Phase based sensing
A method of distributed acoustic sensing (DAS) whereby the derivative or rate of change of a signal backscatted from a fiber is measured. The change, or derivative of the phase measured in this way has a much smaller amplitude than the signal itself if the difference between the two times at which the signal is measured is much less than the period of the signal being measured, resulting in lower sensitivity. Frequency shifts can be applied to temporally displaced return signals to compare the rate of change, for example by employing an output interferometer arranged to modulate the signal in each arm by a different frequency shift.
1. A method of distributed acoustic sensing (DAS) by interrogating a length of optic fibre, said optic fibre providing a change in phase of signal propagation in response to a sensed parameter, said method comprising:
introducing an input signal into a length of optic fibre;
receiving a return signal backscattered from said optical fibre in response to said input signal;
comparing a first return signal backscattered from a portion of said fibre at a first time, and a second return signal backscattered from the same portion of said fibre at a second, different time; and
deriving from said comparison a measure of the rate of change of phase with time of said backscattered signal.
2. A method according to claim 1 , wherein said first return signal is frequency shifted relative to said second return signal.
3. A method according to claim 2 , wherein the first return signal is modulated by a first frequency shift, and the said second return signal is modulated by a second frequency shift.
4. A method according to claim 1 , wherein said method comprises passing received return signals through an output interferometer, said output interferometer arranged to modulate the signal in each arm by a different frequency shift.
5. A method according to claim 1 , wherein said input signal comprises a pair of temporally spaced pulses.
6. A method according to claim 5 , wherein said pulses have different frequency shifts.
7. A method according to claim 1 , further comprising obtaining a value of phase based on said measured rate of change of phase.
8. A method according to claim 1 , further comprising comparing a first return signal backscattered from a first portion of said fibre at a first time, and a second return signal backscattered from a second different portion of said fibre at substantially the same time; and deriving from said comparison a measure of phase of said backscattered signal.
9. A method according to claim 8 , wherein said measure of phase and said measure of rate of change of phase are determined substantially simultaneously in response to a common input signal.
10. A method according to claim 1 , wherein said input signal comprises three temporally spaced pulses.
11. A method according to claim 10 , wherein said input pulses include at least two different wavelengths.
12. A method according to claim 10 , wherein the temporal spacing between said first and third pulses is greater than twice the temporal spacing between the first and second pulses.
13. A distributed acoustic sensing (DAS) system for interrogating a length of optic fibre, said optic fibre providing a change in phase of signal propagation in response to a sensed parameter, said system comprising:
a receiver for receiving a signal backscattered from said optic fibre in response to an input signal,
an output interferometer adapted to combine a first received signal backscattered from a portion of said fibre at a first time, and a second received signal backscattered from the same portion of said fibre at a second, different time, wherein said output interferometer includes a frequency modulator on at least one arm to impose a frequency difference between said first and second return signals; and
a phase detector for receiving said combined signals and determining the rate of change of phase with time of said backscattered signal.
14. A system according to claim 13 , wherein said output interferometer includes a frequency modulator on each arm of said output interferometer, each frequency modulator imposing a different frequency shift.
15. A system according to claim 13 , wherein said system includes a second phase detector for determining the phase of said backscattered signal.
16. A system according to claim 13 , wherein said system includes a demultiplexer for separating signals backscattered from inputs having different wavelengths.
17. A system according to claim 16 , wherein backscattered signals at a first wavelength are passed to said output interferometer, and signals at a second wavelength are passed directly to said second phase detector.