CA2515280A1 - Adaptive optical equalization for chromatic and/or polarization mode dispersion compensation and joint opto-electronic equalizer architecture - Google Patents

Abstract

An adaptive optical parallel equalizer architecture is based on a controllable optical FIR filter device to realize an optical FIR (finite-impulse-response) filter including a plurality of coefficient taps in order to have independent control of each optical FIR filter coefficient. A unique adaptive opto-electronic LMS (least mean square) process is utilized to generate an electronic error signal utilized to control the plurality of parallel tap coefficients of the optical parallel equalizer. The electronic error signal is used as the optimization criterion because the electronic signal after photo-detection is needed to achieve any measurable performance in terms of bit error rate (BER). In a specific embodiment, 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 parallel similar optical signals, controllably adjusting the phase and/or 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 optical signal comprising the vector modulated input optical signal to be transmitted as an output. 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 FIR filter optical output signal utilizing the unique opto-electronic LMS process.

Description

Chen 34/36-4/5-7/8 1 ADAPTIVE OPTICAL EQUALIZATION FOR CHROMATIC AND/OR
POLARIZATION MODE DISPERSION COMPENSATION AND JOINT
OPTO-ELECTRONIC EQUALIZER ARCHITECTURE
Technical Field This invention relates to optical transmission systems and, more particularly, to optical equalization.
Baclt~round of the Inyention Intersymbol interference (ISI) is a problem commonly encountered in high speed fiber-optic communication systems. This ISI problem can introduce bit errors 1 o 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 optical transmission impairments is optical noise.
In a fiber-optic link, a number of optical amplifiers are employed to strengthen 1 s the optical signal. At the same time, such amplifiers add incoherent amplified spontaneous emission (ASE) noise (commonly called optical noise).
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 2o characterize first-order and second-order chromatic dispersion (GVD) of a fiber: a dispersion parameter, in pslkm/nm, and a dispersion slope parameter, in ps/km/nmz .
GVD of any order is linear in the optical domain but becomes nonlinear after square-law photo-detection in the receiver. Usually chromatic dispersion is static and can be effectively compensated by a dispersion compensation module (DClvn comprised of 25 negative dispersion fibers or 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 3o performance of an adaptive equalizer, the first~rder chromatic dispersion is specified Chen 34/36-4/5-7/8 2 in terms of ps/nm without explicitly specifying the fiber type and transmission distance.
Polarization mode dispersion (PMD) is caused by differenttraveling 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 to broadening and may follow 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(lcm). 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.
To evaluate the performance of an equalizer, the instantaneous DGD is used to describe the delay between the fast and slow orthogonal polarization modes (in particular, the principal states of polarization (PSPs) of a fiber). In the wors~case scenario, the input power is split equally between these two orthogonal polari~tion 2o modes, i.e., the power-splitting ratio = 0.5. The performance against the firs~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~urrent 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.
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.

