Optical transmission system

An optical transmission system comprising:

• a transmitter operable to generate an optical signal;
• a modulator operable to modulate the optical signal to obtain a modulated optical signal encoded with a signal data stream;
• a plurality of optical elements constituted by a plurality of optical terminals and at least one optical waveguide means interconnecting the optical terminals and operable to conduct modulated optical signals modulated by the signal data stream therebetween;
• and
• at least one of the optical elements comprising a birefringent element susceptible to degrading the modulated optical signal by polarization mode dispersion;
• wherein at least one of the optical elements is coupled to monitoring means operable to monitor degradation of the modulated optical signal consistent with the effects of polarization mode dispersion in the presence of modulation by the signal data stream and to output monitored data representative of such degradation.

A method of transmitting optical signals in an optical transmission system, comprising the steps of:

• generating an optical signal modulated by a signal data stream by operation of a transmitter of the optical transmission system;
• conducting the optical signal via a plurality of optical elements constituted by at least one optical waveguide means and at least one optical terminal of the optical transmission system, at least one of the optical elements being susceptible to polarization mode dispersion;
• receiving the optical signal at a receiver constituting an optical terminal of the optical transmission system and outputting the signal data stream therefrom;
• monitoring the optical signal by operation of a monitoring means so as to detect degradation of the optical signal consistent with the optical signal having been subject to the effects of polarization mode dispersion during transmission;
• outputting from the monitoring means monitored data representative of the extent to which the optical signal is degraded by polarization mode dispersion;
• comparing the monitored data with a threshold value;
• and
• generating an alarm indication in response to the monitored data exceeding the threshold value.

Monitoring apparatus for use in an optical transmission system, the monitoring apparatus comprising:

• sampling means operable to sample a state of polarization of a received optical signal modulated by a signal data stream to provide sampled data;
• a processor operable to determine monitored data from the sampled data representative of the effect of polarization mode dispersion in the transmission system;
• and
• an optical tap coupled to an optical element of the optical transmission system and operable to extract a sample of the optical signal therefrom;
• wherein the processor is operable to determine the monitored data as a measure of the extent to which the sample of the optical signal is depolarized.

A method of transmitting optical signals in an optical transmission system comprising the steps of:

• generating an optical signal modulated by a signal data stream by operation of a transmitter of the optical transmission system;
• conducting the optical signal via a plurality of optical elements constituted by at least one optical waveguide means and at least one optical terminal of the optical transmission system, at least one of the optical elements being susceptible to polarization mode dispersion;
• receiving the optical signal at a receiver constituting an optical terminal of the optical transmission system and outputting the signal data stream therefrom;
• monitoring the optical signal by operation of a monitoring means so as to detect degradation of the optical signal consistent with the optical signal having been subject to the effects of polarization mode dispersion during transmission;
• outputting from the monitoring means monitored data representative of the extent to which the optical signal is degraded by polarization mode dispersion;
• and
• controlling the operation of the optical transmission system by operation of a control means responsive to the monitored data so as to reduce the effects of polarization mode dispersion on the transmitted optical signal.