Optical signal monitoring method and apparatus

• US 6,538,779 B1
• Filed: 06/14/1999
• Issued: 03/25/2003
• Est. Priority Date: 06/15/1998
• Status: Expired due to Term

• Current Assignee: NEC Corporation
• Original Assignee: NEC Corporation
• Inventors: Shiragaki, Tatsuya, Takeshita, Hitoshi, Shimomura, Hirofumi, Henmi, Naoya
• Primary Examiner(s): Pascal, Leslie
• Assistant Examiner(s): SINGH, DALZID E
• Application Number:
  US09/332,930
• Time in Patent Office: 1,380 Days
• Field of Search: 359/110, 359/158, 359/161, 359/194, 375/222, 375/226, 375/326, 375/327, 375/317
• US Class Current: 398/27
• CPC Class Codes: H04B 10/07953 Monitoring or measuring OSNR, BER or Q
• CPC Class Codes: H04L 1/20 using signal quality detector
• CPC Class Codes: H04L 25/063 Setting decision thresholds using feedback techniques only
• CPC Class Codes: H04Q 11/0005 Switch and router aspects
• CPC Class Codes: H04Q 2011/0039 Electrical control
• CPC Class Codes: H04Q 2011/0045 Synchronisation
• CPC Class Codes: H04Q 2011/0083 Testing; Monitoring

Abstract

A signal quality monitoring method with reliability is disclosed. After extracting a clock signal from the light signal, a decision region is set for each bit of the light signal. The decision region is defined by a plurality of threshold amplitudes corresponding respectively to threshold phases which are produced with respect to the clock signal. The quality of the light signal is monitored depending on whether an amplitude of the light signal falls into the decision region at timings of the N threshold phases.

Claims

1. A method for monitoring a quality of a light signal having a data transmission rate in an optical network, comprising the steps:

a) extracting a clock signal from the light signal;

b) setting a decision region for each bit of the light signal, wherein the decision region is defined by a plurality of threshold amplitudes corresponding respectively to threshold phases which are produced with respect to the clock signal; and

c) monitoring the quality of the light signal depending on whether an amplitude of the light signal falls into the decision region.

2. The method according to claim 1, wherein the step b) comprises the steps of:

producing a plurality of threshold phases with respect to the clock signal, wherein the plurality of threshold phases provide different timings for each bit of the light signal; and

producing a plurality of threshold amplitudes which correspond to the plurality of threshold phases, respectively.

3. The method according to claim 2, wherein the step c) comprises the steps of:

comparing an amplitude of the light signal with the plurality of threshold amplitudes at timings corresponding to the plurality of threshold phases to produce comparison results, respectively;

counting error bits for a period of time based on the comparison results; and

monitoring the quality of the light signal by comparing a count of the error bits with a defined value.

4. The method according to claim 3, wherein in the counting step, an error bit is detected by a logical calculation of the comparison results.

5. The method according to claim 4, wherein the number of threshold phases and amplitudes is equal to 2 and the logical calculation is Exclusive-OR.

6. The method according to claim 3, wherein when the count of the error bits is greater the defined value, a signal quality degradation alarm is generated.

7. A fault recovery method in an optical network comprised of a plurality of codes each providing a plurality of paths, wherein a light signal having a data transmission rate is transmitted through a path of a node, comprising the steps of:

at each of the nodes,

a) extracting a clock signal from the light signal;

b) setting a decision region for each bit of the light signal, wherein the decision region is defined by a plurality of threshold amplitudes corresponding respectively to threshold phases which are produced with respect to the clock signal;

c) monitoring the quality of the light signal depending on whether an amplitude of the light signal falls into the decision region at timings of the threshold phases; and

d) switching the path from a working path to a protection path when the quality of the light signal drops below a defined quality level.