Chen 34/36-4/5-7/8 3 In controlling these optical equalizers, often non-adaptive equalization approaches are used, but these approaches have proven inadequate. What is needed in the art is a better way to compensate for chromatic and/or polarization mode dispersion.
Summary ' In various embodiments; these and other problems and limitations of prior known optical equalization arrangements are overcome in applicants' unique invention by employing a controllable optical FIR filter device to realize an optical FIR (finite-impulse-response) filter.
1o In one aspect, the present invention provides an apparatus for use in an adaptive optical equalizer. In one embodiment, the apparatus includes: (1) a controllable optical FIR filter having an input and an output, and being coupled to receive an incoming optical signal and configured to generate an output optical signal by phase modulation and/or amplitude modulation of the received optical signal, the controllable optical FIR filter including a plurality of similar optical signals in a corresponding plurality of optical paths, each of the parallel optical paths including an opto-electronic controller responsive to electronic control signals for effecting the phase modulation and/or amplitude modulation of the optical signal being transported in the optical path and (2) a control signal generator responsive to an optical output zo signal from the output of the controllable optical FIR filter for generating the electronic control signals in accordance with predetermined criteria.
In another aspect, the present invention provides a method for use in an adaptive optical equalizer including a controllable optical FIR filter. In one embodiment, the method includes: (1) adaptively controlling the controllable optical FIR filter to modulate a supplied optical signal to generate an equalized optical output signal, (2) converting, in accordance with predetermined first criteria, the equalized optical output signal to an electronic signal version, (3) utilizing the electronic signal version to generate, in accordance with second predetermined criteria, amplitude and/or phase control signals, (4) feeding back the control signals to adaptively control the controllable optical FIR filter and (5) employing each control signal to adjust the amplitude and/or phase of a corresponding optical signal propagating on a Chen 34/36-4/5-7/8 4 corresponding optical waveguide of a parallel array of waveguides of the controllable optical FIR filter.
In yet another aspect, the present invention provides an apparatus for joint opto-electronic equalization. In one embodiment, the apparatus includes: (1) an s optical equalizer having an electrical control input, an optical input, an optical output and a state that is fixed by values of a plurality of equalization coefficients, the control input configured to set values of the coefFcients in a manner that is responsive to electrical signals applied to the control input, (2) an optical intensity detector configured to produce an analog electrical output signal in response to the optical to output emitting light, the analog electrical signal being representative of an intensity of the emitted light and (3) an electronic equalizer configured to receive the analog electrical output signal and to produce a stream of digital electrical signals having values that are responsive to the received analog electrical signal, the control input of the optical and electronic equalizers being connected to receive electrical signals t s representative of errors in the digital electrical signals.
In still another aspect, the present invention provides a method of joint opto-electronic equalization. In one aspect, the method includes: (1) producing an output stream of optical signals by passing an input optical signalthrough an optical equalizer,

(2) producing an electrical signal having a value representative of an intensity of the 20 output stream of optical signals, (3) passing the electrical signal through an electronic equalizer to produce an output stream of digital electrical signals and (4) setting equalization coefficients of the optical and electronic equalizers by applying to the optical and electronic equalizers a stream of signals with values representative of errors in the stream of digital electrical signals.
25 Brief Description of the Drawings FIG. 1 shows, in simplified block diagram form, one embodiment of the invention;

