Monitoring phase non-linearities in an optical communication system

1. A method of monitoring phase nonlinearities of an optical communications system, the method comprising steps of:

generating a test signal using a data signal transmitted through at least a portion of the optical communications system and received at a selected monitoring point of the optical communications system, the test signal comprising the received data signal and including a predetermined property that is uniquely associated with at least one phase nonlinearity of the optical communications system; and

detecting the predetermined property of the test signal at the selected monitoring point;

generating the test signal comprising:

receiving the data signal through a wavelength channel of the optical communications system, the data signal including a plurality of flat regions, each flat region having at least a predetermined minimum length; and

varying dispersion of the wavelength channel; and

the predetermined property comprising a variation in a noise metric of the received data signal that correlates with a change in the dispersion of the wavelength channel.

2. A method as claimed in claim 1 , wherein the data signal comprises live data traffic of the optical communications system.

3. A method as claimed in claim 1 , wherein each flat region comprises a plurality of successive bits of equal value.

4. A method as claimed in claim 3 , wherein the predetermined minimum length of each flat region is selected to substantially avoid inter-symbol interference within the respective flat region.

5. A method as claimed in claim 4 , wherein the predetermined minimum length of each flat region is at least three bits.

6. A method as claimed in claim 1 , wherein the step of varying dispersion of the wavelength channel comprises a step of successively imposing each one of a set of predetermined dispersion values on the wavelength channel.

7. A method as claimed in claim 6 , wherein the step of detecting the predetermined property of the test signal comprises, for each imposed dispersion value, steps of:

detecting a flat region within the received data signal;

measuring the noise metric of the received data signal, at a timing of a center portion of the detected flat region; and

storing the measured noise metric value in association with the imposed dispersion value.

8. A method as claimed in claim 7 , wherein the step of measuring the noise metric comprises a step of measuring any one or more of:

a bit error rate;

a signal-to-noise ratio; and

an eye-opening ratio.

9. An apparatus for monitoring phase nonlinearities of an optical communications system, the apparatus comprising:

a receiver configured to receive a data signal through a wavelength channel of the optical communications system, the data signal comprising a plurality of flat regions, each flat region having at least a predetermined minimum length;

a channel monitor configured to generate a test signal comprising the received data signal and including a predetermined property that is uniquely associated with at least one phase nonlinearity of the optical communications system, the channel monitor comprising:

a variable dispersion unit configured to vary dispersion of the wavelength channel;

a flat region detector configured to detect a flat region of the received data signal;

a noise detector configured to detect a noise metric of the received data signal; and

a Signal Processor configured to detect the predetermined property of the test signal, the predetermined property comprising a variation in the noise metric of the received data signal that correlates with a change in the dispersion of the wavelength channel.