Multi-span sensing using wavelength / frequency multiplexing, synchronized receiver, and coherent signal processing

A distributed acoustic sensing (DAS) system, comprising:

• a transmitter, the transmitter comprising:
• a multi-tone source to generate a sensing signal over a plurality of wavelengths;
• a modulator assembly, to perform a frequency modulation of the sensing signal over the plurality of wavelengths to generate a frequency chirp modulated signal;
• and
• a receiver, to receive a reflected signal, derived from the sensing signal, wherein the reflected signal comprises a plurality of optical signals distributed over the plurality of wavelengths,
• wherein the receiver is configured to beat the frequency chirp modulated signal with the reflected signal to generate a plurality of output signals, wherein a frequency of an output signal provides a measure of a location of a disturbance.

A method for distributed acoustic sensing (DAS), comprising:

• receiving a reflected signal, over a plurality of channels corresponding to a plurality of wavelengths, respectively;
• separating the reflected signal according to wavelength into a plurality of optical signals;
• mixing the plurality of optical signals with a plurality of local oscillator signals at the respective plurality of wavelengths;
• converting the plurality of mixed signals into a plurality of analog electrical signals, respectively;
• converting the plurality of analog electrical signals concurrently into a plurality of digital signals;
• receiving the plurality of digital signals concurrently, and processing the plurality of digital signals to convert the plurality of digital signals into phase data associated with the plurality of wavelengths, respectively.

A method of distributed acoustic sensing (DAS), comprising:

• generating a sensing signal over a plurality of wavelengths using a multitone source;
• performing a frequency modulation of the sensing signal over a plurality of channels corresponding to the plurality of wavelengths, respectively, to generate a frequency chirp modulated signal;
• and
• receiving a reflected signal, derived from the sensing signal, wherein the reflected signal comprises a plurality of optical signals distributed over the plurality of channels,
• beating the frequency chirp modulated signal with the reflected signal to generate a plurality of output signals, wherein a frequency of an output signal provides a measure of a location of a disturbance.