Systems and methods for distributed acoustic sensing based on coherent Rayleigh scattering are disclosed herein. A system comprises a pulse generator, an interferometer, a photo detector assembly, and an information handling system. The interferometer comprises a first and second optical switch each comprising a plurality of ports. The information handling system activates one port on each of the first and second optical switches so as to vary the optical path length of the interferometer. A method comprises splitting backscattered light from an optical pulse into a first portion and a second portion, activating one port of a first optical switch and one port of a second optical switch, sending the first portion into a first arm of an interferometer, sending the second portion into a second arm of the interferometer, combining the first and second portions to form an interferometric signal, and receiving the interferometric signal at a photodetector assembly.

A system for sensing phase changes in a medium includes a bidirectional interferometer having a reference arm and a sensing arm, two inputs at each of the two ends of the interferometer, and a circulator disposed between each input and the interferometer. Sources provide squeezed state pulses at an input at each end of the interferometer. Spaced-apart partial reflectors are disposed along the arms. There are detectors associated with each input. The circulators pass the squeezed state pulses into the interferometer and route reflections of the pulses to the detectors. Classical pulses may provided at the other input at each end of the interferometer.

The present invention discloses a distributed device for simultaneously measuring strain and temperature based on optical frequency domain reflection, comprising a tunable laser, a 1:99 beam splitter, a main interferometer system, a light source phase monitoring system based on an auxiliary interferometer, an acquisition device and a computer processing unit, wherein the main interferometer system comprises two Mach-Zehnder interferometers, and two optical fibers having different cladding diameters are arranged in parallel as sensing fibers. Due to the difference in temperature and strain coefficients of optical fibers of the same diameter, the temperature and strain values during changing the temperature and strain simultaneously can be obtained by matrix operation, thereby achieving an effect of eliminating cross sensitivity of temperature and strain sensing in optical frequency domain reflection.

Interferometric measurement signals are detected by a single optical interferometric interrogator for a length of a sensing light guide and an interferometric measurement data set corresponding to the interferometric measurement signals is generated. The interferometric measurement data set is transformed into a spectral domain to produce a transformed interferometric measurement data set. The transformed interferometric measurement data set is compared to a baseline interferometric data set to identify a time-varying signal corresponding to a time-varying disturbance. The baseline interferometric data set is representative of the sensing light guide not being subjected to the time-varying disturbance. A compensating signal is determined from the time-varying signal and used to compensate at least a portion of the interferometric measurement data set for the time-varying disturbance as part of producing a measurement of the parameter.

A distributed optical fiber disturbance positioning system based on the asymmetric dual Mach-Zehnder interference, unlike traditional dual Mach-Zehnder distributed optical fiber disturbance sensing system, the present invention adopts two narrow-bandwidth optical sources (1a, 1b) and adopts corresponding DWDM (3a, 3b) before the detector (4a, 4b) to filter the backscatter noise of the optical fiber, and can solve the problems of having too low SNR due to backscatter influence when the sensing distance is long. The present invention also provides a positioning method for applying the system, which obtains the TFD of the disturbance frame signals by using the time-frequency analysis method based on the short-term average frequency, and takes the points near the point of maximum frequency as the effective signal segment for performing cross-correlation time delay estimation, thus obtaining the delay, and the disturbance position. The method of the invention positions the asymmetric disturbance frame signals in the systems, thus having a high positioning accuracy and reliability.

The present invention discloses a differential COTDR distributed acoustic sensing device based on heterogeneous double-sideband chirped-pulses of the invention, comprising a light source (1), a 1?2 polarization-maintaining optical-fiber coupler (2), a dual Mach-Zehnder electro-optical modulator (3), an arbitrary waveform generator (4), a first low noise microwave amplifier (5), a second low noise microwave amplifier (6), an electro-optical modulator bias control panel (7), a 1?2 optical-fiber coupler (8), an erbium-doped optical-fiber amplifier (9), an optical-fiber filter (10), an optical-fiber circulator (11), a sensing optical fiber (12), a tricyclic polarization controller (13), a 2?2 optical-fiber coupler (14), a balanced photoelectric detector (15), a data acquisition card (16) and a processing unit (17). The present invention combines heterogeneous double-sideband chirped-pulse modulation and coherent light time-domain reflection technology, so as to double the sensitivity of the to-be-measured acoustic wave signal and to suppress common-mode noise, and further improves SNR.

A method for distributed acoustic sensing includes sending a first optical pulse down an optical fiber, wherein light from the first optical pulse is backscattered from positions along a length of the optical fiber according to coherent Rayleigh scattering; splitting backscattered light from the first optical pulse into a first portion for a first interferometer and a second portion for a second interferometer, the first interferometer having a first gauge length and the second interferometer having a second gauge length, wherein the first gauge length is different from the second gauge length; detecting a first interferometric signal from the first interferometer responsive to the first portion of backscattered light; detecting a second interferometric signal from the first interferometer responsive to the second portion of backscattered light; and processing the first and second interferometric signals for two different sensing applications adapted for the first and second gauge lengths, respectively.

An optical interrogation system, e.g., an OFDR-based system, measures local changes of index of refraction of a sensing light guide subjected to a time-varying disturbance. Interferometric measurement signals detected for a length of the sensing light guide are transformed into the spectral domain. A time varying signal is determined from the transformed interferometric measurement data set. A compensating signal is determined from the time varying signal which is used to compensate the interferometric measurement data set for the time-varying disturbance. The compensation technique may be applied along the length of the light guide.

An optical sensor (10) comprises an optical cavity defined by a dielectric body and responsive to one or more physical environmental conditions, and a waveguide (70) having a terminal end spaced apart from the optical cavity such that light is optically coupled from the terminal end of the waveguide (70) to the optical cavity. The waveguide (70) is arranged such that, in use, it is maintained at a first temperature that would not damage the optical coupling to the optical cavity when the dielectric body is maintained at a second temperature sufficient to damage the optical coupling to the optical cavity.

The application describes methods and apparatus for distributed fiber sensing, especially distributed acoustic/strain sensing. The method involves launching interrogating radiation in to an optical fiber and sampling radiation backscattered from within said fiber at a rate so as to acquire a plurality of samples corresponding to each sensing portion of interest. The plurality of samples are divided into separate processing channels and processed to determine a phase value for that channel. A quality metric is then applied to the processed phase data and the data combined to provide an overall phase value for the sensing portion based on the quality metric. The quality metric may be a measure of the degree of similarity of the processed data from the channels. The interrogating radiation may comprise two relatively narrow pulses separated by a relatively wide gap and the sampling rate may be set such that a plurality of substantially independent diversity samples are acquired.