Signal processing methods for an optical detection system

A signal processing method for a distributed acoustic sensing system, the method comprising:

• transmitting a spread spectrum pulsed test signal along an optical path;
• receiving, at a detector stage, a scattered signal that was scattered from a plurality of locations along the optical path;
• receiving, at the detector stage, a local oscillator signal;
• generating, based on an interference of the scattered signal and the local oscillator signal, a first complex carrier signal that is modulated by a phase difference between the local oscillator signal and the scattered signal;
• processing the first complex carrier signal to generate a spread spectrum second complex carrier signal that is modulated by a spatial differential of the phase difference between the local oscillator signal and the scattered signal, the spatial differential being taken along a length of the optical path;
• pulse compressing the second complex carrier signal;
• and
• determining, based on the pulse compressed second complex carrier signal, a value representative of the spatial differential of the phase difference for the location along the optical path;
• wherein processing the first complex carrier signal to generate the second complex carrier signal comprises:
• digitally simulating interference of the first complex carrier signal with a delayed copy of the first complex carrier signal, or
• multiplying a complex conjugate of the first complex carrier signal with a delayed copy of the first complex carrier signal, and
• wherein the spread spectrum pulsed test signal is configured to provide a pulse compressed second complex carrier signal having a predetermined sidelobe rejection threshold and central lobe width.