Polarization diversity combining method in coherent DAS maintaining phase continuity

1. 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.

2. 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).