Chen 34/36-4/5-7/8 5 FIG. 2 shows, in simplified block diagram form, details of a controllable optical FIR filter that may be employed in the practice of the invention of the invention;
FIG. 3 shows, in simplified block diagram form, details of another embodiment of the invention;
FIG. 4 shows, in simplified block diagram form, details of yet another embodiment of the invention;
FIG. 5 shows, in simplified block diagram form, details of still another embodiment of the invention; and 1o FIG. 6 shows, in flow diagram form, a method incorporating a technique carried out according to the principles of the present invention.
Detailed Description of Embodiments of the Invention FIG. 1 shows, in simplified block diagram form, one embodiment of the invention. Specifically, shown is optical 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 2.3x10'4 Hertz to about 1.8x10'4Hertz, i.e., a wavelength of about 1.3 microns to about 1.7 microns. In one example, an optical carrier signal having a wavelength of approximately 1.55 microns, i.e., a frequency of 1.93 x10'4 Hertz is supplied via input terminal 101 to controllable optical FIR filter 102. Also supplied to controllable optical FIR filter 102, via circuit path 112, is a control signal, which is used to phase and/or amplitude modulate, i.e., vector modulate the supplied optical signal from input terminal 101 to generate the desire optical signal at output terminal 103. The control signal at time, k, is responsive to the electrical control signal e(k). The controllable optical FIR filter 102 may, e.g., be essentially a controllable optical FIR filter or equalizer. One embodiment of an optical FIR filter that may be advantageously employed as controllable optical FIR
filter 102 in the embodiment of the invention of FIG. 1 is a controllable optical vector modulator shown in FIG. 2 and described below. As indicated above, other embodiments for optical FIR filter 102 may also be equally employed in practicing Chen 34/36-4/5-7/8 6 the invention. One such embodiment is an array of controllable optical waveguide gratings.
For a received optical signal E(t) supplied to controllable optical FIR filter via input terminal 101 the output optical signal Eo (t) from controllable optical FIR
filter 102 at output terminal 103 is n n Eo(t)=~a;e'B'E(t-z;)=~c;E(t-z;), (1) rm where n is the number of taps for the optical equalizer, a, is amplitude parameter, B, and c, = a;eie' is the i'" f Iter coefficient. In one embodiment, for a tap delay of 1 / fs , z; = (i -1) l fs for i = 1,..., n. The optical output signalEo (t) from controllable optical FIR filter 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 I q(k)I Z in response to detection of Eo (t) , where q(k) = Eo (k l fs ) .
Transimpedance amplifier 105 converts the current from photodiode 104 to a voltage signal, in well known fashion. The electronic voltage signal from transimpedance t5 amplifier 105 is supplied to dicer unit 106 and to a negative input of algebraic adder, i.e., subtractor 108. An automatic threshold control signal is also supplied to dicer 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 sficer 106. The output from slicer 106 is the desired compensated received data signal 2o 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 subtractor 108 is supplied to WUD( a , 9 ) unit 109, where the electronic control signal amplitude (a ) and phase ( 9 ) values are generated, in accordance with an opto-electronic least-mean~quare (OE-LMS) process. The amplitude (a) values and phase (9) values are supplied via 25 circuit path 110 to adjust the tap coeffcients in controllable optical FIR
filter 102.
Note that although a single electronic feedback path 110 is shown, it will be understood that as many circuit paths are included equal to the number of controllable , CA 02515280 2005-11-04 Chen 34/36-4/5-7/8 7 taps in the FIR filter embodiment of controllable optical FIR filter 102. In this example, there may be N such circuit paths. Again, the values of (a ) and ( 9 ), in this embodiment of the invention, are generated in accordance with a single OE-LMS
process. It is further noted that when only the amplitude of the received optical signal is modulated only the amplitude adjustment values (a) are supplied from WUD(a, 6) unit 109 to controllable optical FIR filter 102. Similarly, when only the phase of the received optical signal is being modulated only the phase adjustment values (B ) are supplied from unit 109 to controllable optical FIR filter 102. Finally, when both the amplitude and phase of the received optical signal are being modulated both the amplitude adjustment values (a) and the phase adjustment values (9) are supplied from unit 109 to controllable optical FIR filter 102.
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 FIR filter 102. Operating in the optical domain, this process, however, minimizes the electronic error, e(k), between the compensated 2o signal, d(k), and the desired signal in the mean square sense in a similar fashion to the least-mean-square (LMS) algorithm for pure electronic equalization. Thus, the ISI
elimination process in this invention utilizes a single OE-LMS process.
FIG. 2 shows, in simplified block diagram form, details of an optical vector modulator that may be utilized as controllable optical FIR filter 102 employed in FIG.
1 in the embodiment of the invention. The optical vector modulator 102 is based on the summing of multiple optical tapped delay lines. The principle of operation is as follows: The input optical signal E(t) to be phas~shifted and/or to be amplitude-modulated is a modulated optical carrier. Input optical signal E(t) is supplied to optical vector modulator 102 via input terminal 101 where it is split via input Chen 34/36-4/5-7/8 8 multimode interference (MMI) coupler 201 into a plurality of similar branches.
Input MMI 102 is essentially a power splitter. Each of the plurality of branches is equipped with an amplitude and/or phase modulator 202-1 through 202-N to adjust the amplitude and/or phase of the input optical carrier E(t). In this example, not to be construed as limiting the scope of the invention, both the amplitude and phase is adjusted in each branch of the optical vector modulator 102. Each of the amplitude and phase modulators 202-1 through 202-N is followed by an optical delay line, namely, delay units 203-1 through 203-N, respectively. The delays T~ through T" in each of the modulator branches including phase modulators 202-1 through 202-N
are generated by delay units 203-1 through 203-N respectively. Each of these delay lines in delay units 203-1 through 203-N changes the phase of the sub-carrier of the optical signal from amplitude and/or phase modulators 201-1 through 201-N, respectively, by a fixed amount. For example, the delay Iine in unit 203-1 provides a delay of i, delay unit 203-2 provides a delay of 2i, and delay unit 203-N provides a delay of Ni.
~5 Typically, a delay i of 1/(N x carrier frequency) is required. In one embodiment, delay unit 203-1 supplies a zero (0) delay interval, delay unit 203-2 supplies a delay of i and so on until delay unit 203-N supplies a delay of i(N-1). Thus, if the carrier frequency is 40 GHz, the delay range should be 0,..., 25 picoseconds (ps).
Delay i can be equal to one (1) bit period, i.e., T=25 ps for the instance of 40 Gbps.
Therefore, the delay range is 0,..., T(N-1 ). Alternatively, delay i can be a fraction of a bit period, for example, T/2=12.5 ps. for 40 Gbps. Thus, for the example that z = T/2 = 12.5 ps., the delay range is 0,..., (N-1)* 12.5 ps. Another MMI 204 coupler, which is for example a power combiner, combines all of the amplitude and phase adjusted, and delayed optical signals from all branches to produce a modulated output optical signal at output 106, which will interfere constructively or destructively depending on the summing optical phases from all tributary branches. Therefore, by interfering signals with different carrier phase, the phase and the amplitude of the carrier of the summing signal can be set to an arbitrary selected state. These interfered optical carriers will produce microwave phasors with prescribed amplitude and phase at the remote optical detector, namely, photodiode 104 of FIGS. 1 and 3.

