Method and system for determining origin of optical signal quality degradation

• US 7,200,328 B2
• Filed: 06/19/2002
• Issued: 04/03/2007
• Est. Priority Date: 07/13/2001
• Status: Expired due to Fees

• Current Assignee: Nippon Telegraph and Telephone Corporation
• Original Assignee: Nippon Telegraph and Telephone Corporation
• Inventors: Shake, Ippei, Takara, Hidehiko, Uchiyama, Kentaro
• Primary Examiner(s): SINGH, DALZID E
• Application Number:
  US10/173,752
• Publication Number:
  US 20030011837A1
• Time in Patent Office: 1,749 Days
• Field of Search: 398 10- 21, 398 26- 28, 398/30
• US Class Current: 398/10
• CPC Class Codes: H04B 10/07951 Monitoring or measuring chromatic dispersion or PMD
• CPC Class Codes: H04B 10/07953 Monitoring or measuring OSNR, BER or Q

Abstract

A monitoring system includes first and second evaluation sections for obtaining an averaged Q-factor parameter and a waveform distortion parameter from an optical signal amplitude histogram collected from optical signals under measurement. The monitoring system further includes a third evaluation section for determining both averaged Q-factor parameter and waveform distortion parameter, and for making a decision as to whether the main factor of the optical signal quality degradation is waveform distortion or not by comparing the averaged Q-factor parameter and waveform distortion parameter with their initial values or initial characteristics which are obtained when no optical signal quality degradation is present.

Claims

1. A method for determining the origin of optical signal quality degradation comprising:

an optical signal amplitude histogram measurement step of obtaining an optical signal amplitude histogram from an optical signal under measurement;

an averaged Q-factor parameter measurement step of obtaining an averaged Q-factor parameter, which is an optical signal quality parameter calculated from the optical signal amplitude histogram obtained in said optical signal amplitude histogram measurement step;

a waveform distortion parameter measurement step of obtaining a waveform distortion parameter, which is another optical signal quality parameter calculated from the optical signal amplitude histogram obtained in said optical signal amplitude histogram measurement step; and

an optical signal quality evaluation step of evaluating an optical signal quality degradation factor using a measurement value of the averaged Q-factor parameter and a measurement value of the waveform distortion parameter obtained in said averaged Q-factor parameter measurement step and said waveform distortion parameter measurement step,

wherein said optical signal quality evaluation step makes a decision that a main factor of the optical signal quality degradation is the waveform distortion in the optical signal under measurement when the averaged Q-factor parameter is reduced and the waveform distortion parameter is increased, and makes a decision that the main factor of the optical signal quality degradation is a factor other than waveform distortion when the waveform distortion parameter is not increased.

9. The method for determining the origin of optical signal quality degradation as claimed in any one of claims 1–

8, wherein said optical signal amplitude histogram measurement step comprises;

an opto-electric conversion step of converting the optical signal under measurement with a bit rate of f₀ (bits per second) into an electric intensity modulated signal;

an electrical sampling step of sampling the electric intensity modulated signal obtained in said opto-electric conversion step with a sampling clock having a repetition frequency of

$f_1<br><br>\left(\mathrm{Hz}\right)=\frac{n}{m}*f_0+a,$

where n and m are natural numbers, and a is an offset frequency; and

a histogram evaluation step of obtaining an optical signal intensity distribution from the sampled signal obtained in said electrical sampling step, and of obtaining the optical signal amplitude histogram from the optical signal intensity distribution within an average time period.

10. The method for determining the origin of optical signal quality degradation as claimed in any one of claims 1–

8, wherein said optical signal amplitude histogram measurement step comprises;

an optical combining step of combining the optical signal under measurement of a bit rate f₀ (bits per second) with a sampling optical pulse train whose repetition frequency is

$f_1<br><br>\left(\mathrm{Hz}\right)=\frac{n}{m}*f_0+a,$

where n and mare a natural numbers, and a is an offset frequency, and whose pulse width is substantially narrower than a time slot with a bit rate of f₀ (bits per second);

a cross-correlation optical signal generating step of obtaining a cross-correlation optical signal by launching the combined light which is combined in said optical combining step into a nonlinear optical medium for inducing nonlinear interaction between the optical signal under measurement and the sampling optical pulse train;

an optical splitting step of splitting the cross-correlation optical signal obtained in said cross-correlation optical signal generating step from the optical signal under measurement and the sampling optical pulse train;

an opto-electric conversion step of converting the cross-correlation optical signal obtained by said splitting in said optical splitting step into an electric intensity modulated signal; and

a histogram evaluation step of obtaining an optical signal intensity distribution from the electric intensity modulated signal obtained in said opto-electric conversion step, and obtaining the optical signal amplitude histogram from the optical signal intensity distribution within the average time period.

11. The method for determining the origin of optical signal quality degradation as claimed in any one of claims 1–

8, wherein said optical signal amplitude histogram measurement step comprises;

an optical gating step of sampling the optical signal under measurement with a bit rate of f₀ (bits per second) with a sampling clock, said sampling clock having a repetition frequency of

$f_1<br><br>\left(\mathrm{Hz}\right)=\frac{n}{m}*f_0+a,$

where n and mare a natural numbers, and a is an offset frequency;

an opto-electric conversion step of converting the sampled optical signal obtained in said optical gating step into a sampled electric signal; and

a histogram evaluation step of obtaining an optical signal intensity distribution from the sampled electric signal obtained in said opto-electric conversion step, and of obtaining the optical signal amplitude histogram from the optical signal intensity distribution within an average time period.

