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 .- 10 . (canceled)
11 . A method of using an optical fiber distributed acoustic sensor system for surveillance, the sensor system having an optical sensing fiber deployed along a structure or in an area to be monitored, the method comprising:
isolating a reference section of the optical sensing fiber in a quiet location;
transmitting light into the optical sensing fiber;
receiving light backscattered from along the optical sensing fiber; and
processing the received light to measure the phase, frequency and amplitude data of the received light to provide quantitative measurements of acoustic perturbations along the length of the fiber, the processing including using light from the reference section as a reference.
12 . A method of using an optical fiber distributed acoustic sensor system for surveillance, the sensor system having an optical sensing fiber deployed along a structure or in an area to be monitored, the method comprising:
isolating a reference section of the optical sensing fiber in a quiet location;
transmitting light into the optical sensing fiber;
receiving light reflected from along the optical sensing fiber; and
processing the received light to measure the phase, frequency and amplitude data of the received light to provide quantitative measurements of acoustic perturbations along the length of the fiber, the processing including using light from the reference section as a reference.
13 . A surveillance apparatus, the apparatus comprising a distributed acoustic sensor system, the system including:
an optical sensing fibre deployed in use along a structure or in an area to be monitored, a section of the optical sensing fibre being isolated in a quiet location such that in use it is used as a reference section, the reference section being formed between respective reflectors or a combination of beam splitters and reflectors;
a means for receiving light backscattered and/or reflected from along the optical sensing fiber; and
a means for processing the received light to measure the phase, frequency and amplitude data of the received light to provide quantitative measurements of acoustic perturbations along the length of the fiber.
14 . A surveillance apparatus according to claim 13 , wherein the optical sensing fiber is deployed in use so as to monitor one or more of the group comprising: boreholes, pipelines, the perimeter of an area, ports and/or geographical borders.
15 . A surveillance apparatus according to claim 13 , wherein the means for receiving is an interferometer arranged in use to receive backscattered and/or reflected light from along the isolated reference fibre and the optical sensing fibre, the interferometer comprising at least two optical paths with a path length difference therebetween, in use the backscattered and/or reflected light interfering in the interferometer to produce interference components, and wherein the means for processing comprises plural photodetectors arranged in use to measure the interference components, and a processor arranged in use to determine optical phase angle data therefrom to measure the phase, frequency and amplitude data of the received light.
16 . A surveillance apparatus according to claim 15 , wherein the interferometer further comprises an optical coupler arranged in use to introduce a relative phase shift between the interference components.
17 . A surveillance apparatus according to claim 15 , wherein the interferometer further comprises an optical amplifier arranged in use to amplify the received backscattered and/or reflected light to produce an amplified light signal.
18 . A surveillance apparatus according to claim 17 , wherein the interferometer further comprises an optical filter arranged in use to filter out the out of band Amplified Spontaneous Emission (ASE) noise generated by the optical amplifier from the amplified light signal.
19 . A surveillance apparatus according to claim 15 , wherein the processor is further arranged in use to process the optical phase angle data to identify acoustic perturbations experienced by the optical fiber.