Method and apparatus for optical sensing
The present invention provides novel apparatus and methods for fast quantitative measurement of perturbation of optical fields transmitted, reflected and/or scattered along a length of an optical fibre. The present invention can be used for point sensors as well as distributed sensors or the combination of both. In particular this technique can be applied to distributed sensors while extending dramatically the speed and sensitivity to allow the detection of acoustic perturbations anywhere along a length of an optical fibre while achieving fine spatial resolution. The present invention offers unique advantages in a broad range of acoustic sensing and imaging applications. Typical uses are for monitoring oil and gas wells such as for distributed flow metering and/or imaging, seismic imaging, monitoring long cables and pipelines, imaging within large vessel as well as for security applications.
1. An optical fiber distributed acoustic sensing system, comprising:
a light source arranged in use to generate optical signals;
means for varying an optical frequency of the optical signals;
an optical sensing fiber arranged in use to be deployed in an environment in which acoustic perturbations are to be monitored, the optical sensing fiber being arranged in use to receive light of a frequency corresponding to that of the frequency-varied optical signals;
an interferometer arranged in use to receive light backscattered and/or reflected from along the optical sensing fiber, the light being backscattered and/or reflected in dependence on the acoustic perturbations incident long the length of the optical sensing fiber causing the optical sensing fiber to expand and contract, the backscattered and/or reflected light interfering in the interferometer to produce interference components; and
means for processing the interference components to quantitatively measure properties of the received backscattered and/or reflected light.
2. A system according to claim 1 , wherein the means for processing is arranged to determine a relative phase, frequency and amplitude of the received light.
3. A system according to claim 1 , wherein the light source includes a laser, and the optical signals are optical pulses.
4. A system according to claim 1 , wherein the means for varying the optical frequency comprises:
an optical modulator arranged in use to modulate the optical signals with a microwave frequency to thereby generate modulation sidebands in the optical signals; and
an optical filter arranged in use to controllably select at least one of the modulation sidebands, and thereby vary the frequency of the light input to the optical sensing fiber to that of the at least one selected modulation sideband.
5. A system according to claim 4 , wherein the optical modulator is arranged in use to be driven by a microwave frequency of 10 to 40 GHz.
6. A system according to claim 1 , wherein the interferometer comprises at least two optical paths with a path length difference therebetween.
7. A system according to claim 1 , wherein the means for processing comprises plural photodetectors to measure the interference components, and a processor arranged to determine the relative optical phase modulation along the sensing fiber therefrom.