12. The method for determining the origin of optical signal quality degradation as claimed in any one of claims 1, 2 and 5–

8, wherein said averaged Q-factor parameter measurement step and said waveform distortion parameter measurement step comprise the steps of;

estimating an amplitude histogram distribution function g1 associated with a “

level 1”

from a portion of the amplitude histogram greater than a first predetermined intensity threshold value;

estimating an amplitude histogram distribution function g0 associated with a “

level 0”

from a portion of the amplitude histogram less than a second predetermined intensity threshold vaIue;

obtaining mean intensity values and standard deviations of the “

level 1” and

the “

level 0”

from the functions g1 and g0;

calculating the averaged Q-factor parameter as a ratio of a difference between the mean intensity values of the “

level 1” and

the “

level 0”

to the sum of the standard deviations of the “

level 1” and

the “

level 0”

; and

adopting at least one of the standard deviation of the “

level 1”

, the standard deviation of the “

level 0”

, and the difference between the mean intensity values of the “

level 1” and

the “

level 0”

as the waveform distortion parameter.

13. The method for determining the origin of optical signal quality degradation as claimed in any one of claims 1, 2 and 5–

8, wherein said optical signal amplitude histogram measurement step comprises;

an opto-electric conversion step of converting the optical signal under measurement with a bit rate of f₀ (bits per second) into an electric intensity modulated signal;

an electrical sampling step of obtaining a sampled signal by sampling the electric intensity modulated signal obtained in said opto-electric conversion step with a sampling clock having a repetition frequency of

$\frac{n}{m}*f_0+a,$

where n and m are natural numbers and a is an offset frequency; and

a histogram evaluation step of obtaining an optical signal intensity distribution from the sampled signal obtained in said electrical sampling step, and of obtaining the optical signal amplitude histogram from the optical signal intensity distribution within an average time period,

wherein said averaged Q-factor parameter measurement step and said waveform distortion parameter measurement step comprise the steps of;

estimating an amplitude histogram distribution function g1 associated with a “

level 1”

from a portion of the amplitude histogram greater than a first predetermined intensity threshold value,

estimating an amplitude histogram distribution function g0 associated with a “

level 0”

from a portion of the amplitude histogram less than another predetermined intensity threshold value,

obtaining mean intensity values and standard deviations of the “

level 1” and

the “

level 0”

from the functions g1 and g0,

calculating the averaged Q-factor parameter as a ratio of a difference between the mean intensity values of the “

level 1” and

the “

level 0”

to the sum of the standard deviations of the “

level 1” and

the “

level 0, and

adopting at least one of the standard deviation of the “

level 1”

, the standard deviation of the “

level 0”

, and the difference between the mean intensity values of the “

level 1” and

the “

level 0”

as the waveform distortion parameter.

14. The method for determining the origin of optical signal quality degradation as claimed in any one of claims 1, 2 and 5–

8, wherein said optical signal amplitude histogram measurement step comprises;

an optical combining step of combining the optical signal under measurement of a bit rate f₀ (bits per second) with a sampling optical pulse train whose repetition frequency is

$f_1<br><br>\left(\mathrm{Hz}\right)=\frac{n}{m}*f_0+a,$

where n and m are natural numbers, and a is an offset frequency whose pulse width is substantially narrower than a time slot with a bit rate of f₀ (bits per second);

a cross-correlation optical signal generating step of obtaining a cross-correlation optical signal by launching the combined light which is combined in said optical combining step into a nonlinear optical medium for inducing nonlinear interaction between the optical signal under measurement and the sampling optical pulse train;

an optical splitting step of splitting the cross-correlation optical signal obtained in said cross-correlation optical signal generating step from the optical signal under measurement and the sampling optical pulse train;

an opto-electric conversion step of converting the cross-correlation optical signal, obtained by said splitting in said optical splitting step, into an electric intensity modulated signal; and

a histogram evaluation step of obtaining an optical signal intensity distribution from the electric intensity modulated signal obtained in said opto-electric conversion step, and of obtaining the optical signal amplitude histogram from the optical signal intensity distribution within the average time period,

wherein said averaged Q-factor parameter measurement step and said waveform distortion parameter measurement step comprise the steps of;

estimating an amplitude histogram distribution function g1 associated with a “

level 1”

from a portion of the amplitude histogram greater than a first predetermined intensity threshold value,

estimating an amplitude histogram distribution function g0 associated with a “

level 0”

from a portion of the amplitude histogram less than a second predetermined intensity threshold value,

obtaining mean intensity values and standard deviations of the “

level 1” and

the “

level 0”

from the functions g1 and g0,

calculating the averaged Q-factor parameter as a ratio of a difference between the mean intensity values of the “

level 1” and

the “

level 0”

to the sum of the standard deviations of the “

level 1” and

the “

level 0”

, and

adopting at least one 6f the standard deviation of the “

level 1”

, the standard deviation of the “

level 0”

, and the difference between the mean intensity values of the “

level 1” and

the “

level 0”

as the waveform distortion parameter.

15. The method for determining the origin of optical signal quality degradation as claimed in any one of claims 1, 2 and 5–

8, wherein said optical signal amplitude histogram measurement step comprises;

an optical gating step of sampling the optical signal under measurement with a bit rate of f₀ (bits per second) with a sampling clock, said sampling clock having a repetition frequency of

$\left(\mathrm{Hz}\right)=\frac{n}{m}*f_0+a,$

where n and m are natural numbers, and a is an offset frequency;

an opto-electric conversion step of converting the sampled optical signal obtained in said optical gating step into a sampled electric signal; and

a histogram evaluation step of obtaining an optical signal intensity distribution from the sampled electric signal obtained in said opto-electric conversion step, and of obtaining the optical signal amplitude histogram from the optical signal intensity distribution within an average time period,

wherein said averaged Q-factor parameter measurement step and said waveform distortion parameter measurement step comprise the steps of;

estimating an amplitude histogram distribution function g1 associated with a “

level 1”

from a portion of the amplitude histogram greater than a first predetermined intensity threshold value,

estimating an amplitude histogram distribution function g0 associated with a “

level 0”

from a portion of the amplitude histogram less than a second predetermined intensity threshold value,

obtaining mean intensity values and standard deviations of the “

level 1” and

the “

level 0”

from the functions g1 and g0,

calculating the averaged Q-factor parameter as a ratio of a difference between the mean intensity values of the “

level 1” and

the “

level 0”

to the sum of the standard deviations of the “

level 1” and

the “

level 0”

, and

adopting at least one of the standard deviation of the “

level 1”

, the standard deviation of the “

level 0”

, and the difference between the mean intensity values of the “

level 1” and

the “

level 0”

as the waveform distortion parameter.

