Direct-coupling single-mode fiber ring resonator

A single-mode resonator is constructed by joining together the ends of the coupled fiber of a 2 × 2 directional single-mode optical fiber coupler. Analytical expressions are obtained for the resonance conditions. The effects of different kinds of losses are investigated. The variation of the phase at resonance and the problem of mismatch are discussed. A formula for the finesse of the resonator is also given. Unlike reentry fiber resonators, this kind of resonator needs a directional coupler with only a small coupling coefficient. It is possible to achieve a high finesse and symmetrical outputs. Applications of such a resonator are discussed.