Polarization diversity combining method in coherent DAS maintaining phase continuity

A distributed optical fiber sensing (DOFS)/distributed acoustic sensing (DAS) method employing polarization diversity combining and spatial diversity combining for a DOFS/DAS system having

• a length of optical fiber;
• and
• an optical interrogator unit that generates optical pulses, introduces them into the optical fiber and receives Rayleigh reflected signals from the fiber;
• and
• a coherent receiver unit configured to extract information from the Rayleigh reflected signals,
• the method comprising:
• operating the DOFS/DAS system to obtain a respective set of beating products for a plurality of locations along the length of the optical fiber;
• performing a polarization diversity combining for each one of the sets of the beating products;
• and
• using the polarization diversity combining for each one of the sets of the beating products, performing a spatial diversity for each of the plurality of locations along the optical fiber;
• wherein the polarization diversity combining comprises:
• determining a temporal average product for each beating product
• determining one having a max average power;
• rotating that one having max average power for its phase shift to produce a reference;
• determining a phase difference for each beating product as compared to the reference;
• compensating any phase difference such that all beating products exhibit a well-aligned phase;
• and combining the beating products;
• wherein the spatial diversity combining comprises:
• using the combined beating products for each location, determine a temporal average power;
• determine a location having a greatest average power;
• and
• combining the results and
• providing an indicia of the combined result(s).