Chen 34/36-4/5-7/8 9 The electrically controllable amplitude and phase modulator 202 of each branch of the optical vector modulator 102 is fabricated, for example, in a material system with linear electro-optic effect, as InP, GaAs or LiNb03. The effective refractive index of an optical waveguide changes in proportion to the electrical field s applied perpendicular to this waveguide via control circuit path 110. A high frequency distributed electrical waveguide is engineered to co-propagate with the optical wave with matched propagating velocity to deliver the local control electrical field with high modulation bandwidth. The different branches will delay flee optical signal by a different length of time. This results in different sub-carrier phases at the outputs of these delay lines in units 203. In the combiner 204, these different output signals from the various branches interfere coherently with different carrier phases due to the different time delays these signals experienced. The carrier of the signal after the MMI coupler, i.e., power combiner 204, is the sum of all carriers of the signals that interfere coherently.
is FIG. 3 shows, in simplified form, details of another embodiment of the invention. The embodiment of the invention illustrated in FIG. 3 is similar to that shown in FIG. 1 except it specifically employed the optical vector modulator shown in FIG. 2 for controllable optical FIR filter 102 of FIG 1. It also employs interferometer 113 (FIG. 3) for generating a signal employed in the O)rLMS
process.
2o Thus, elements similar to those shown in FIG. 1 have been similarly numbered and will not be described again in detail.
Tn the embodiment of FIG. 3 an optical interferometer 113 is supplied via optical path 111 with the optical signal supplied via input 101 to optical vector modulator 102, and via optical path 112 with the output optical signal at output 103 of z5 optical vector modulator 102. As is well known, optical interferometer 113 in response to the supplied optical signals develops optical output signals, which are representative of the sum and difference of the supplied optical signals.
These sum and difference signals are supplied to photodiodes 114 and 115. Photodiodes 114 and 11 S generate electronic signals which are supplied to differential amplifier 116, which 3o generates a correlated signal of the optical vector modulator 102, i.e., the optical FIR

Chen 34/36-4/5-7/8 10 filter, input signal and output q~(k)r(k+i)signal, as described below in relation to Equation (5) which is supplied to WUD(a, 9) unit 109. The "*" denotes the complex conjugate.
Operation of this embodiment of the invention, is described for an incoming s optical signal E(t) of a single polarization is sampled at a sampling rate f, =1 /TJ
equal to or being a multiple of the bit rate fb . When fs = fb , controllable optical vector modulator 102 (which is a FIR filter having a plurality of parallel legs) is synchronous (SYN). On the other hand, when fs is a multiple of the bit rate fb, controllable optical vector modulator 102 is said to be fractionally spaced (FS).
to Denote the sampled data vector as r (k) _ [r(k+L)..x(k-L)]T , where r(k) =
E(kT ) and the superscript T denote a transpose function. The controllable optical vector modulator 102 is a FIR filter with a coefficient vector of a length N = 2L+1 is denoted as c (k) _ [c-~ (k), ..., c; (k), ...,cL (k )]~ , where the coefficient indices are rearranged to i=-L,...,L to center the middle tap of the FIR filter for the sake of "easy"
~5 mathematical manipulation. It should be noted that c(k) is complex in general. The output of the FIR filter is then q(k) =cH(k)rH(k) _ ~~ -~c; (k)r(k-i). Here the superscript H implies Hermitian conjugate transpose and the superscript T
implies transpose. Then, photodetector 104 (FIG. l, FIG. 3) converts the optical output signal q(k) from controllable optical vector modulator 102 to an electronic signal, namely, 20 I q(k)I2 = q(k)q~ (k) = c" (k)R(k)c(k) , where R(k) = i~ (k)rH (k) . It can be shown that R(k) is a Hermitian matrix and, therefore, can be diagonalized by a unitary matrix.
Error signal e(k) is generated in conjunction with the output from TIA 105 q(k)I Z and the output from sficer 106 d (k) being supplied to the negative and positive inputs, respectively, of algebraic adder, i.e., subtractor 108 (FIG. 1, FIG.

