An apparatus includes a polarization beam splitter (PBS) and an optical detector. The PBS is configured to receive a polarized optical signal transported via an optical communication path of an optical network. The detector is configured to receive from the PBS a first polarization component of the optical signal, and to produce a first electrical measure of the first polarization component. A processor is configured to determine a dynamic metric of the optical communication path based at least on the first electrical measure. Some embodiments also include a second detector configured to receive from the PBS a second polarization component of the optical signal. The second detector produces a second electrical measure of the second polarization component, and the processor is configured to determine the dynamic metric based on both the first and second electrical measures.

This disclosure provides systems, methods, and apparatus for mitigating the effects of an optical link in a communication system on polarization angles of signals transmitted over the optical link. The communication system can include an optical transmitter transmitting polarization-division multiplexed (PDM) optical signals over the optical link and a direct-detection receiver for receiving the PDM optical signals. The transmitter can include a polarization compensation unit for receiving estimated values of link polarization parameters from the receiver. The receiver uses the estimated values to pre-distort modulator drive and bias signals and to adjust the relative phase between modulated optical signals. The transmitter can also transmit training optical signals to the receiver over the optical link. The receiver uses the training optical signals to estimate the values of link polarization parameters. The receiver also can include a crosstalk mitigation unit for mitigating effects of crosstalk between the PDM optical signals.

Methods, systems, and apparatus, for an external cavity FP laser. In one aspect, an apparatus is provided that includes a FP laser diode; a Faraday rotator (FR) coupled to receive an optical output of the FP laser diode and that rotates a polarization of the optical output; an optical fiber coupled at a first end to receive the output of the FR; a WDM filter coupled to a second end of the optical fiber to receive the optical signal from the optical fiber; and a FRM coupled directly or indirectly to an output of the WDM filter, wherein an optical output of the WDM filter is partially reflected by the FRM such that the polarization of a reflected beam is rotated, and wherein the reflected optical signal then passes through the FR with its polarization being rotated by the FR before it is injected back into the FP laser diode.

In accordance with an example embodiment of the present invention, there is provided an apparatus comprising a dual-rail encoder (120) configured to receive light from a light source and to output dual-rail encoded light, a combiner (130) configured to convert the dual-rail encoded light into polarization encoded light, and at least one processing core configured to obtain compensation adjustment information concerning a fibre (145) and to control the dual-rail encoder (120) based at least in part on the compensation adjustment information.

There is provided an optical source. The optical source comprises a reflective optical amplifier configured to generate an optical signal, the optical signal comprising an amplified spontaneous light emission having a plurality of light modes each having a respective wavelength. The optical source further comprises a reflective mirror, spaced from the reflective optical amplifier, and arranged to receive the optical signal and to rotate a polarisation of each light mode in the received optical signal, to form a further optical signal. The optical source further comprises an optical power splitter arranged to receive the further optical signal and to split the further optical signal into a first optical signal which is directed to the reflective optical amplifier for amplification thereby, and a second optical signal. The optical source further comprises an output arranged to output the second optical signal. There is also provided a communications network optical apparatus and a communications network base station.

In a coherent optical receiver receiving a polarization multiplexed optical signal through an optical communications network, a method of compensating noise due to polarization dependent loss (PDL). A Least Mean Squares (LMS) compensation block processes sample streams of the received optical signal to generate symbol estimates of symbols modulated onto each transmitted polarization of the optical signal. A decorrelation block de-correlates noise in the respective symbol estimates of each transmitted polarization and generating a set of decorrelated coordinate signals. A maximum likelihood estimator soft decodes the de-correlated coordinate signals generated by the decorrelation block.

A signal equalizer for compensating impairments of an optical signal received through a link of a high speed optical communications network. At least one set of compensation vectors are computed for compensating at least two distinct types of impairments. A frequency domain processor is coupled to receive respective raw multi-bit in-phase (I) and quadrature (Q) sample streams of each received polarization of the optical signal. The frequency domain processor operates to digitally process the multi-bit sample streams, using the compensation vectors, to generate multi-bit estimates of symbols modulated onto each transmitted polarization of the optical signal. The frequency domain processor exhibits respective different responses to each one of the at least two distinct types of impairments.

Provided is an optical communication system comprising a polarization-diversity optical power supply capable of supplying light over a non-polarization-maintaining optical fiber to a polarization-sensitive modulation device. In an example embodiment, the polarization-diversity optical power supply operates to accommodate random polarization fluctuations within the non-polarization-maintaining optical fiber and enables an equal-power split at a passive polarization splitter preceding the polarization-sensitive modulation device.

An optical transmission method wavelength-multiplexes and transmits multiple channels. Data transmitted on the wavelength-multiplexed channels are composed of data areas independent between the channels and data areas non-independent between the channels. Data patterns of the data areas non-independent between the channels are variable. This reduces the time period during which polarization states of the multiple channels are correlated, thereby reducing non-linear optical effects occurring between the channels and reducing deterioration in signal quality at a receiving end.

A radio-over-fibre transmitter comprising: an optical splitter arranged to receive an optical carrier signal having a carrier optical frequency, and split it into a plurality of portions; electro-optic modulation apparatus each arranged to receive a respective optical carrier signal portion and a respective modulated radio frequency subcarrier signal, and arranged to modulate the respective optical carrier signal portion with the respective modulated radio frequency subcarrier signal and arranged to suppress onward transmission of the respective optical carrier signal portion, to form a respective carrier suppressed optical subcarrier signal; an optical combiner arranged to receive the carrier suppressed optical subcarrier signals and one of the optical carrier signal portions and arranged to combine them to form a subcarrier multiplexed optical signal; and polarisation apparatus arranged to ensure that the carrier suppressed optical subcarrier signals and said optical carrier signal portion each have the same polarisation state at the optical combiner.