Abstract: In a coherent optical receiver, a method of at least partially compensating Polarization Dependent Loss (PDL) of an optical signal received through an optical communications system. A respective multi-bit sample stream of each one of a pair of orthogonal received polarizations of the optical signal is tapped, and used to derive a respective metric value indicative of a quality of each multi-bit sample stream. A gain of an analog front end of the coherent optical receiver is adjusted based on the derived metric values.

Abstract: Methods and systems for compensating a frequency mismatch ?f between a local Oscillator (LO) of a coherent optical receiver and a carrier of a received optical signal. An average frequency of the LO is controlled to compensate at least long-period variations of the frequency mismatch. An electrical carrier recovery circuit for compensating short period variations of the frequency mismatch.

Abstract: In a coherent optical receiver of an optical communications network, a method of recovering a clock signal from a high speed optical signal received through an optical link. A set of compensation vectors are adaptively computed for compensating Inter-symbol Interference (ISI) due to at least polarization impairments of the optical signal. A channel delay is estimated based on the computed compensation vectors. The estimated channel delay is subtracted from the computed compensation vectors to generate corresponding modified compensation vectors. Finally, the modified compensation vectors are used to derive a recovered clock signal.

Abstract: In a coherent optical receiver, a frequency domain engine digitally processes at least two multi-bit sample streams of a received optical signal. The frequency domain engine includes a Fast Fourier Transform (FFT) filter for computing a complex vector representative of a frequency-domain spectrum of the received optical signal. A transpose and add block computes a vector sum of the complex vector and a transposed version of the complex vector, and an Inverse Fast Fourier Transform (IFFT) filter computes a complex output vector from the addition result. With this arrangement, parallel real filter operations are efficiently performed on each of the multi-bit sample streams, using a single back-to-back FFT-IFFT filter structure.

Abstract: A method of suppressing effects of aliasing in a system for digitally processing a high speed signal having a symbol rate of 1/T. The high speed signal is sampled at a fractional multiple (N) of the symbol rate, wherein 1<N<2, to generate a corresponding sample stream, and filtered using a low-pass filter characteristic having a cut-off frequency corresponding to 1/2T. Phase distortions due to the filtering are compensated by digitally processing the sample stream.

Abstract: A method of suppressing effects of aliasing in a system for digitally processing a high speed signal having a symbol rate of 1/T. The high speed signal is sampled at a fractional multiple (N) of the symbol rate, wherein 1<N<2, to generate a corresponding sample stream, and filtered using a low-pass filter characteristic having a cut-off frequency corresponding to 1/2T. Phase distortions due to the filtering are compensated by digitally processing the sample stream.

Abstract: A method of recovering a most likely value of each symbol transmitted through an optical communications network using a high speed optical signal. A stream of multi-bit digital samples of the optical signal is processed to generate a respective multi-bit estimate X?(n) of each transmitted symbol. A first function is applied to each symbol estimate X?(n) to generate a corresponding soft decision value {tilde over (X)}(n). Each soft decision value {tilde over (X)}(n) is processed to generate a corresponding hard decision value. {circumflex over (X)}(n) having an ideal amplitude and phase. A plurality of successive soft decision values and hard decision values are processed to determine a second function, which is applied to each soft decision value {tilde over (X)}(n) to generate a most likely symbol value {circumflex over ({circumflex over (X)}(n).

Abstract: Described is a method of reducing transmitter error in an optical communications channel. An optical signal transmitted from an optical transmitter that has impairment due to transmitter error is processed to generate a digitally-equalized signal. A nonlinear characteristic of the digitally-equalized signal that relates to the transmitter error is determined. An optical control signal comprising data that are based on the nonlinear characteristic is transmitted to the optical transmitter. The optical transmitter modifies a transmitter parameter in response to the optical control signal to change the nonlinear characteristic and thereby reduce the impairment.

Abstract: A method and system for initializing a coherent optical receiver. Upon detection of an optical signal, a multi-bit digital sample stream of the optical signal is digitally processed to initialize each one of a plurality of adaptive control blocks of the coherent optical receiver. The adaptive control blocks include at least a dispersion compensation block and a clock recovery block. The dispersion compensation block is initialized before initializing the clock recovery block.

Abstract: A method of recovering a clock signal from an optical signal received through an optical communications system. A digital sample stream is processed to generate a dispersion compensated signal. The dispersion compensated signal is then tapped to obtain upper side band and lower side band signals of each received polarization of the optical signal. The upper side band and lower side band signals are then processed to compensate polarization dependent impairments, and the clock recovered from the resulting optimized.

Abstract: A method of carrier recovery from a high speed optical signal received through an optical communications network. A stream of multi-bit digital samples of the optical signal is processed to generate a multi-bit estimate X?(n) of each one of a plurality of transmitted symbols. A phase of each symbol estimate X?(n) is rotated, and a respective symbol phase error ??(n) of the rotated symbol estimate determined.