3), namely, 25 e(k) = d (k) - I q(k)i2 . It is noted that d (k) is generated during normal operation of the invention and is the desired output. It is further noted that a training sequence can be employed to train feedback-controlled optical FIR filter 102 of FIG. 1 and optical Chen 34/36-4/5-7/8 11 vector modulator 102 of FIG. 3 or any other arrangement that realizes the desired FIR
filter function.
The OE-LMS process tends to minimize deterministically the cost function defined here as J(k) = le(k)I2. Therefore, taking a step in the negative gradient s direction for minimizing the cost function, the OE-LMS process determines the optimized c recursively as follows:
c(k + I) = c(k) - 4 Oc{~ e(k), } , (2) where /3 is a preset step size and Dc{[e(k)]2} is the gradient of the cost function. In this example, Dc{[e(k)]z} = 2e(k)Oc{e(k)} =-2e(k)~c{c" (k)R(k)c(k)} . Since it Io can be shown that ~c{cH(k)R(k)c(k)}=2R(k)c(k), the OE-LMS process updates the FIR coefficients in the manner that follows:
c(k + 1) = c (k) + ~3e(k)R(k)c (k) (3) = c (k) + /3e(k)q' (k)i-(k) . (4) Thus, the f" FIR filter coefficient is updated as follows:
15 c;(k+1)=c; (k)+/3e(k)q'(k)r(k+i). ($) The additional product term q' (k) results directly from the square-law detection via photodetector 104 converting the optical signal output from controllable optical FIR filter (optical vector modulator) 102 to an electronic signal. In other words, the inner product qt (k)r(k - i) between the un-equalized and equalized 2o signals is used for the adjustment of the coefficients of controllable optical vector modulator 102. Alternatively, in equation (3), the sole information required for optical equalization is the optical input correlation matrix R, since the FIR
filter coefficients c are already known. To obtain the correlated signal of q(k) and r(k-i), interferometer 113 (FIG. 3) is employed. To this end, the optical input signal E(t) to 25 and the optical output signal Eo (t) from controllable optical FIR filter 102 (optical vector modulator (FIG.3)) are supplied to first and second inputs, respectively, of optical interferometer 113. In known fashion, optical interferometer 113 generates Chen 34/36-4/5-7/8 12 optical signals at its outputs, which are representative of the sum and difference of the supplied optical signals from optical vector modulator 102. These optical sum and difference signals are supplied to photodiodes 114 and 115, respectively.
Photodetectors 114 and 115, which are photodiodes, convert the optical output from s optical interferometer l 13 to electronic signals. These electronic signals are supplied to differential amplifier 116 that generates a difference signal, which is supplied to WUD(a, B) 109 for use in generating the amplitude and phase control signals a, 8 , respectively, for each leg, i.e., tap, of optical vector modulator 102.
The above discussion assumes a polarized incoming optical signal E(t) and, to thus, leads to a single-polarization OE-LMS process, which can effectively mitigate GVD-induced ISI. However, for the instance of first-order PMD, two orthogonal polarizations and involved, namely, E,, (t) and E" (t) representing the optical signals of vertical and horizontal polarizations, respectively. In consideration of both the vertical and horizontal polarizations, the electronic output from photodiode 104 is t s ~q(k)~Z = ~q~ (k)~2 +~qH (k)~2 ~ where qv (k) _ ~H (k)r~ (k) and qH (k) _ ~H (k)rx (k) Wider the assumption of the controllable optical FIR filter, i.e., optical vector modulator 102, of FIG. 3, being insensitive to polarization, i.e., c,, = cH = c . Hence, q(k) = cH (k)[R,, (k) +R" (k)]c(k) and ~c{[e(k)]2}=2e(k)Dc{e(k)}=-4e(k)[R~,(k)+RH(k)]c(k). Thus, the OE-LMS
2o process tap weight-date procedure becomes:
c (k + 1) = c (k) + ~3e(k)[R~, (k) + RH (k)]c (k) (6) _ ~ (k) '~ ~e(k)[qv (k)rv (k) + qe (k)rH (k)]
In scalar form, the i'" FIR filter tap coefficient is updated as follows:
c; (k + 1) = c; (k) + ~3e(k)[q~ (k)r~ (k - i) + qH (k)rH (k - i)] . (8) 2s If we denote q(k)=[q~(k)~qH(k)]T~u(k-i)=[rv(k-i)~re(k-i)]T
then, ci (k + 1) = c; (k) + ~3e(k)qH (k)u(k - i) . (9) Chen 34/36-4/5-7/8 13 Here q"(k)u(k-1)-Ilq(k)Illlu(k-III ~os(eq,~)~
where Ilqll is the Euclidean norm of q and B9,u is the angle between q and a .
In both equations (5) and (9), the knowledge of the inner product of the input a and the equalized q is required for the optimization of the optical FIR filter coefficients.
