Phase based sensing
A method of interrogating a phase based transducer by providing a pulsed input including two different wavelengths in which the different wavelength components can be used to derive a phase change experienced by a synthetic wavelength, and by arranging for the synthetic wavelength to be significantly greater that the component wavelengths, the phase so detected has a reduced sensitivity, and is less susceptible to overscaling effects.
1. A method of interrogating a phase based transducer, said transducer adapted to provide a phase output in response to a sensed parameter, said method comprising:
inputting first and second input pulses to said transducer, said pulses having a time delay therebetween, and wherein at least one of said input pulses contains components of at least two different wavelengths;
receiving an output from said transducer corresponding to a combination of said first and second input pulses; and
performing interferometric detection on the output to produce a signal representative of the phase at a synthetic wavelength which is a combination of said two different wavelengths, said signal providing a measure of the sensed parameter.
2. A method according to claim 1 , wherein both input pulses contain components of at least two different wavelengths.
3. A method according to claim 1 , wherein one input pulse contains first and second different frequency components.
4. A method according to claim 3 , wherein the other input pulse has a third single frequency.
5. A method according to claim 1 , wherein the components of different wavelengths are combined using wavelength division multiplexing.
6. A method according to claim 1 , wherein one pulse is formed of a first component having wavelength λ 1 and shifted by frequency f 1 , and a second component having wavelength λ 2 and shifted by frequency f 2 , and wherein the other pulse is formed of a first component having wavelength λ 1 and shifted by frequency f 3 and a second component having wavelength λ 2 and shifted by frequency f 3 .
7. A method according to claim 6 , wherein both f 1 -f 3 and f 2 -f 3 >>f 1 -f 2 .
8. A method according to claim 6 , wherein the output is processed to produce a signal representative of the synthetic phase having a carrier frequency at substantially f 1 -f 2 .
9. A method according to claim 1 wherein the output is processed to produce a signal at a single input wavelength, representative of the phase at a single wavelength.
10. A method according to claim 1 , wherein said transducer comprises at least one intrinsic fibre optic sensor.
11. A method according to claim 1 , wherein said transducer is adapted to produce a single output pulse having components derived from both said input pulses.
12. A system for interrogating a phase based transducer, said system comprising
a first signal source for generating a signal at a first wavelength,
a second signal source for generating a signal at a second wavelength,
a wavelength division multiplexer for combining signals at said first and second wavelengths, and
an acousto-optic modulator for pulsing signals,
wherein the system is arranged to output to said transducer at least two pulses, at least one of which contains both said first and second wavelengths and wherein said system further comprises a detector for receiving an output returned from said transducer in response to said at least two pulses and for using said output to produce a signal representative of the phase at a synthetic wavelength which is a combination of said first and second wavelengths.
13. A system according to claim 12 , wherein the phase based sensor or transducer is one of a hydrophone, a geophone or an accelerometer.
14. A method of two-wavelength interrogation of a fibre optic phase based sensor, the method comprising:
inputting at least two pulses to said sensor, said two pulses including at least two different wavelengths and three different frequencies, and
using the resulting output to determine a measure of phase at a third synthetic wavelength which is a combination of said first and second wavelengths.
15. A method according to claim 14 , wherein one pulse contains first and second different frequencies, and the other pulse contains a third frequency different from said first and second frequencies.
16. A method for interrogating a multiplexed fibre optic seismic array according to claim 14 .