' unique invention by employing a parallel adaptive equalizer architecture based on a controllable optical modulator device to realize an optical FIR (finite-impulse-response) filter including a plurality of parallel coefficient taps in order to have independent control of each optical filter coefficient. [0010] Additionally, a unique adaptive opto-electronic LMS (least mean squares) process is utilized to generate an electronic error signal utilized to control the plurality of parallel tap coefficients of the parallel optical equalizer. The electronic error signal is used as the optimization criterion to generate control signals to adapt the adaptive optical equalizer because the electronic signal after photo-detection is needed to achieve any measurable performance in terms of bit error rate (BER). [0011] In a specific embodiment of the invention, the controllable optical parallel FIR filter is realized by employing an optical vector modulator. The optical vector modulator is realized by splitting a supplied input optical signal into a plurality of similar parallel optical signals, controllably adjusting the phase and/amplitude of each of the plurality of optical signals and delaying the resulting optical signals in a prescribed manner relative to one another. Then, the "delayed" signals are combined to yield the signal comprising the vector modulated input optical signal to be transmitted as an output. Wherein a signal can be "delayed" by a zero (0) delay interval. [0012] In one particular embodiment, both the phase and amplitude is adjusted of each of the plurality of parallel optical signals, and the error control signals for effecting the adjustments are generated in response to the optical modulator output signal utilizing the unique Opto-Electronic LMS process. BRIEF DESCRIPTION OF THE DRAWING [0013] FIG. 1 shows, in simplified block diagram form, one embodiment of the invention; [0014] FIG. 2 shows, in simplified block diagram form, details of a controllable optical modulator that may be employed in the practice of the invention of the invention; and [0015] FIG. 3 illustrates, in simplified block diagram form, details of another embodiment of the invention. DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION [0016] FIG. 1 shows, in simplified block diagram form, one embodiment of the invention. Specifically, shown is optical light input terminal to which an optical input signal from an optical channel is supplied. Exemplary optical carrier signals to be processed have optical frequencies of about 2.3.times.10.sup.14 Hertz to about 1.8.times.10.sup.14 Hertz, i.e., a wavelength of about 1.3 microns to about 1.7 microns. In one example, an optical carrier signal having a wavelengti of approximately 1.55 micronns, i.e., a frequency of 1.93.times.10.sup.14 Hertz is supplied via input terminal 101 to controllable optical modulator 102. Also supplied to controllable optical modulator 102, via circuit path 112, is error signal e(k), which is used to phase and/or amplitude modulated, i.e., vector modulate the supplied optical signal from input terminal 101 to generate the desire transport signal at output terminal 103. As indicated above, controllable optical modulator 102 is essentially a controllable optical FIR filter. One embodiment of an optical FIR filter that may be advantageously employed as controllable optical modulator 102 in the embodiment of the invention of FIG. 1 is shown in FIG. 2 and described below. As indicated above, other embodiments for optical modulator 102 may also be equally employed in practicing the invention. One such embodiment is an array of optical waveguide gratings. [0017] For a received optical signal E(t) supplied to controllable modulator 102 via input terminal 101 the output optical signal E.sub.o (t) from controllable modulator 102 at output terminal 103 is E O .function. ( t ) = i = 1 n .times. .alpha. i .times. e j .times. .times. .theta. i .times. E .function. ( t - .tau. i ) = i = 1 n .times. c i .times. E .function. ( t - .tau. i ) , ( 1 ) where n is the number of taps for the optical equalizer, .alpha..sub.i is amplitude parameter, .theta..sub.i and c.sub.i=.alpha..sub.ie.sup.j.sup..theta., is the i.sup.th filter coefficient. In one embodiment, for a tap delay of 1/f.sub.s, .tau..sub.i=(i-1)/f.sub.s for i=1, . . . , n. The optical output signal E.sub.o(t) from controllable modulator 102 is transported to an optical receiver and therein to photodiode 104. As is well known, photodiode 104 is a square-law detector and generates a current |q(k)|.sup.2 in response to detection of E.sub.o(t). Transimpedance amplifier 105 converts the current from photodiode 104 to a voltage signal, in well known fashion. The electronic voltage signal from transimpedance amplifier 105 is supplied to slicer unit 106 and to a negative input of algebraic combiner, i.e., algebraic adder 108. An automatic threshold control signal is also supplied to slicer unit 106. The threshold control is such as to slice the voltage signal from transimpedance amplifier 105 in such a manner to realize a desired output level from slicer 106. The output from slicer 106 is the desired compensated received data signal {circumflex over (d)} (k) and is supplied as an output from the receiver and to a positive input to algebraic adder 108. The error signal output from alegbraic combiner 108 is supplied to WUD ({overscore (.alpha.)}, {overscore (.theta.)}) unit 109, where the electronic control signal amplitude ({overscore (.alpha.)}) and phase ({overscore (.theta.)}) values are generated, in accordance with an aspect of the invention, utilizing the unique opto-electronic LMS process. The amplitude ({overscore (.alpha.)}) values and phase ({overscore (.theta.)}) values are supplied via circuit path 110 to adjust the tap coefficients in controllable modulator 102. Note that although a single circuit path 110 is shown, it will be understood that as many circuit paths are included equal to the number of controllable taps or legs included in controllable optical modulator 102. In this example, there may be N such circuit paths. Again, the values of ({overscore (.alpha.)}) and ({overscore (.theta.)}), in this embodiment of the invention, are generated in accordance with the unique opto-electronic LMS process. It is further noted that when only the amplitude of the received optical signal is modulated only the amplitude adjustment values ({overscore (.alpha.)}) are supplied from unit 109 to controllable optical modulator 102. Similarly, when only the phase of the received optical signal is being modulated only the phase adjustment values ({overscore (.theta.)}) are supplied from unit 109 to controllable optical modulator 102. Finally, when both the amplitude and phase of the received optical signal are being modulated both the amplitude adjustment values ({overscore (.alpha.)}) and the phase adjustment values ({overscore (.theta.)}) are supplied from unit 109 to controllable optical modulator 102. [0018] Not shown in the above embodiment is the typical clock data recovery circuitry (CDR). Just before the CDR, an uncompensated detected signal may contain a certain amount of ISI induced by optical impairments along the optical path, such as GVD and PMD. To remove the ISI present in the electronic signal before recovering the bit stream, a coefficient-updating process is employed, in accordance with the invention, to control controllable optical modulator 102. Operating in the optical domain, this process, however, minimizes the electronic error between the compensated signal and the desired signal in the mean square sense in a similar fashion to the least-mean-square (LMS) algorithm for pure electronic equalization. [0019] Thus, the ISI elimination process in this invention utilizes a unique opto-electronic LMS process. Continue reading... Full patent description for Adaptive optical equalization for chromatic and/or polarization mode dispersion compensation Brief Patent Description - Full Patent Description - Patent Application Claims Click on the above for other options relating to this Adaptive optical equalization for chromatic and/or polarization mode dispersion compensation patent application. ###  How KEYWORD MONITOR works... a FREE service from FreshPatents1. Sign up (takes 30 seconds). 2. Fill in the keywords to be monitored. 3. Each week you receive an email with patent applications related to your keywords. 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