Note that once the values for all c, are known, the corresponding values for a; and Bj are readily generated, since c; =a;e'B~ , as shown in Equation (1) above.
FIG. 4 shows, in simplified block diagram fonm, details of yet another embodiment of the invention. The embodiment of the invention illustrated in FIG. 4 1o is similar to that shown in FIG. 3, but includes a WUD(B,C,F) unit 109 that performs both optical and electronic equalization. The embodiment of FIG. 4 includes both feedforward and feedback electronic equalizers (401, 402). The embodiment includes the interferometer 113, photodiodes 114, 115 and differential amplifier 116, which connect to the optical vector modulator 102 and WUD(B,C,F) unit 109 as shown in FIG. 3: These elements are left out of FIG. 4 for clarity. Here, elements similar to those shown in FIG. 3 have been similarly numbered and will not be described again in detail.
In the embodiment of FIG. 4, the optical output signalEo(t) from controllable optical vector modulator 102 is transported to an optical receiver and therein to 2o photodiode 104. As is well known, photodiode 104 is a square-law detector and generates a current Iq(k)IZ in response to detection of Eo(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 feedforward filter F(x) section 401 which is controlled by WUD(B,C,F) 2s unit 109. The output of feedforward filter F(x) section 401 is provided via subtractor 403 to sficer unit 106 and to a negative input of algebraic adder, i.e., subtractor 108.
An automatic threshold control signal is also supplied to sficer 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 dicer 106: The output Chen 34/36-4/5-7/8 14 from dicer 106 is the desired compensated received data signal d (k) and is supplied as an output from the receiver and to a positive input to algebraic adder 108.
The subtractor 108 produces an error signal e(k), which is supplied to WUD(B,C,F) unit 109, where feedback filter B(x) section signal B, feedforward filter F(x) section signal F and the electronic control signal C for the optical vector modulator 102 are generated utilizing a single OE-LMS process. Signal B and signal F are the control inputs for the electronic equalizer. Feedback filter B(x) section 402 receives signal B, along with the output of sficer 106 and generates an output signal that is provided to a negative input of an algebraic adder, i.e., subtractor 403. The amplitude (a ) values 1o and phase (6) components from WUD(B,C,F) unit I09 are supplied via electrical feedback path 110 to adjust the tap coefficients in controllable optical vector modulator 102. Note that although a single electrical feedback 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 vector modulator 102. In this example, there may be N such circuit paths. Again, the values of (a ) and/or ( 9 ) components are generated in accordance with a single OE-LMS process. It is further noted that when only the amplitude of the received optical signal is modulated only the amplitude adjustment value (a) components are supplied from unit 109 to controllable optical vector modulator 102. - Similarly, when only the phase of the 2o received optical signal is being modulated only the phase adjustment value (6 ) components are supplied from unit 109 to controllable optical vector modulator 102.
Finally, when both the amplitude and phase of the received optical signal are being modulated both the amplitude adjustment value (a ) components and the phase adjustment value (B) components are supplied from unit 109 to controllable optical vector modulator 102.
FIG. 5 shows, in simplified block diagram form, details of still another embodiment that produces joint optical and electronic equalization. FIG. 5 is similar to FIG. 4, except that feedforward filter F(x) section 401 is absent, simplifying the overall architecture. However, as has been discovered, the embodiment of FIG.
5 is Chen 34/36-4/5-7/8 15 still remarkably effective at increasing performance with respect to devices that do not perform optical and electronic equalization together.
In the embodiment of FIG. 5, the optical output signalEo(t) from controllable optical vector 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 I q(k)I Z in response to detection of Eo (t) , i.e., q(k) = Eo (k l f~ ) .