Abstract: A method of suppressing effects of aliasing in a system for digitally processing a high speed signal having a symbol rate of 1/T. The high speed signal is sampled at a fractional multiple (N) of the symbol rate, wherein 1<N<2, to generate a corresponding sample stream, and filtered using a low-pass filter characteristic having a cut-off frequency corresponding to 1/2T. Phase distortions due to the filtering are compensated by digitally processing the sample stream.

Abstract: A method of processing a stream of digital samples of an optical signal received by a coherent optical receiver. The digital sample stream is processed to generate a dispersion compensated sample stream. The dispersion compensated sample stream is then processed to compensate polarization dependent impairments of the optical signal.

Abstract: A method of recovering a clock signal from an optical signal received through an optical communications system. A digital sample stream is processed to generate a dispersion compensated signal. The dispersion compensated signal is then tapped to obtain upper side band and lower side band signals of each received polarization of the optical signal. The upper side band sand lower side band signals are then processed to compensate polarization dependent impairments and the clock recovered from the resulting optimized.

Abstract: A frequency-agile optical transceiver includes a shared local oscillator (LO), a coherent optical receiver and an optical transmitter. The LO operates to generate a respective LO optical signal having a predetermined LO wavelength. The coherent optical receiver is operatively coupled to the LO, and uses the LO signal to selectively receive traffic of an arbitrary target channel of an inbound broadband optical signal. The optical transmitter is also operatively coupled to the LO, and uses the LO to generate an outbound optical channel signal having a respective outbound channel wavelength corresponding to the LO wavelength.

Abstract: A method of a conveying data through an optical communications system. An optical signal is received through the optical communication system, the optical signal comprising data symbols and SYNC bursts, each SYNC burst having a predetermined symbol sequence. The received optical signal is oversampled to generate a multi-bit sample stream. The sample stream is partitioned into blocks of contiguous samples, wherein each block of samples partially overlaps at least one other block of samples and encompasses at least one SYNC burst and a plurality of data symbols. Each block of samples is independently processed to detect a value of each data symbol.

Abstract: A commercially viable All-Raman system, is implemented by removing the dispersion compensating Fiber (DCF) and two stage amplifier at each span, and including a transmission path dispersion compensator which performs dispersion compensation on a transmission path basis. For example, by pre-compensating for the accumulated dispersion in the electrical domain at the transmitter, the gain of the Raman pumps at each span amplifier need only compensate for the loss within the span, without needing to compensate for the loss of a DCF. In addition there is provided a low-cost method for implementing a bidirectional Service Channel by modulating/demodulating low-rate data on the Raman pump. For example, a Raman amplifier can include an information source for producing a service channel signal which includes information to be communicated; and a modulator for modulating the Raman pump signal with the service channel signal.

Abstract: A method is provided for an equalization strategy for compensating channel distortions in a dual-polarization optical transport system wherein the received signal includes a complex signal of a first transmitted polarization component and a complex signal of a second transmitted polarization component. In a first step, a blind self recovery mode used a blind adaptation algorithm in calculating and modifying multiple complex equalizer transfer function coefficients to enables recovery of only the complex signal of the first transmitted polarization component. In a second step, equalization is performed in a training mode for recovery of the complex signals of the first and second transmitted polarization components. In a third step, equalization is performed in a data directed mode. The method is suited for a digital signal processing implementation in a coherent receiver.

Abstract: A carrier lock detector for a QPSK or 4-QAM system implements a lock detection algorithm that maps detected signals onto one of first and second areas associated with nominal states defined by (I2?I3)·(Q2?Q3) and Q1?Q2·(I1 I2I3+ I1I2I3)+( I1?I2)·(Q1 Q2Q3+ Q1Q2Q3), or alternatively, by ( I1?I2· Q2?Q3)+( Q1?Q2· I2?I3), respectively. When detected signals map onto one of the first areas, a first signal is generated. When detected signals map onto one of the second areas, a second signal is generated. When a difference between the first and second signals exceeds a threshold, a carrier lock detection signal is generated to enable a decoder. The carrier lock detector is able to detect carrier lock at a raw BER of 1e-2 or greater at a very low signal-to-noise ratio.

Abstract: A method is provided for correcting a quadrature angle error that exists in the coherent receiver hardware of a dual-polarization optical transport system. The receiver hardware that causes the quadrature angle error is a 90 degree optical hybrid mixing device. The method involves generating an estimate of the quadrature angle error and compensating for the quadrature angle error by multiplying the first and second detected baseband signals by coefficients that are a function of the estimate of the quadrature angle error. The method is robust to severe channel distortion encountered within an optical fiber transmission channel as well as temperature effects and ageing of the 90 degree optical hybrid. The method is suited for a digital signal processing implementation in the coherent receiver when a modulation scheme used on a transmitted signal is quadriphase-shift keying (QPSK).