Transmission line monitoring device and transmission line monitoring method

1. A transmission line monitoring device comprising:

a memory; and

a processor coupled to the memory and configured to:

compensate an electric field signal alternately for a chromatic dispersion and a nonlinear distortion produced in an optical signal in each of virtual sections, the electric field signal indicating an optical electric field component of the optical signal input from a transmission line virtually divided into the virtual sections;

evaluate a quality of the electric field signal that is compensated for the chromatic dispersion and the nonlinear distortion;

set a first compensation quantity of the chromatic dispersion according to a length of each of the virtual sections to search for a second compensation quantity of the nonlinear distortion for each of the virtual sections when the quality satisfies a predetermined condition; and

monitor a power distribution of the optical signal on the transmission line on a basis of a relationship between the first compensation quantity and the second compensation quantity,

wherein the processor is further configured to, when searching for the second compensation quantity,

select the virtual sections in sequence,

set the first compensation quantity, and

search for a third compensation quantity of the nonlinear distortion for a selected virtual section when the quality satisfies the predetermined condition under an assumption that no nonlinear distortion is produced in virtual sections other than the selected virtual section out of the virtual sections, and

search for the second compensation quantity for each of the selected virtual section by setting an initial value of the second compensation quantity to the third compensation quantity.

2. The transmission line monitoring device according to claim 1 , wherein the predetermined condition for the quality is that a Q-factor of the electric field signal is maximum.

3. The transmission line monitoring device according to claim 1 , wherein the processor is further configured to, when searching for the third compensation quantity, search for the third compensation quantity for each of the virtual sections in parallel.

4. The transmission line monitoring device according to claim 1 , wherein the processor is further configured to, when searching for the third compensation quantity, search for the third compensation quantity by dividing the first compensation quantity into a fourth compensation quantity for virtual sections upstream of a location within the selected virtual section and a fifth compensation quantity for virtual sections downstream of the location within the selected virtual section, and setting the fourth compensation quantity and the fifth compensation quantity.

5. The transmission line monitoring device according to claim 1 , wherein the processor is further configured to, when searching for the second compensation quantity,

search for the second compensation quantity by varying a compensation quantity of the nonlinear distortion for each of the virtual sections,

determine whether the quality has increased or decreased every time when the compensation quantity of the nonlinear distortion is varied, and

terminate searching for the second compensation quantity when an increment in the quality is lower than a lower limit a predetermined number of times consecutively.

6. The transmission line monitoring device according to claim 1 , wherein the processor is configured to, when searching for the third compensation quantity, increase a compensation quantity of the nonlinear distortion for the selected virtual section from a minimum value to a maximum value in increments of a predetermined amount, and search for the third compensation quantity for the selected virtual section by determining whether the quality has increased or decreased every time when the compensation quantity of the nonlinear distortion is increased.

7. A transmission line monitoring method comprising:

compensating an electric field signal alternately for a chromatic dispersion and a nonlinear distortion produced in an optical signal in each of virtual sections, the electric field signal indicating an optical electric field component of the optical signal input from a transmission line virtually divided into the virtual sections;

evaluating a quality of the electric field signal that is compensated for the chromatic dispersion and the nonlinear distortion;

setting a first compensation quantity of the chromatic dispersion according to a length of each of the virtual sections and searching for a second compensation quantity of the nonlinear distortion for each of the virtual sections when the quality satisfies a predetermined condition; and

monitoring a power distribution of the optical signal on the transmission line on a basis of a relationship between the first compensation quantity and the second compensation quantity,

wherein the searching of the second compensation quantity includes:

selecting the virtual sections in sequence,

setting the first compensation quantity, and

searching for a third compensation quantity of the nonlinear distortion for a selected virtual section when the quality satisfies the predetermined condition under an assumption that no nonlinear distortion is produced in virtual sections other than the selected virtual section out of the virtual sections, and

searching for the second compensation quantity by setting an initial value of the second compensation quantity to the third compensation quantity.

8. The transmission line monitoring method according to claim 7 , wherein the predetermined condition for the quality is that a Q-factor of the electric field signal is a maximum value.

9. The transmission line monitoring method according to claim 7 , wherein the searching of the third compensation quantity for each of the virtual sections includes searching for the third compensation quantity for each of the virtual sections in parallel.

10. The transmission line monitoring method according to claim 7 , wherein the searching of the third compensation quantity includes searching for the third compensation quantity by dividing the first compensation quantity into a fourth compensation quantity for virtual sections upstream of a location within the selected virtual section and a fifth compensation quantity for virtual sections downstream of the location within the selected virtual section, and setting the fourth compensation quantity and the fifth compensation quantity.

11. The transmission line monitoring method according to claim 7 , wherein the searching of the second compensation quantity includes:

searching for the second compensation quantity by varying a compensation quantity of the nonlinear distortion for each of the virtual sections,

determining whether the quality has increased or decreased every time when the compensation quantity of the nonlinear distortion is varied, and

terminating searching for the second compensation quantity when an increment in the quality is lower than a lower limit a predetermined number of times consecutively.

12. The transmission line monitoring method according to claim 7 , wherein the searching of the third compensation quantity for the selected virtual section includes increasing a compensation quantity of the nonlinear distortion for the selected virtual section from a minimum value to a maximum value in increments of a predetermined amount, and searching for the third compensation quantity for the selected virtual section by determining whether the quality has increased or decreased every time when the compensation quantity of the nonlinear distortion is increased.