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 algebraic adder 403 and then to sficer unit 106 and to a to negative input of algebraic adder, i.e., subtractor 108. An automatic threshold control signal is also supplied to sficer 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 sficer 106. The output from sficer 106 is the desired compensated received data signal d (k) and is supplied as an output from the receiver ~ s and to a positive input to algebraic adder 108. The error signal, e(k), output from subtractor 108 is supplied to WUD(B,C) unit 109 where feedback filter B(x) section signal B and electronic control signal C (having amplitude (a) and phase (9) components) are generated utilizing a single O&LMS process. Signal B is the control inputs for the electronic equalizer. In the exemplary embodiment, WUD(B,C) unit 20 109 determines B as follows: B(k+1) = B(k)-ae(k)d(k). In the exemplary embodiment, WLJD(B,C) unit 109 determines C as follows: C(k+1) -C(k)+(3e(k)q*(k)r(k). Thus, the WUD(B,C) unit 109 jointly optimizes both the optical and electronic equalizers by setting both the C(k) and B(k) coefficients based on the same LMS process.
25 Feedback filter B(x) section 402 receives signal B, along with the output of sficer 106 and generates an output signal that is provided to a negative input of algebraic adder, i.e., subtractor 403. The amplitude (a) values and phase (9) components from WUD(B,C,F) unit 109 are supplied via electronic feedback path 110 to adjust the tap coefficients in controllable optical vector modulator 102. Note Chen 34/36-4/5-7/8 16 that although a single electronic feedback 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 vector modulator I02. Again, in this example, there may be N such circuit paths. The values of (a) and/or (9) components, in this embodiment of the invention, are again generated in accordance with a single OE-LMS process. It is also noted again that when only the amplitude of the received optical signal is modulated only the amplitude adjustment value (a) components are supplied from unit 109 to controllable optical vector modulator 102.
Similarly, when only the phase of the received optical signal is being modulated only the phase to adjustment value (6) components are supplied from unit 109 to controllable optical vector modulator 102. Finally, when both the amplitude and phase of the received optical signal are being modulated both the amplitude adjustment value (a ) components and the phase adjustment value (9) components are supplied from unit 109 to controllable optical vector modulator 102.
As stated above, the signal coming out of feedback filter B(x) section 402 is subtracted from the post-photodetection electronic signal x(k) (from photodiode 104).
An uncompensated signal in front of sficer I05 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, 2o OE-LMS is used to control both the O-EQ and the E~EQ in a unified fashion.
This gains advantages of both equalizer types without causing conflict between optimization of the O-EQ and the E EQ. In essence, OE~LMS minimizes the electronic error between the compensated signal and the desired signal in the mean square sense, which is compatible with the least-mean-square (LMS) algorithm conventionally used for electronic equalization.
FIG. 6 shows, in flow diagram form, a method incorporating a technique carried out according to the principles of the present invention. The method begins in start step 610 and proceeds to step 620 wherein input signals pass through an optical equalizer. As a result, an output stream of optical signals is produced in step 630.
3o Then in a step 640, an electrical signal is produced. The electrical signal has a value Chen 34/36-4/5-7/8 17 representative of an intensity of the output stream of optical signals. Next, in step 650, the electrical signal is passed through an electronic equalizer to produce an output stream of digital electrical signals. Then, in step 660, equalization coefficients of the optical and electronic equalizers are set by applying to the optical and electronic equalizers a stream of signals with values representative of errors in the stream of digital electrical signals. The method ends in end yep 670. Those skilled in the pertinent art will understand that although these steps have been set forth sequentially, they are advantageously performed concurrently to effect equalization of the input signals to yield the output stream of optical signals.
The above-described embodiments are, of course, merely illustrative of the principles of the invention. Indeed, numerous other methods or apparatus may be devised by those skilled in the art without departing from the spirit and scope of the invention. Specifically, other arrangements may be equally employed for realizing the controllable optical FIR filter.