2. A method for determining the origin of optical signal quality degradation comprising:

an optical signal amplitude histogram measurement step of obtaining an optical signal amplitude histogram from an optical signal under measurement;

an averaged Q-factor parameter measurement step of obtaining an averaged Q-factor parameter, which is an optical signal quality parameter calculated from the optical signal amplitude histogram obtained in said optical signal amplitude histogram measurement step;

a waveform distortion parameter measurement step of obtaining a waveform distortion parameter, which is another optical signal quality parameter calculated from the optical signal amplitude histogram obtained in said optical signal amplitude histogram measurement step;

an initial state storing step of storing into a storage medium at least one of an initial value and initial characteristic of the averaged Q-factor parameter and the waveform distortion parameter at a system installation without optical signal quality degradation, which are obtained using said averaged Q-factor parameter measurement step and said waveform distortion parameter measurement step; and

an optical signal quality evaluation step of evaluating an optical signal quality degradation factor using a measurement value of the averaged Q-factor parameter and a measurement value of the waveform distortion parameter obtained in said averaged Q-factor parameter measurement step and said waveform distortion parameter measurement step, and using at least one of the initial values and initial characteristics stored in the storage medium in said initial state storing step,

wherein said optical signal quality evaluation step makes a decision that a main factor of the optical signal quality degradation is the waveform distortion in the optical signal under measurement when the averaged Q-factor parameter is reduced and the waveform distortion parameter is increased, and makes a decision that the main factor of the optical signal quality degradation is a factor other than waveform distortion when the waveform distortion parameter is not increased.

3. A method for determining the origin of optical signal quality degradation comprising:

an optical splitting step of splitting an optical signal under measurement;

an optical signal amplitude histogram measurement step of obtaining an optical signal amplitude histogram from a first optical signal under measurement which is obtained by splitting the optical signal under measurement in said optical splitting step;

an averaged Q-factor parameter measurement step of obtaining an averaged Q-factor parameter, which is an optical signal quality parameter calculated from the optical signal amplitude histogram obtained in said optical signal amplitude histogram measurement step;

an optical signal to optical noise power measurement step of measuring optical signal to optical noise power from a second optical signal under measurement which is obtained by splitting the optical signal under measurement in said optical splitting step;

an optical signal-to-noise ratio parameter obtaining step of obtaining an optical signal-to-noise ratio parameter, which is another optical signal quality parameter from the optical signal to optical noise power obtained in said optical signal to optical noise power measurement step; and

an optical signal quality evaluation step of evaluating an optical signal quality degradation factor using a measurement value of the averaged Q-factor parameter obtained in said averaged Q-factor parameter measurement step and a measurement value of the optical signal-to-noise ratio parameter obtained in said optical signal-to-noise ratio parameter obtaining step,

wherein said optical signal quality evaluation step makes a decision as to whether a main factor of the optical signal quality degradation is the degradation in the optical signal-to-noise ratio or not by determining and evaluating both the averaged Q-factor parameter and the optical signal-to-noise ratio parameter.

4. A method for obtaining the origin of optical signal quality degradation comprising:

an optical splitting step of splitting an optical signal under measurement;

an optical signal amplitude histogram measurement step of obtaining an optical signal amplitude histogram from a first optical signal under measurement which is obtained by splitting the optical signal under measurement in said optical splitting step;

an averaged Q-factor parameter measurement step of obtaining an averaged Q-factor parameters which is an optical signal quality parameter calculated from the optical signal amplitude histogram obtained in said optical signal amplitude histogram measurement step;

an optical signal to optical noise power measurement step of measuring optical signal to optical noise power from a second optical signal under measurement which is obtained by splitting the optical signal under measurement in said optical splitting step;

an optical signal-to-noise ratio parameter obtaining step of obtaining an optical signal-to-noise ratio parameter, which is another optical signal quality parameter from the optical signal to optical noise power obtained in said optical signal to optical noise power measurement step;

an initial state storing step of storing into a storage medium at least one of an initial value and initial characteristic of the averaged Q-factor parameter and of the optical signal-to-noise ratio parameter at a system installation without optical signal quality degradation, which are obtained using said averaged Q-factor parameter measurement step and said optical signal-to-noise ratio parameter obtaining step; and

an optical signal quality evaluation step of evaluating an optical signal quality degradation factor using a measurement value of the averaged Q-factor parameter obtained in said averaged Q-factor parameter measurement step and a measurement value of the optical signal-to-noise ratio parameter obtained in said optical signal-to-noise ratio parameter obtaining step, and using at least one of the initial values and initial characteristics of the averaged Q-factor parameter and the optical signal-to-noise ratio parameter at the system installation without optical signal quality degradation, which are stored in the storage medium in said initial state storing step,

wherein said optical signal quality evaluation step makes a decision as to whether a main factor of the optical signal quality degradation is the degradation in the optical signal-to-noise ratio or not by determining and evaluating both the averaged Q-factor parameter and the optical signal-to-noise ratio parameter.

5. A method for determining the origin of optical signal quality degradation comprising:

an optical splitting step of splitting an optical signal under measurement;

an optical signal amplitude histogram measurement step of obtaining an optical signal amplitude histogram from a first optical signal under measurement which is obtained by splitting the optical signal under measurement in said optical splitting step;

an optical signal to optical noise power measurement step of measuring optical signal to optical noise power from a second optical signal under measurement which is obtained by splitting the optical signal under measurement in said optical splitting step;

a waveform distortion parameter measurement step of obtaining a waveform distortion parameter, which is an optical signal quality parameter from the optical signal amplitude histogram obtained in said optical signal amplitude histogram measurement step;

an optical signal-to-noise ratio parameter obtaining step of obtaining an optical signal-to-noise ratio parameter which is an optical signal quality parameter from the optical signal to optical noise power obtained in said optical signal to optical noise power measurement step; and

an optical signal quality evaluation step of evaluating an optical signal quality degradation factor using a measurement value of the waveform distortion parameter obtained in said waveform distortion parameter measurement step, and a measurement value of the optical signal-to-noise ratio parameter obtained in said optical signal-to-noise ratio parameter obtaining step, wherein said optical signal quality evaluation step makes a decision as to whether a main factor of the optical signal quality degradation is the degradation in the optical signal-to-noise ratio or the waveform distortion by determining and evaluating both the waveform distortion parameter and the optical signal-to-noise ratio parameter.

