Determining in-band optical signal-to-noise ratio in polarization-multiplexed optical signals using signal correlations
A method and apparatus for determining in-band OSNR in optical information signals, e.g. in polarization-multiplexed QPSK and higher-order M-ary QAM signals, are disclosed. A correlation measurement of the signal amplitude or power at two distinct optical frequencies of the signal may be used to determine the in-band optical noise in the signal. A measurement of the signal power may be used to determine the OSNR based on the determined in-band noise.
1. A method for determining a group velocity dispersion accumulated due to chromatic dispersion of a modulated optical signal comprising a plurality of wavelength channels, the method comprising:
measuring time-varying amplitudes and phases of the modulated optical signal in two mutually orthogonal polarization states simultaneously at first and second predetermined optical frequencies separated by a non-zero frequency interval, in at least one of the plurality of wavelength channels;
introducing a differential time and phase delay between signals representing the time-varying optical signal amplitudes and phases at the first and second optical frequencies;
determining a correlation between the time-varying optical signal amplitudes and phases at the predetermined optical frequencies by calculating a correlation coefficient between the time-varying amplitudes and phases of the modulated optical signal;
varying the differential time and phase delay;
determining the correlation between the time-varying optical signal amplitudes and phases at the predetermined optical frequencies and varying the differential time and phase delay until the correlation coefficient reaches a maximum; and
calculating the group velocity dispersion from the differential time and phase delay.
2. The method according to claim 1 , wherein the frequency interval is substantially equal to a symbol repetition frequency of the modulated optical signal in the selected wavelength channel, or an integer multiple thereof.
3. The method according to claim 1 , wherein differences between each of the first and second optical frequencies and a carrier frequency of the modulated optical signal in the selected wavelength channel are substantially of equal magnitude.
4. The method according to claim 1 , determining a correlation between the time-varying parameters measured for the first and second optical frequencies based on at least one of:
removing differential phase and time delays introduced by chromatic dispersion in the transmission link between the time-varying parameters at the first and second optical frequencies; and
removing the differential group delay.
5. A method for determining a differential group delay accumulated due to polarization mode dispersion of a modulated optical signal comprising a plurality of wavelength channels, the method comprising:
measuring time-varying optical signal power of the modulated optical signal in two mutually orthogonal polarization states simultaneously at first and second predetermined optical frequencies separated by a non-zero frequency interval, in at least one of the plurality of wavelength channels;
introducing a differential group delay between signals representing the time-varying optical signal powers at the first and second optical frequencies;
determining a correlation between the time-varying optical signal powers at the first and second optical frequencies by calculating a correlation coefficient between the time-varying optical signal powers;
varying the differential group delay;
determining a correlation between the time-varying optical signal powers at the first and second optical frequencies and varying the differential group delay until the correlation coefficient reaches a maximum;
and
using the last value of the differential group delay varied to obtain the differential group delay accumulated due to polarization mode dispersion of the modulated optical signal.
6. The method according to claim 5 , wherein the frequency interval is substantially equal to a symbol repetition frequency of the modulated optical signal in the selected wavelength channel, or an integer multiple thereof.
7. The method according to claim 5 , wherein differences between each of the first and second optical frequencies and a carrier frequency of the modulated optical signal in the selected wavelength channel are substantially of equal magnitude.
8. The method according to claim 5 , determining a correlation between the time-varying parameters measured for the first and second optical frequencies based on at least one of:
removing differential phase and time delays introduced by chromatic dispersion in the transmission link between the time-varying parameters at the first and second optical frequencies; and
removing the differential group delay.