Nonlinear distortion detecting circuit, optical receiver, optical transmission system, and method for detecting nonlinear distortion

1. A module comprising:

a first estimator that estimates a first signal quality based on a number of error corrections of an electrical signal obtained by photoelectrically converting a received optical signal;

a second estimator that estimates a second signal quality from which an influence of nonlinear effects is removed based on signals upstream and downstream of an identification calculator identifying the electrical signal; and

a calculator that calculates a difference between the first and second signal qualities estimated by the first and second estimators to calculate an amount of nonlinear effects,

wherein the second estimator estimates the second signal quality, from which the influence of nonlinear effects is removed, using a signal from which a noise component having no correlation between symbols is removed by averaging on a plurality of symbols.

2. The module according to claim 1 , wherein the second estimator obtains, as a first average, an average of squares of signals obtained by subtracting a signal downstream of the identification calculator from a signal upstream thereof, obtains, as a second average, an average of products of signals, each obtained by subtracting a signal downstream of the identification calculator from a signal upstream thereof, of adjacent symbols, and subtracts the second average from the first average to estimate the signal quality from which the influence of nonlinear effects is removed.

3. The module according to claim 1 , wherein the first and second estimators each uses a Q value as the signal quality.

4. An optical receiver comprising:

a first estimator that estimates a first signal quality based on a number of error corrections of an electrical signal obtained by photoelectrically converting a received optical signal;

a second estimator that estimates a second signal quality from which an influence of nonlinear effects is removed based on signals upstream and downstream of an identification calculator identifying the electrical signal;

a calculator that calculates a difference between the first and second signal qualities estimated by the first and second estimators to calculate an amount of nonlinear effects; and

a controller that controls an equalizer based on the difference output by the calculator so that nonlinear distortion detected by the module decreases, wherein the second estimator estimates the second signal quality, from which the influence of nonlinear effects is removed, using a signal from which a noise component having no correlation between symbols is removed by averaging on a plurality of symbols.

5. An optical transmission system comprising:

a first estimator that estimates a first signal quality based on a number of error corrections of an electrical signal obtained by photoelectrically converting a received optical signal;

a second estimator that estimates a second signal quality from which an influence of nonlinear effects is removed based on signals upstream and downstream of an identification calculator identifying the electrical signal; and

a calculator that calculates a difference between the first and second signal qualities estimated by the first and second estimators to calculate an amount of nonlinear effects;

a controller that controls an equalizer based on the difference output by the calculator so that nonlinear distortion detected by the module decreases; and

an optical transmitter that transmits a signal to the optical receiver through an optical transmission path,

wherein the second estimator estimates the second signal quality, from which the influence of nonlinear effects is removed, using a signal from which a noise component having no correlation between symbols is removed by averaging on a plurality of symbols.

6. A method for detecting nonlinear effects, the method comprising:

estimating a first signal quality based on the number of error corrections of an electrical signal obtained by photoelectrically converting an optical signal received through an optical transmission path;

estimating a second signal quality, from which the influence of nonlinear effects is removed, based on signals upstream and downstream of an identification calculator identifying the electrical signal; and

obtaining a difference between the first and second signal qualities to calculate an amount of nonlinear effects,

wherein in the estimating the second signal quality, the second signal quality is estimated using a signal from which a noise component having no correlation between symbols is removed by averaging on a plurality of symbols.

7. The method according to claim 6 , wherein the estimating the second signal quality includes a sub-operation of obtaining, as a first average, a squares average of signals obtained by subtracting a signal downstream of the identification calculator from a signal upstream thereof, a sub-operation of obtaining, as a second average, a products average of signals, obtained by subtracting a signal downstream of the identification calculator from a signal upstream thereof, of adjacent symbols, and a sub-operation of subtracting the second average from the first average to estimate the signal quality from which the influence of nonlinear effects is removed.

8. The method according to claim 6 , wherein in each of the estimating the first signal quality and the second signal quality, a Q value is used as the signal quality.