6. A method for determining the origin of optical signal quality degradation comprising:

an optical splitting step of splitting an optical signal under measurement;

an optical signal amplitude histogram measurement step of obtaining an optical signal amplitude histogram from a first optical signal under measurement which is obtained by splitting the optical signal under measurement in said optical splitting step;

an optical signal to optical noise power measurement step of measuring optical signal to optical noise power from a second optical signal under measurement which is obtained by splitting the optical signal under measurement in said optical splitting step;

a waveform distortion parameter measurement step of obtaining a waveform distortion paramete, which is an optical signal quality parameter calculated from the optical signal amplitude histogram obtained in said optical signal amplitude histogram measurement step;

an optical signal-to-noise ratio parameter obtaining step of obtaining an optical signal-to-noise ratio parameter, which is another optical signal quality parameter from the optical signal to optical noise power obtained in said optical signal to optical noise power measurement step;

an initial state storing step of storing into a storage medium at least one of an initial value and initial characteristic of the waveform distortion parameter and that of the optical signal-to-noise ratio parameter at a system installation without optical signal quality degradation, which are obtained using said waveform distortion parameter measurement step and said optical signal-to-noise ratio parameter obtaining step; and

an optical signal quality evaluation step of evaluating an optical signal quality degradation factor using a measurement value of the waveform distortion parameter obtained in said waveform distortion parameter measurement step and a measurement value of the optical signal-to-noise ratio parameter obtained in said optical signal-to-noise ratio parameter obtaining step, and using at least one of the initial values and initial characteristics of the waveform distortion parameter and that of the optical signal-to-noise ratio parameter at the system installation without optical signal quality degradation,

wherein said optical signal quality evaluation step makes a decision as to whether a main factor of the optical signal quality degradation is the degradation in the optical signal-to-noise ratio or the waveform distortion by determining and evaluating both the waveform distortion parameter and the optical signal-to-noise ratio parameter.

7. A method for determining the origin of optical signal quality degradation comprising:

an optical splitting step of splitting an optical signal under measurement;

an optical signal amplitude histogram measurement step of obtaining an optical signal amplitude histogram from a first optical signal under measurement which is obtained by splitting the optical signal under measurement in said optical splitting step;

an averaged Q-factor parameter measurement step of obtaining an averaged Q-factor parameter, which is an optical signal quality parameter calculated from the optical signal amplitude histogram obtained in said optical signal amplitude histogram measurement step;

an optical signal to optical noise power measurement step of measuring optical signal to optical noise power from a second optical signal under measurement which is obtained by splitting the optical signal under measurement in said optical splitting step;

a waveform distortion parameter measurement step of obtaining a waveform distortion parameter, which is another optical signal quality parameter calculated from the optical signal amplitude histogram obtained in said optical signal amplitude histogram measurement step;

an optical signal-to-noise ratio parameter obtaining step of obtaining an optical signal-to-noise ratio parameter, which is another optical signal quality parameter from the optical signal to optical noise power obtained in said optical signal to optical noise power measurement step; and

an optical signal quality evaluation step of evaluating an optical signal quality degradation factor using a measurement value of the averaged Q-factor parameter obtained in said averaged Q-factor parameter measurement step, a measurement value of the waveform distortion parameter obtained in said waveform distortion parameter measurement step, and a measurement value of the optical signal-to-noise ratio parameter obtained in said optical signal-to-noise ratio parameter obtaining step,

wherein said optical signal quality evaluation step makes a decision as to whether a main factor of the optical signal quality degradation is the degradation in the optical signal-to-noise ratio or the waveform distortion or not by determining and evaluating all of the averaged Q-factor parameter, the waveform distortion parameter1 and the optical signal-to-noise ratio parameter.

8. A method for determining the origin of optical signal quality degradation comprising:

an optical splitting step of splitting an optical signal under measurement;

an optical signal amplitude histogram measurement step of obtaining an optical signal amplitude histogram from a first optical signal under measurement, which is obtained by splitting the optical signal under measurement in said optical splitting step;

an averaged Q-factor parameter measurement step of obtaining an averaged Q-factor parameter1, which is an optical signal quality parameter calculated from the optical signal amplitude histogram obtained in said optical signal amplitude histogram measurement step;

an optical signal to optical noise power measurement step of measuring optical signal to optical noise power from a second optical signal under measurement which is obtained by splitting the optical signal under measurement in said optical splitting step;

a waveform distortion parameter measurement step of obtaining a waveform distortion parameter, which is another optical signal quality parameter calculated from the optical signal amplitude histogram obtained in said optical signal amplitude histogram measurement step;

an optical signal-to-noise ratio parameter obtaining step of obtaining an optical signal-to-noise ratio parameter, which is another optical signal quality parameter from the optical signal to optical noise power obtained in said optical signal to optical noise power measurement step;

an initial state storing step of storing into a storage medium at least one of an initial value and an initial characteristic of the averaged Q-factor parameter, that of the waveform distortion parameter, and that of the optical signal-to-noise ratio parameter at a system installation without optical signal quality degradation, which are obtained using said averaged Q-factor parameter measurement step, said waveform distortion parameter measurement step and said optical signal-to-noise ratio parameter obtaining step; and

an optical signal quality evaluation step of evaluating an optical signal quality degradation factor using a measurement value of the averaged Q-factor parameter obtained in said averaged Q-factor parameter measurement step, a measurement value of the waveform distortion parameter obtained in said waveform distortion parameter measurement step, and a measurement value of the optical signal-to-noise ratio parameter obtained in said optical signal-to-noise ratio parameter obtaining step, and using at least one of the initial values and initial characteristics of the averaged Q-factor parameter, that of the waveform distortion parameter, and the optical signal-to-noise ratio parameter at the system installation without optical signal quality degradation, which are stored into the storage medium by said initial state storing step,

wherein said optical signal quality evaluation step makes a decision as to whether a main factor of the optical signal quality degradation is degradation in the optical signal-to-noise ratio, the waveform distortion or not by determining and evaluating all the averaged Q-factor parameter, the waveform distortion parameter, and the optical signal-to-noise ratio parameter.

