Monitoring phase non-linearities in an optical communications system
View Patent ↗Phase nonlinearities of an optical communications system are monitored by generating a test signal which includes a predetermined property that is uniquely associated with at least one phase nonlinearity of the optical communications system. The predetermined property of the test signal is then detected at a monitoring point of the optical communications system, and used to estimate the associated phase nonlinearity.
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;
wherein the step of generating the test signal comprises steps of:
transmitting a probe signal through a first wavelength channel of the optical communications system, the probe signal having a predetermined frequency and transition time; and
transmitting the data signal through a second wavelength channel of the optical communications system, such that the test signal is generated by cross-phase modulation (XPM)-induced distortion of the data signal;
wherein the predetermined property comprises a phase jitter of the test signal at a timing of self-phase modulation (SPM)-induced distortions of the probe signal arriving at the selected monitoring point.
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 the predetermined frequency is selected to minimize probe signal distortions due to either one or both of inter-symbol interference and dispersion.
4. A method as claimed in claim 3 , wherein the predetermined frequency is less than a data rate of the co-propagating data traffic.
5. A method as claimed in claim 1 , wherein the predetermined transition time is based on a data rate of data traffic of the optical communications system.
6. A method as claimed in claim 5 , wherein the predetermined transition time substantially corresponds with the transition time of the data traffic.
7. A method as claimed in claim 2 , wherein the step of detecting the predetermined property of the test signal comprises steps of:
recovering a clock signal from the test signal;
monitoring a phase jitter of the clock signal; and
correlating the monitored phase jitter with SPM-induced distortion components of the probe signal arriving at the selected monitoring point.
8. A method as claimed in claim 1 , wherein the step of generating the test signal comprises steps of:
receiving the data signal through a channel of the optical communications system;
storing sample data indicative of a waveform of a portion of the received data signal; and
storing recovered data bits corresponding to the stored sample data;
wherein the predetermined property comprises non-linear inter-symbol interference within the stored sample data.
9. A method as claimed in claim 8 , wherein the step of detecting the predetermined property of the test signal comprises steps of:
digitally processing the stored sample data and the stored recovered data bits to remove effects of each one of: linear inter-symbol interference, polarization and dispersion from the stored sample data; and
correlating remaining distortions in the waveform to the recovered data bits.