8. An apparatus for monitoring a quality of a light signal having a data transmission rate in an optical network, comprising:

a light receiver for receiving the light signal and to produce an input signal which is an electric signal corresponding to the light signal;

N bit decision circuits for making respective bit decisions with respect to decision thresholds for each bit of the input signal to produce N bit decision results, respectively; and

a monitor for monitoring the quality of the light signal based on the N bit decision results,

each of the N bit decision circuits comprising;

a clock recovery circuit for extracting a clock signal from the input signal;

a phase shifter for phase-shifting the clock signal by a defined amount to produce a phase-shifted clock signal; and

a bit discriminator for making a binary discrimination of an amplitude of the input signal with respect to a threshold voltage at timings based on the phase-shifted clock signal to produce a bit decision result, wherein the decision thresholds are defined by the phase-shifted clock signal and the threshold voltage.

9. The apparatus according to claim 8, wherein the bit discriminator is a D-type flip-flip circuit having differential input terminals and at least one output terminal, wherein the input signal is applied to a first input terminal and the threshold voltage is applied to the second input terminal.

10. The apparatus according to claim 8, wherein the monitor counts error bits for a time period based on the N decision results and determines the quality of the light signal by comparing a count of the error bits with another threshold value.

11. The apparatus according to claim 10, wherein N-2 and the monitor monitors the quality of the light signal by performing an Exclusive-OR of the bit decision results.

12. The apparatus according to claim 10, wherein the monitor comprises an interface to a host controller for generating a signal quality degradation alarm when the count of the error bits is greater than another threshold value.

13. A photonic node in an optical network composed of a plurality of photonic nodes, the photonic node providing a plurality of paths, wherein a light signal having a data transmission rate is transmitted through a path of the photonic node, comprising:

a light receiver for receiving the light signal and to produce an input signal which is an electric signal corresponding to the light signal;

N bit decision circuits for making respective bit decisions with respect to decision thresholds for each bit of the input signal to produce N bit decision results, respectively;

a monitor for monitoring the quality of the light signal based on the N bit decision results; and

a switch controller for switching the path from a working path to a protection path when the quality of the light signal drops below a quality level,

each of the N bit decision circuits comprising;

a clock recovery circuit for extracting a clock signal from the input signal;

a phase shifter for phase-shifting the clock signal by a defined amount to produce a phase-shifted clock signal; and

a bit discriminator for making a binary discrimination of an amplitude of the input signal with respect to a threshold voltage at a timings based on the phase-shifted clock signal to produce a bit decision result, wherein the decision thresholds are defined by the phase-shifted clock signal and the threshold voltage.

14. An apparatus for monitoring a quality of a light signal having a data transmission rate in an optical network, comprising:

a light receiver for receiving the light signal and to produce an input signal which is an electric signal corresponding to the light signal;

first and second bit decision circuits for making respective bit decisions with respect to decision thresholds for each bit of the input signal to produce first and second bit decision results, each of the bit decision circuits comprising;

a clock recovery circuit for extracting a clock signal from the input signal;

a phase shifter for phase-shifting the clock signal by a defined amount to produce a phase-shifted clock signal; and

a bit discriminator for making a binary discrimination of an amplitude of the input signal with respect to a threshold voltage at timings based on the phase-shifted clock signal to produce a bit decision result, wherein the decision thresholds are defined by the phase-shifted clock signal and the threshold voltage;

a logic circuit for calculating an exclusive OR of the bit decision results to produce an error bit;

an error counter for counting error bits for a defined time period;

a voltage controller for controlling the threshold voltage of each of the bit decision circuits so as to minimize a count of the error bits; and

a circuit for calculating an extinction ratio obtained by dividing the threshold voltage of the first bit decision circuit by the threshold voltage of the second bit decision circuit.

15. A method for monitoring a quality of a light signal having data, the method comprising:

receiving the light signal;

determining a clock rate of the light signal;

for a particular bit of the light signal, defining a phase range during which the particular bit of the light signal should not have any data;

for the particular bit of the light signal, defining a voltage range, the voltage range defining a range of voltages that the value of the particular bit of the light signal should not equal during the phase range; and

determining the quality of the light signal based on whether the value of the particular bit is within the voltage range.