16. A system for determining the origin of optical signal quality degradation comprising:

optical signal amplitude histogram measurement means for obtaining an optical signal amplitude histogram from an optical signal under measurement;

averaged Q-factor parameter measurement means for obtaining an averaged Q-factor parameter, which is an optical signal quality parameter calculated from the optical signal amplitude histogram obtained by said optical signal amplitude histogram measurement means;

waveform distortion parameter measurement means for obtaining a waveform distortion parameter, which is another optical signal quality parameter calculated from the optical signal amplitude histogram obtained by said optical signal amplitude histogram measurement means; and

optical signal quality evaluation means for evaluating an optical signal quality degradation factor using a measurement value of the averaged Q-factor parameter and a measurement value of the waveform distortion parameter obtained by said averaged Q-factor parameter measurement means and said waveform distortion parameter measurement means,

wherein said optical signal quality evaluation means makes a decision that a main factor of the optical signal quality degradation is the waveform distortion in the optical signal under measurement when the averaged Q-factor parameter is reduced and the waveform distortion parameter is increased, and makes a decision that the main factor of the optical signal quality degradation is a factor other than waveform distortion when the waveform distortion parameter is not increased.

24. The system for determining the origin of optical signal quality degradation as claimed in any one of claims 16–

23, wherein said optical signal amplitude histogram measurement means comprises;

opto-electric conversion means for converting the optical signal under measurement with a bit rate of f₀ (bits per second) into an electric intensity modulated signal;

electrically sampling means for sampling the electric intensity modulated signal obtained by said opto-electric conversion means with a sampling clock having a repetition frequency of

$f_1<br><br>\left(\mathrm{Hz}\right)=\frac{n}{m}*f_0+a,$

where n and m are natural numbers, and a is an offset frequency; and

histogram evaluation means for obtaining an optical signal intensity distribution from the sampled signal obtained by said electrical sampling means, and of obtaining the optical signal amplitude histogram from the optical signal intensity distribution within an average time period.

25. The system for determining the origin of optical signal quality degradation as claimed in any one of claims 16–

23, wherein said optical signal amplitude histogram measurement means comprises;

optical combining means for combining the optical signal under measurement of a bit rate f₀ (bits per second) with a sampling optical pulse train whose repetition frequency is

$f_1<br><br>\left(\mathrm{Hz}\right)=\frac{n}{m}*f_0+a,$

where n and m are natural numbers, and a is an offset frequency, and whose pulse width is substantially narrower than a time slot with a bit rate of f₀ (bits per second);

cross-correlation optical signal generating means for obtaining a cross-correlation optical signal by providing the combined light which is combined in said optical combining means into a nonlinear optical medium for inducing nonlinear interaction between the optical signal under measurement and the sampling optical pulse train;

optical splitting means for splitting the cross-correlation optical signal obtained by said cross-correlation optical signal generating means from the optical signal under measurement and the sampling optical pulse train;

opto-electric conversion means for converting the cross-correlation optical signal, which is obtained by said splitting in said optical splitting means, into an electric intensity modulated signal; and

histogram evaluation means for obtaining an optical signal intensity distribution from the electric intensity modulated signal obtained by said opto-electric conversion means, and of obtaining the optical signal amplitude histogram from the optical signal intensity distribution within the average time period.

26. The system for determining the origin of optical signal quality degradation as claimed in any one of claims 16–

23, wherein said optical signal amplitude histogram measurement means comprises;

optical gating means for sampling the optical signal under measurement with a bit rate of f₀ (bits per second) by with a sampling clock, said sampling clock having a repetition frequency of

$\left(\mathrm{Hz}\right)=\frac{n}{m}*f_0+a,$

where n and m are natural numbers, and a is an offset frequency;

opto-electric conversion means for converting the sampled optical signal obtained by said optical gating means into a sampled electric signal; and

histogram evaluation means for obtaining an optical signal intensity distribution from the sampled electric signal obtained by said opto-electric conversion means, and of obtaining the optical signal amplitude histogram from the optical signal intensity distribution within an average time period.

27. The system for determining the origin of optical signal quality degradation as claimed in any one of claims 16, 17 and 20–

23, wherein said averaged Q-factor parameter measurement means and said waveform distortion parameter measurement means comprise;

means for estimating an amplitude histogram distribution function g1 associated with a “

level 1”

from a portion of the amplitude histogram greater than a first predetermined intensity threshold value;

means for estimating an amplitude histogram distribution function g0 associated with a “

level 0”

from a portion of the amplitude histogram less than a second predetermined intensity threshold value;

means for obtaining mean intensity values and standard deviations of the “

level 1” and

the “

level 0”

from the functions g1 and g0;

means for calculating the averaged Q-factor parameter as a ratio of a difference between the mean intensity values of the “

level 1” and

the “

level 0”

to the sum of the standard deviations of the “

level 1” and

the “

level 0”

; and

means for adopting at least one of the standard deviation of the “

level 1“

, the standard deviation of the “

level 0” and

the difference between the mean intensity values of the “

level 1” and

the “

level 0”

as the waveform distortion parameter.

