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Single-mode optical-fiber double-ring resonator with a planar 3 X 3 fiber coupler.

I present what is to my knowledge the first theoretical analysis of an optical-fiber double-ring resonator connected to a planar (collinear) 3 × 3 fiber coupler in which the cores of the three identical fibers inside the coupler lie in the same plane. Expressions for the circulating (resonant) and output fields are obtained. This fiber resonator has potential device applications as a spectral filter and a sensor with one of the highest phase sensitivities among various passive fiber ring and loop resonators.