Method and system for high sensitivity in distributed fiber sensing applications

1. A distributed fiber sensing system, comprising:

a laser source, for generating at least two ultra-narrow linewidth interrogating pulses;

a circulator, coupled with said laser source;

a detector, coupled with said circulator, said detector comprising a predetermined detection bandwidth, for detecting back-scattered signals; and

an optical fiber, coupled with said circulator,

said laser source comprising:

an ultra-narrow linewidth laser; and

a modulator, coupled with said ultra-narrow linewidth laser and said circulator,

wherein said modulator generates a plurality of pairs of interrogating pulses having a time delay;

wherein a frequency difference between two pulses of the same interrogating pair is less than said predetermined detection bandwidth;

wherein a frequency difference between two pulses not of the same interrogating pair is larger than said predetermined detection bandwidth; and

wherein said optical fiber can be characterized based on beat-notes between back-scattered signals originating from said pair of interrogating pulses detected by said detector.

2. The distributed fiber sensing system according to claim 1 , wherein said plurality of pairs of interrogating pulses have different carrier frequencies.

3. The distributed fiber sensing system according to claim 2 , wherein said different carrier frequencies are generated by a technique selected from the list consisting of:

electro-optic modulation; and

acousto-optic modulation.

4. The distributed fiber sensing system according to claim 1 , wherein said modulator generates said plurality of pairs of interrogating pulses by a technique selected from the list consisting of:

modulating a phase of a first one of said at least two ultra-narrow linewidth interrogating pulses at a first frequency and a phase of a second one of said at least two ultra-narrow linewidth interrogating pulses at a second frequency; and

generating said plurality of pairs of interrogating pulses at different times by creating sidebands at a required frequency at each moment.

5. The distributed fiber sensing system according to claim 1 , wherein said time delay is selected from the list consisting of:

being different for each one of said plurality of pairs of interrogating pulses;

being the same for each one of said plurality of pairs of interrogating pulses; and

being different for some of said plurality of pairs of interrogating pulses and being the same for others of said plurality of pairs of interrogating pulses.

6. The distributed fiber sensing system of claim 1 , wherein each one of said plurality of pairs of interrogating pulses is transmitted having a parameter selected from the list consisting of:

being transmitted at the same time;

being transmitted at different times;

being transmitted having the same frequency difference;

being transmitted having a different frequency difference;

being transmitted having the same pulse duration; and

being transmitted having a different pulse duration.

7. The distributed fiber sensing system of claim 1 , wherein said detector is selected from the list consisting of:

a photo-detector;

an avalanche photodiode; and

a photodiode.

8. The distributed fiber sensing system of claim 1 , further comprising at least one of:

at least one amplifier, to ensure high signal-to-noise ratio; or

at least one variable optical attenuator, to use a full dynamic range of said detector at each instant.

9. The distributed fiber sensing system of claim 8 , wherein said at least one amplifier is selected from the list consisting of:

an Erbium doped fiber amplifier; and

a distributed Raman amplifier.

10. A distributed fiber sensing system, comprising:

a laser source, for generating at least two ultra-narrow linewidth interrogating pulses and at least one local oscillation signal;

a circulator, coupled with said laser source;

a detector, coupled with said circulator and said laser source, said detector for detecting back-scattered signals; and

an optical fiber, coupled with said circulator,

said laser source comprising:

an ultra-narrow linewidth laser, coupled with said detector; and

a modulator, coupled with said ultra-narrow linewidth laser and said circulator,

wherein said modulator generates consecutive interrogating pulses having a time delay;

wherein said ultra-narrow linewidth laser provides said at least one local oscillation signal to said detector to beat back-scattered signals originating from said consecutive interrogating pulses; and

wherein said optical fiber can be characterized based on a phase difference between beat-notes between said back-scattered signals originating from said consecutive interrogating pulses detected by said detector.

11. The distributed fiber sensing system according to claim 10 , wherein a DC term in said detector can be eliminated using a technique selected from the list consisting of:

balanced detection; and

using a low-pass filter.

12. The distributed fiber sensing system according to claim 10 , wherein said detector detects said back-scattered signals by a technique selected from the list consisting of:

baseband in-phase and quadrature-phase detection;

heterodyne detection; and

homodyne detection.

13. The distributed fiber sensing system according to claim 10 , wherein said time delay of each one of said consecutive interrogating pulses is not the same for all of said consecutive interrogating pulses.

14. The distributed fiber sensing system according to claim 10 , wherein said consecutive interrogating pulses are transmitted at different times and wherein said time delay of said consecutive interrogating pulses determines a resolution of said distributed fiber sensing system.

15. The distributed fiber sensing system according to claim 10 , wherein said back-scattered signals are detected coherently by said detector using said at least one local oscillation signal and analyzed pair-wise.

16. The distributed fiber sensing system of claim 10 , further comprising at least one of:

at least one amplifier, to ensure high signal-to-noise ratio; or

at least one variable optical attenuator, to use a full dynamic range of said detector at each instant.

17. The distributed fiber sensing system of claim 16 , wherein said at least one amplifier is selected from the list consisting of:

an Erbium doped fiber amplifier; and

a distributed Raman amplifier.