28. The system for determining the origin of optical signal quality degradation as claimed in any one of claims 16, 17 and 20–

23, wherein said optical signal amplitude histogram measurement means comprises;

opto-electric conversion means for converting the optical signal under measurement with a bit rate of f₀ (bits per second) into an electric intensity modulated signal;

electric sampling means for sampling the electric intensity modulated signal obtained by said opto-electric conversion means with a sampling clock having a repetition frequency of

$f_1<br><br>\left(\mathrm{Hz}\right)=\frac{n}{m}*f_0+a,$

where n and m are natural numbers, and a is an offset frequency; and

histogram evaluation means for obtaining an optical signal intensity distribution from the sampled signal obtained by said electrically sampling means, and of obtaining the optical signal amplitude histogram from the optical signal intensity distribution within an average time period; and

wherein said averaged Q-factor parameter measurement means and said waveform distortion parameter measurement means comprise;

means for estimating an amplitude histogram distribution function g1 associated with a “

level 1”

from a portion of the amplitude histogram greater than a predetermined intensity threshold value,

means for estimating an amplitude histogram distribution function g0 associated with a “

level 0”

from a portion of the amplitude histogram less than a second predetermined intensity threshold value,

means for obtaining mean intensity values and standard deviations of the “

level 1” and

the “

level 0”

from the functions g1 and g0,

means for calculating the averaged Q-factor parameter as a ratio of a difference between the mean intensity values of the “

level 1” and

the “

level 0”

to the sum of the standard deviations of the “

level 1” and

the “

level 0, and

means for adopting at least one of the standard deviation of the “

level 1”

, the standard deviation of the “

level 0”

, and the difference between the mean intensity values of the “

level 1” and

the “

level 0”

as the waveform distortion parameter.

29. The system for determining the origin of optical signal quality degradation as claimed in any one of claims 16, 17 and 20–

23, wherein said optical signal amplitude histogram measurement means comprises;

optical combining means for combining the optical signal under measurement of a bit rate f₀ (bits per second) with a sampling optical pulse train whose repetition frequency is

$f_1<br><br>\left(\mathrm{Hz}\right)=\frac{n}{m}*f_0+a,$

where n and m are natural numbers, and a is an offset frequency, and whose pulse width is substantially narrower than a time slot with a bit rate of f₀ (bits per second);

cross-correlation optical signal generating means for obtaining a cross-correlation optical signal by providing the combined light which is combined in said optical combining means into a nonlinear optical medium for inducing nonlinear interaction between the optical signal under measurement and the sampling optical pulse train;

optical splitting means for splitting the cross-correlation optical signal obtained by said cross-correlation optical signal generating means from the optical signal under measurement and the sampling optical pulse train;

opto-electric conversion means for converting the cross-correlation optical signal, which is obtained by said splitting in said optical splitting means, into an electric intensity modulated signal; and

histogram evaluation means for obtaining an optical signal intensity distribution from the electric intensity modulated signal obtained by said opto-electric conversion means, and of obtaining the optical signal amplitude histogram from the optical signal intensity distribution within the average time period; and

wherein said averaged Q-factor parameter measurement means and said waveform distortion parameter measurement means comprise;

means for estimating an amplitude histogram distribution function g1 associated with a “

level 1”

from a portion of the amplitude histogram greater than a predetermined intensity threshold value,

means for estimating an amplitude histogram distribution function g0 associated with a “

level 0”

from a portion of the amplitude histogram less than another predetermined intensity threshold value,

means for obtaining mean intensity values and standard deviations of the “

level 1” and

the “

level 0”

from the functions g1 and g0;

means for calculating the averaged Q-factor parameter as a ratio of a difference between the mean intensity values of the “

level 1” and

the “

level 0”

to the sum of the standard deviations of the “

level 1” and

the “

level 0”

, and

means for adopting at least one of the standard deviation of the “

level 1”

, the standard deviation of the “

level 0”

, and the difference between the mean intensity values of the “

level 1” and

the “

level 0”

as the waveform distortion parameter.

30. The system for determining the origin of optical signal quality degradation as claimed in any one of claims 16, 17 and 20–

23, wherein said optical signal amplitude histogram measurement means comprises;

optical gating means for sampling the optical signal under measurement with a bit rate of f₀ (bits per second) by a sampling clock, said sampling clock generating means having a repetition frequency of

$f_1<br><br>\left(\mathrm{Hz}\right)=\frac{n}{m}*f_0+a,$

where n and m are natural numbers, and a is an offset frequency;

opto-electric conversion means for converting the sampled optical signal obtained by said optical gating means into a sampled electric signal; and

histogram evaluation means for obtaining an optical signal intensity distribution from the sampled electric signal obtained by said opto-electric conversion means, and of obtaining the optical signal amplitude histogram from the optical signal intensity distribution within an average time period; and

wherein said averaged Q-factor parameter measurement means and said waveform distortion parameter measurement means comprise;

means for estimating an amplitude histogram distribution function g1 associated with a “

level 1”

from a portion of the amplitude histogram greater than a first predetermined intensity threshold value,

means for estimating an amplitude histogram distribution function g0 associated with a “

level 0”

from a portion of the amplitude histogram less than a second predetermined intensity threshold value,

means for obtaining mean intensity values and standard deviations of the “

level 1” and

the “

level 0”

from the functions g1 and g0,

means for calculating the averaged Q-factor parameter as a ratio of a difference between the mean intensity values of the “

level 1” and

the “

level 0”

to the sum of the standard deviations of the “

level 1” and

the “

level 0”

, and

means for adopting at least one of the standard deviation of the “

level 1”

, the standard deviation of the “

level 0” and

the difference between the mean intensity values of the “

level 1” and

the “

level 0”

as the waveform distortion parameter.

