Determining in-band optical signal-to-noise ratio in polarization-multiplexed optical signals using signal correlations

Patent metadata

Field	Value
Publication number	US-2016164599-A1
Application number	US-201414564580-A
Country	US
Kind code	A1
Filing date	Dec 9, 2014
Priority date	Dec 9, 2014
Publication date	Jun 9, 2016
Grant date	—

Abstract

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.

First claim

What is claimed is:

1 . A method for determining an optical signal-to-noise ratio of a modulated optical signal propagating in a transmission link, the modulated optical signal comprising a plurality of wavelength channels, the method comprising:
(a) measuring an optical power spectrum of the modulated optical signal, the optical power spectrum comprising at least one of the plurality of wavelength channels; (b) measuring a time-varying parameter comprising at least one of:
time-varying optical signal amplitudes and phases in two mutually orthogonal polarization states, and
time-varying optical signal power levels in two mutually orthogonal polarization states;
wherein the time-varying parameter is measured simultaneously at first and second predetermined optical frequencies in a selected one of the plurality of wavelength channels, wherein the first and second predetermined optical frequencies are separated by a non-zero frequency interval;
(c) determining a correlation between the time-varying parameters measured in step (b) at the first and second optical frequencies by calculating a correlation coefficient between the time-varying parameters at the first and second optical frequencies; and (d) determining an optical signal-to-noise ratio from the optical power spectrum measured in step (a) and the correlation coefficient calculated in step (c).

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 , wherein step (c) comprises at least one of:
(i) 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 (ii) removing a differential group delay introduced by polarization mode dispersion in the transmission link between the time-varying parameters at the first and second optical frequencies.

5 . The method according to claim 1 , wherein the time-varying parameter comprises the time-varying optical signal amplitudes and phases.

6 . 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:
(a) 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; (b) 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; (c) 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; and (d) varying the differential time and phase delay of step (b); (e) repeating steps (c) and (d) until the correlation coefficient reaches a maximum; and (f) calculating the group velocity dispersion from the differential time and phase delay introduced in step (b) and varied in step (d), and the frequency interval of step (a).

7 . 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:
(a) 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; (b) introducing a differential group delay between signals representing the time-varying optical signal powers at the first and second optical frequencies; (c) 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; (d) varying the differential group delay of step (b); (e) repeating steps (c) and (d) until the correlation coefficient reaches a maximum; and (f) using the last value of the differential group delay varied in step (d) to obtain the differential group delay accumulated due to polarization mode dispersion of the modulated optical signal.

8 . An apparatus for determining an optical signal-to-noise ratio of a modulated optical signal comprising a plurality of wavelength channels, the apparatus comprising:
a spectrum analyzer for measuring an optical power spectrum of the modulated optical signal, the optical power spectrum comprising at least one of the plurality of wavelength channels; a frequency selective splitter for selecting first and second portions of the modulated optical signal at first and second predetermined optical frequencies, respectively, in a selected one of the plurality of wavelength channels, wherein the first and second predetermined optical frequencies are separated by a non-zero frequency interval; a measuring unit for measuring a time-varying parameter comprising at least one of: time-varying optical amplitudes and phases; and time-varying optical power levels of the first and second portions of the modulated optical signal; and a signal processor for determining a correlation between the time-varying parameters of the first and second portions of the modulated optical signal, and for calculating the optical signal-to-noise-ratio from the correlation of the time-varying parameters and the power spectrum of the modulated optical signal.

9 . The apparatus according to claim 8 , wherein the time-varying parameter comprises the time-varying optical signal amplitudes and phases; wherein the amplitude and phase detector comprises a coherent receiver; and wherein the frequency selective splitter comprises a tunable local oscillator light source.

10 . The apparatus according to claim 9 , wherein the coherent receiver has phase and polarization diversity.

11 . The apparatus according to claim 9 , wherein the tunable local oscillator light source comprises a tunable laser.

12 . The apparatus according to claim 9 , wherein the tunable local oscillator light source comprises a laser operating at a predetermined optical frequency, and a tunable optical frequency shifter optically coupled to the laser, for shifting the optical frequency of the laser.

13 . The apparatus according to claim 12 , wherein the laser operating at predetermined optical frequency has a linewidth of no greater than 100 kHz.

14 . The apparatus according to claim 12 , further comprising an optical power splitter for splitting output light of the laser into first and second portions,
wherein the first portion of the output light is coupled to the tunable frequency shifter, whose output signal is coupled to the coherent receiver for detecting an amplitude and phase of the first portion of the modulated optical signal, whereas the second portion of the output light is coupled directly to the coherent receiver for detecting an am

Assignees

• Jds Uniphase Corp

Inventors

• Heismann Fred L

Classifications

• H04B10/07953 Primary

  Monitoring or measuring OSNR, BER or Q · CPC title

• H04B10/07955

  Monitoring or measuring power · CPC title

• H04B10/07951

  Monitoring or measuring chromatic dispersion or PMD · CPC title