A compensation function mitigates a substantial portion of the chromatic dispersion imparted to a communications signal by an optical communications system. A digital input signal is digitally processed using the compensation function to generate a predistorted signal. An amplitude and a phase of an optical signal are modulated using a pair of orthogonal signal components to generate a predistorted optical signal for transmission. In one implementation, the pair of orthogonal signal components are components of the predistorted signal. In another implementation, the predistorted signal is processed using a non-linear compensator to generate a further distorted signal and the pair of orthogonal signal components are components of the further distorted signal. In that implementation, the non-linear compensator is configured to substantially compensate for nonlinearities in one or both of an optical modulator of a transmitter of the system and an optical-to-electrical converter of a receiver of the system.

A forward optical intensity modulation signal, generated by optical intensity-modulating a laser signal using a forward microwave phase modulation signal, is transmitted from a base to a remote station. A backward microwave phase modulation signal, in which frequency of the forward microwave phase modulation signal is changed by demodulating the forward optical intensity modulation signal, is generated, and a backward optical intensity modulation signal, generated by optical intensity-modulating the laser signal using the backward microwave phase modulation signal, is transmitted from the remote station to the base. The backward microwave phase modulation signal is extracted by photoelectric converting the backward optical intensity modulation signal, a round trip timing is extracted by demodulating the backward microwave phase modulation signal, and transmission delay is determined from a difference between the timing and the round trip timing.

Mixing between I and Q components in coherent homodyne optical signals can occur due to phase shifts, e.g. relative to the local oscillator, relative to the other signal components. In some examples, the phase shifts can arise due to thermal expansion of the optical waveguides and/or can include polarization mixing. A descrambler functions to correct for mixing between multiple signal components. The descrambler may be configured to at least partially correct for a phase difference between a first plurality of modulated optical data signals and a first local oscillator reference signal; and a controller may be configured for determining a first correction parameter for at least partially correcting for the phase difference, and for transmitting the first correction parameter to the descrambler. The controller may be configured for determining the first correction parameter from a first pilot signal transmitted with the first plurality of modulated optical data signals.

Optical dispersion imposed on a communications signal conveyed through an optical communications system is compensated by modulating the communications signal in the electrical domain. A compensation function is determined that substantially mitigates the chromatic dispersion. The communications signal is then modulated in the electrical domain using the compensation function. In preferred embodiments, compensation is implemented in the transmitter, using a look-up-table and digital-to-analog converter to generate an electrical predistorted signal. The electrical predistorted signal is then used to modulate an optical source to generate a corresponding predistorted optical signal for transmission through the optical communications system.

An optical line terminal transmitter front-end, an optical network terminal receiver front-end and a bit-interleaved passive optical network (BIPON). In one embodiment, the transmitter front-end includes: (1) a bit interleaver configured to group and interleave a plurality of user bit-streams to yield a combined single bit-stream, (2) an encoder coupled to the bit interleaver and configured to encode multiple bits of the single bit-stream into a multi-level code corresponding to a 2.sup.m-level multi-level signal and (3) a multi-level modulator coupled to the encoder and configured to modulate the multi-level code into the 2.sup.m-level multi-level signal.

Mixing between I and Q components in coherent homodyne optical signals can occur due to phase shifts, e.g. relative to the local oscillator, relative to the other signal components. In some examples, the phase shifts can arise due to thermal expansion of the optical waveguides and/or can include polarization mixing. A descrambler functions to correct for mixing between multiple signal components. The descrambler may be configured to at least partially correct for a phase difference between a first plurality of modulated optical data signals and a first local oscillator reference signal; and a controller may be configured for determining a first correction parameter for at least partially correcting for the phase difference, and for transmitting the first correction parameter to the descrambler. The controller may be configured for determining the first correction parameter from a first pilot signal transmitted with the first plurality of modulated optical data signals.

A semiconductor IQ optical modulator in which a phase modulation unit is configured by a differential capacitively loaded traveling-wave electrode structure based on an SS line configuration, phase modulation units of adjacent channels are spaced apart from each other by 400 m or more, a distance between main signal lines of the capacitance loading type structure is 60 m or less, a DC phase adjustment electrode and a PAD are provided between an I side phase modulation unit and a Q side phase modulation unit, the DC phase adjustment electrode is spaced apart by at least 80 m or more from a signal line of the phase adjustment unit, and a crosstalk characteristic between the adjacent channels is 30 dB or less in a required frequency band.