17. A system for determining the origin of optical signal quality degradation comprising:

optical signal amplitude histogram measurement means for obtaining an optical signal amplitude histogram from an optical signal under measurement;

averaged Q-factor parameter measurement means for obtaining an averaged Q-factor parameter, which is an optical signal quality parameter calculated from the optical signal amplitude histogram obtained by said optical signal amplitude histogram measurement means;

waveform distortion parameter measurement means for obtaining a waveform distortion parameter, which is another optical signal quality parameter calculated from the optical signal amplitude histogram obtained by said optical signal amplitude histogram measurement means;

initial state storing means for storing into a storage medium at least one of an initial value and an initial characteristic of the averaged Q-factor parameter and that of the waveform distortion parameter at a system installation without optical signal quality degradation, which are obtained using said averaged Q-factor parameter measurement means and said waveform distortion parameter measurement means; and

optical signal quality evaluation means for evaluating an optical signal quality degradation factor using a measurement value of the averaged Q-factor parameter and a measurement value of the waveform distortion parameter obtained by said averaged Q-factor parameter measurement means and said waveform distortion parameter measurement means, and using at least one of the initial values and initial characteristics stored in said storage medium in said initial state storing means,

wherein said optical signal quality evaluation means makes a decision that a main factor of the optical signal quality degradation is the waveform distortion in the optical signal under measurement when the averaged Q-factor parameter is reduced and the waveform distortion parameter is increased, and makes a decision that the main factor of the optical signal quality degradation is a factor other than waveform distortion when the waveform distortion parameter is not increased.

18. A system for determining the origin of optical signal quality degradation comprising:

optical splitting means for splitting an optical signal under measurement;

optical signal amplitude histogram measurement means for obtaining an optical signal amplitude histogram from a first optical signal under measurement which is obtained by splitting the optical signal under measurement in said optical splitting means;

averaged Q-factor parameter measurement means for obtaining an averaged Q-factor parameter, which is an optical signal quality parameter calculated from the optical signal amplitude histogram obtained by said optical signal amplitude histogram measurement means;

optical signal to optical noise power measurement means for measuring optical signal to optical noise power from a second optical signal under measurement which is obtained by splitting the optical signal under measurement in said optical splitting means;

optical signal-to-noise ratio parameter obtaining means for obtaining an optical signal-to-noise ratio parameter, which is another optical signal quality parameter from the optical signal to optical noise power obtained by said optical signal to optical noise power measurement means; and

optical signal quality evaluation means for evaluating an optical signal quality degradation factor using a measurement value of the averaged Q-factor parameter obtained by said averaged Q-factor parameter measurement means and a measurement value of the optical signal-to-noise ratio parameter obtained by said optical signal-to-noise ratio parameter obtaining means,

wherein said optical signal quality evaluation means makes a decision as to whether a main factor of the optical signal quality degradation is the degradation in the optical signal-to-noise ratio or not by determining and evaluating both the averaged Q-factor parameter and the optical signal-to-noise ratio parameter.

19. A system for determining the origin of optical signal quality degradation comprising:

optical splitting means for splitting a optical signal under measurement;

optical signal amplitude histogram measurement means for obtaining an optical signal amplitude histogram from a first optical signal under measurement which is obtained by splitting the optical signal under measurement in said optical splitting means;

averaged Q-factor parameter measurement means for obtaining an averaged Q-factor parameter, which is an optical signal quality parameter calculated from the optical signal amplitude histogram obtained by said optical signal amplitude histogram measurement means;

optical signal to optical noise power measurement means for measuring optical signal to optical noise power from a second optical signal under measurement which is obtained by splitting the optical signal under measurement in said optical splitting means;

optical signal-to-noise ratio parameter obtaining means for obtaining an optical signal-to-noise ratio parameter, which is an optical signal quality parameter from the optical signal to optical noise power obtained by said optical signal to optical noise power measurement means;

initial state storing means for storing into a storage medium at least one of an initial value and initial characteristic of the averaged Q-factor parameter and that of the optical signal-to-noise ratio parameter at a system installation without optical signal quality degradation, which are obtained using said averaged Q-factor parameter measurement means and said optical signal-to-noise ratio parameter obtaining means; and

optical signal quality evaluation means for evaluating an optical signal quality degradation factor using a measurement value of the averaged Q-factor parameter obtained by said averaged Q-factor parameter measurement means and a measurement value of the optical signal-to-noise ratio parameter obtained by said optical signal-to-noise ratio parameter obtaining means, and using at least one of the initial values and initial characteristics of the averaged Q-factor parameter and that of the optical signal-to-noise ratio parameter at the system installation without optical signal quality degradation, which are stored in the storage medium in said initial state storing means,

wherein said optical signal quality evaluation means makes a decision as to whether a main factor of the optical signal quality degradation is the degradation in the optical signal-to-noise ratio or not by determining and evaluating both the averaged Q-factor parameter and the optical signal-to-noise ratio parameter.

20. A system for determining the origin of optical signal quality degradation comprising:

optical splitting means for splitting an optical signal under measurement;

optical signal amplitude histogram measurement means for obtaining an optical signal amplitude histogram from a first optical signal under measurement which is obtained by splitting the optical signal under measurement in said optical splitting means;

optical signal to optical noise power measurement means for measuring optical signal to optical noise power from a second optical signal under measurement which is obtained by splitting the optical signal under measurement by said optical splitting means;

waveform distortion parameter measurement means for obtaining a waveform distortion parameter1 which is an optical signal quality parameter calculated from the optical signal amplitude histogram obtained by said optical signal amplitude histogram measurement means;

optical signal-to-noise ratio parameter obtaining means for obtaining an optical signal-to-noise ratio parameter, which is another optical signal quality parameter from the optical signal to optical noise power obtained by said optical signal to optical noise power measurement means; and

optical signal quality evaluation means for evaluating an optical signal quality degradation factor using a measurement value of the waveform distortion parameter obtained by said waveform distortion parameter measurement means and a measurement value of the optical signal-to-noise ratio parameter obtained by said optical signal-to-noise ratio parameter obtaining means,

wherein said optical signal quality evaluation means makes a decision as to whether a main factor of the optical signal quality degradation is the degradation in the optical signal-to-noise ratio or the waveform distortion by determining and evaluating both the waveform distortion parameter and the optical signal-to-noise ratio parameter.

