Patent ID: 7492977

Claim:
A device comprising: a three-by-three (3×3) optical coupler made of polarization-maintaining (PM) fibers and thus being a 3×3 PM fiber coupler; a light source and at least one photon-detector connected to a first side of said 3×3 PM fiber coupler; and a fiber coil connected to a second side of said 3×3 PM fiber coupler, wherein said 3×3 PM fiber coupler comprises monolithically fused PM fibers and has a phase difference between at least two of a plurality of input/output ports of said first and second sides, said phase difference being substantially close to 120 degrees, and wherein said 3×3 PM fiber coupler is adapted to split an input light from said light source into first and second optical signals while maintaining their respective polarization directions; and is adapted to cause coherent interference of third and fourth optical signals related respectively to the first and second optical signals and received from said fiber coil.