Claims (10)

Claims:

1. Apparatus for use in an adaptive optical equalizer comprising:

a controllable optical FIR filter having an input and an output, and being coupled to receive an incoming optical signal and configured to generate an output optical signal by phase modulation and/or amplitude modulation of the received optical signal, said controllable optical FIR filter including a plurality of similar optical signals in a corresponding plurality of optical paths, each of said parallel optical paths including an opto-electronic controller responsive to electronic control signals for effecting said phase modulation and/or amplitude modulation of said optical signal being transported in said optical path; and a control signal generator responsive to an optical output signal from said output of said controllable optical FIR filter for generating sail electronic control signals in accordance with predetermined criteria.

2. The apparatus as defined in claim 1 wherein said controllable optical FIR
filter comprises an arrayed waveguide grating.

3. The apparatus as defined in claim 1 wherein said control signet generator is configured to update said adjustable control signals at a predetermined sampling rate.

4. A method for use in an adaptive optical equalizer including a controllable optical FIR filter comprising the steps of:
adaptively controlling said controllable optical FIR filter to modulate a supplied optical signal to generate an equalized optical output signal;
converting, in accordance with predetermined first criteria, said equalized optical output signal to an electronic signal version;
utilizing said electronic signal version to generate, in accordance with second predetermined criteria, amplitude and/or phase control signals;
feeding back said control signals to adaptively control said controllable optical FIR filter; and employing each control signal to adjust the amplitude and/or phase of a corresponding optical signal propagating on a corresponding optical waveguide of a parallel array of waveguides of said controllable optical FIR alter.

5. The method as defined in claim 12 wherein said controllable optical FIR
filter is configured to operate as a controllable optical finite impulse response (FIR) filter, and wherein said parallel array of waveguides form parallel optical taps of said controllable optical FIR filter.

6. The method as defined in claim 12 wherein said controllable optical FIR
filter is configured to operate as a controllable optical vector modulator.

7. An apparatus for joint opto-electronic equalization, comprising:
an optical equalizer having an electrical control input, an optical input, an optical output and a state that is fixed by values of a plurality of equalization coefficients, the control input configured to set values of the coefficients in a manner that is responsive to electrical signals applied to the control input:
an optical intensity detector configured to produce an analog electrical output signal in response to the optical output emitting light, the analog electrical signal being representative of an intensity of the emitted light; and an electronic equalizer configured to receive the analog electrical output signal and to produce a stream of digital electrical signals having values that are responsive to the received analog electrical signal, the control input of the optical and electronic equalizers being connected to receive electrical signals representative of errors in the digital electrical signals.

8. The apparatus as defined in claim 21 wherein said optical equalizer comprises arrayed waveguide gratings.

9. The apparatus as defined in claim 21 wherein said electronic egualizer is configured to update said stream of digital electrical signals at a predetermined sampling rate.

10. A method of joint opto-electronic equalisation, comprising:
producing an output stream of optical signals by passing an input optical signal through an optical equalizer;
producing an electrical signal having a value representative of an intensity of the output stream of optical signals;
passing the electrical signal through an electronic equalizer to produce an output stream of digital electrical signals; and setting equalization coefficients of the optical and electronic equalizers by applying to the optical and electronic equalizers a stream of signals with values representative of errors in the stream of digital electrical signals.