21. A system for determining the origin of optical signal quality degradation comprising:

optical splitting means for splitting an optical signal under measurement;

optical signal amplitude histogram measurement means for obtaining an optical signal amplitude histogram from a first optical signal under measurement obtained by splitting the optical signal under measurement in said optical splitting means;

optical signal to optical noise power measurement means for measuring optical signal to optical noise power from a second optical signal under measurement which is obtained by splitting the optical signal under measurement by said optical splitting means;

waveform distortion parameter measurement means for obtaining a waveform distortion parameter, which is an optical signal quality parameter calculated from the optical signal amplitude histogram obtained by said optical signal amplitude histogram measurement means;

optical signal-to-noise ratio parameter obtaining means for obtaining an optical signal-to-noise ratio parameter, which is another optical signal quality parameter from the optical signal to optical noise power obtained by said optical signal to optical noise power measurement means;

initial state storing means for storing into a storage medium at least one of an initial value and initial characteristic of the waveform distortion parameter and that of the optical signal-to-noise ratio parameter at a system installation without optical signal quality degradation, which are obtained using said waveform distortion parameter measurement means and said optical signal-to-noise ratio parameter obtaining means; and

optical signal quality evaluation means for evaluating an optical signal quality degradation factor using a measurement value of the waveform distortion parameter obtained by said waveform distortion parameter measurement means and a measurement value of the optical signal-to-noise ratio parameter obtained by said optical signal-to-noise ratio parameter obtaining means, and using at least one of the initial values and initial characteristics of the waveform distortion parameter and that of the optical signal-to-noise ratio parameter at the system installation without optical signal quality degradation,

wherein said optical signal quality evaluation means makes a decision as to whether a main factor of the optical signal quality degradation is the degradation in the optical signal-to-noise ratio or the waveform distortion by determining and evaluating both the waveform distortion parameter and the optical signal-to-noise ratio parameter.

22. A system for determining the origin of optical signal quality degradation comprising:

optical splitting means for splitting an optical signal under measurement;

optical signal amplitude histogram measurement means for obtaining an optical signal amplitude histogram from a first optical signal under measurement which is obtained by splitting the optical signal under measurement in said optical splitting means;

averaged Q-factor parameter measurement means for obtaining an averaged Q-factor parameters which is an optical signal quality parameter calculated from the optical signal amplitude histogram obtained by said optical signal amplitude histogram measurement means;

optical signal to optical noise power measurement means for measuring optical signal to optical noise power from a second optical signal under measurement which is obtained by splitting the optical signal under measurement in said optical splitting means;

waveform distortion parameter measurement means for obtaining a waveform distortion parameter, which is another optical signal quality parameter calculated from the optical signal amplitude histogram obtained by said optical signal amplitude histogram measurement means;

optical signal-to-noise ratio parameter obtaining means for obtaining an optical signal-to-noise ratio parameter, which is another optical signal quality parameter from the optical signal to optical noise power obtained by said optical signal to optical noise power measurement means; and

optical signal quality evaluation means for evaluating an optical signal quality degradation factor using a measurement value of the averaged Q-factor parameter obtained by said averaged Q-factor parameter measurement means, a measurement value of the waveform distortion parameter obtained by said waveform distortion parameter measurement means, and a measurement value of the optical signal-to-noise ratio parameter obtained by said optical signal-to-noise ratio parameter obtaining means,

wherein said optical signal quality evaluation means makes a decision as to whether a main factor of the optical signal quality degradation is the degradation in the optical signal-to-noise ratio, the waveform distortion or not by determining and evaluating all the averaged Q-factor parameter, the waveform distortion parameter and the optical signal-to-noise ratio parameter.

23. A system for determining the origin of optical signal quality degradation comprising:

optical splitting means for splitting an optical signal under measurement;

optical signal amplitude histogram measurement means for obtaining an optical signal amplitude histogram from a first optical signal under measurement, which is obtained by splitting the optical signal under measurement in said optical splitting means;

averaged Q-factor parameter measurement means for obtaining an averaged Q-factor parameter1, which is an optical signal quality parameter calculated from the optical signal amplitude histogram obtained by said optical signal amplitude histogram measurement means;

optical signal to optical noise power measurement means for measuring optical signal to optical noise power from a second optical signal under measurement obtained by splitting the optical signal under measurement in said optical splitting means;

waveform distortion parameter measurement means for obtaining a waveform distortion parameter, which is another optical signal quality parameter calculated, from the optical signal amplitude histogram obtained by said optical signal amplitude histogram measurement means;

optical signal-to-noise ratio parameter obtaininq means for obtaining an optical signal-to-noise ratio parameter, which is another optical signal quality parameter from the optical signal to optical noise power obtained by said optical signal to optical noise power measurement means;

initial state storing means for storing into a storage medium at least one of an initial value and initial characteristic of the averaged Q-factor parameter, that of the waveform distortion parameter, and that of the optical signal-to-noise ratio parameter at a system installation without optical signal quality degradation, which are obtained using said averaged Q-factor parameter evaluation measurement means, said waveform distortion parameter evaluation measurement means and said optical signal-to-noise ratio parameter obtaining means; and

optical signal quality evaluation means for evaluating an optical signal quality degradation factor using a measurement value of the averaged Q-factor parameter obtained by said averaged Q-factor parameter measurement means, a measurement value of the waveform distortion parameter obtained by said waveform distortion parameter measurement means, and a measurement value of the optical signal-to-noise ratio parameter obtained by said optical signal-to-noise ratio parameter obtaining means, and at least one of the initial values and initial characteristics of the averaged Q-factor parameter, the waveform distortion parameter, and the optical signal-to-noise ratio parameter at the system installation without optical signal quality degradation, which are stored into the storage medium by said initial state storing means,

wherein said optical signal quality evaluation means makes a decision as to whether a main factor of the optical signal quality degradation is the degradation in the optical signal-to-noise ratio, the waveform distortion or not by determining and evaluating all the averaged Q-factor parameter, the waveform distortion parameter and the optical signal-to-noise ratio parameter.