Line monitoring system having heterodyne coherent detection

1. A line monitoring system, comprising:

a laser source to launch a probe signal over a first bandwidth;

a polarization maintaining tap to receive the probe signal, and to split the probe signal into a first portion and a second portion;

a polarization rotator to receive the first portion of the probe signal and send the first portion to a transmission system;

a return tap to receive the second portion of the probe signal and to receive a return signal from the transmission system, wherein the return signal being derived from the first portion of the probe signal,

a photodetector coupled to receive an interference signal from the return tap, wherein the interference signal is generated by a mixing the return signal and the second portion of the probe signal, the photodetector arranged to output a power signal based upon the interference signal; and

a power measurement system to measure the power signal at a given measurement frequency, wherein the given measurement frequency is derived from a frequency of the polarization rotator.

2. The line monitoring system of claim 1 , wherein the polarization rotator further comprises an on-off-keying (OOK) component to modulate the first portion with OOK data.

3. The line monitoring system of claim 1 , further comprising a second polarization rotator to receive the second portion of the probe signal, to apply a rotation and to send the second portion after rotation to the return tap.

4. The line monitoring system of claim 1 , further comprising a frequency shifter assembly, arranged to receive the second portion and apply a frequency shift to the second portion, and transmit the second portion to the return tap.

5. The line monitoring system of claim 4 , wherein the frequency shifter assembly comprises a first frequency shifter and a second frequency shifter, the line monitoring system further comprising:

an intermediate tap, arranged to receive the second portion from the polarization maintaining tap, to output a first part of the second portion to the first frequency shifter, and to output a second part of the second portion to the second frequency shifter; and

a polarization beam combiner to receive a first frequency shifted beam from the first frequency shifter and to receive a second frequency shifted beam from the second frequency shifter, and output the second portion as a combined beam to the return tap.

6. The line monitoring system of claim 5 , wherein the intermediate tap is a second polarization maintaining tap.

7. The line monitoring system of claim 1 , further comprising at least one filter amplifier, disposed to intercept at least a portion of the probe signal, between the laser source and the return tap.

8. The line monitoring system of claim 1 , wherein the probe signal comprises a bandwidth of 25 MHz.

9. The line monitoring system of claim 1 , wherein the power signal comprises a frequency of 25 MHz to 5 GHz, and wherein the power measurement system is configured to measure over a sampling period of 50 μs to 500 μs.

10. The line monitoring system of claim 1 , wherein a rotation frequency of the polarization rotator is 1 GHz.

11. The line monitoring system of claim 1 , wherein the given measurement frequency corresponds to a frequency of the polarization rotator.

12. The line monitoring system of claim 3 , wherein the given measurement frequency corresponds to:

a difference between a rotation frequency of the polarization rotator and a rotation frequency of the second polarization rotator; or

a sum of the rotation frequency of the polarization rotator and the rotation frequency of the second polarization rotator.

13. The line monitoring system of claim 5 , wherein the given measurement frequency corresponds to two different frequencies.

14. A method of measuring a fault in a transmission system, comprising:

launching a probe beam from a laser probe source, the probe beam having a first bandwidth;

directing a first portion of the probe beam through a polarization rotator and to a transmission system;

directing a second portion of the probe beam to a return tap;

receiving at the return tap, a return signal from the transmission system, the return signal being based upon the first portion of the probe beam;

mixing the second portion of the probe beam and the return signal to generate an interference signal;

generating a power signal based upon the interference signal; and

measuring the power signal at a given measurement frequency, wherein the given measurement frequency is derived from a frequency of the polarization rotator.

15. The method of claim 14 , wherein the directing the first portion comprises:

directing the first portion through the polarization rotator; and

modulating the first portion using an on-off-keying (OOK) component, for sending to the transmission system.

16. The method of claim 14 , wherein the probe beam comprises a bandwidth of 25 MHz.

17. The method of claim 14 , wherein the measuring the power signal comprises measuring at a frequency of 25 MHz to 5 GHz and a sampling period of 50 μs to 500 μs.

18. The method of claim 14 , the directing the second portion comprising directing the second portion through a second polarization rotator before the second portion is received at the return tap.

19. A method of measuring a fault in a transmission system, comprising:

launching a probe beam from a laser probe source, the probe beam having a narrow bandwidth;

directing a first portion of the probe beam through a polarization rotator and to a transmission system;

directing a second portion of the probe beam to a frequency shifter assembly to form a frequency shifted beam;

receiving at a return tap, the frequency shifted beam and a return signal from the transmission system, the return signal being based upon the first portion of the probe beam;

mixing the frequency shifted beam and the return signal to generate an interference signal;

generating a power signal based upon the interference signal; and

measuring the power signal at a given measurement frequency over a second bandwidth, equal to the narrow bandwidth or equal to a larger one of: the first bandwidth and a sampling rate of the measuring the power signal.

20. The method of claim 19 , wherein the directing the second portion of the probe beam comprises:

shifting a first part of the second portion of the probe beam by a first frequency shifter to form a first frequency shifted beam;

shifting a second part of the second portion of the probe beam by a second frequency shifter to form a second frequency shifted beam; and

combining the first frequency shifted beam and the second frequency shifted beam, in order to form the frequency shifted beam.

21. The method of claim 19 , wherein the measuring the power signal comprises measuring at a first frequency of the first frequency shifted beam and at a second frequency of the second frequency shifted beam.