Abstract: Disclosed are signal processing methods for an optical detection system, and corresponding systems. An example is a signal processing method for a distributed acoustic sensing system which utilizes spread spectrum pulses transmitted along an optical path, where a scattered signal that was scattered at a scattering location along an optical path is received and interfered with a local oscillator signal to generate a first carrier signal that is modulated by a phase difference between the local oscillator and scattered signals. The first carrier signal is then processed to generate a second carrier signal that is modulated by a spatial differential of the phase difference. Pulse compression is then performed on the second carrier signal. The spatial differential of the phase difference is directly related to the strain (or acoustic environment) of the optical path at the scattering location, and so enables the strain at the scattering location to be estimated.

Abstract: This application described methods and apparatus for distributed fibre optic sensing. A sensing apparatus has a modulator which modulates radiation from an optical source to interrogate a sensing optical fibre with a first interrogation pulse at a first frequency (F1) and a second interrogation pulse at a second, different, frequency (F2), both different in frequency from a local oscillator (LO). A mixer mixes backscatter from the sensing optical fibre with the local oscillator and supplies the mixed signal to a detector that provides a corresponding digital signal. A processor processes the digital signal (DX, DY) in a first and second processing channels to demodulate respective first and second phase signals based on the respective frequency difference between the first and second frequency and the local oscillator and determines a temporal difference between the first and second phase signals.

Abstract: This application described methods and apparatus for distributed fibre optic sensing. A sensing apparatus has a modulator which modulates radiation from an optical source to interrogate a sensing optical fibre with a first interrogation pulse at a first frequency (F1) and a second interrogation pulse at a second, different, frequency (F2), both different in frequency from a local oscillator (LO). A mixer mixes backscatter from the sensing optical fibre with the local oscillator and supplies the mixed signal to a detector that provides a corresponding digital signal. A processor processes the digital signal (DX, DY) in a first and second processing channels to demodulate respective first and second phase signals based on the respective frequency difference between the first and second frequency and the local oscillator and determines a temporal difference between the first and second phase signals.