Spectral characteristics of coupled-waveguide Fabry-Perot resonators and filters

Conventional Fabry-Perot resonators provide a narrow spectral width but lack the capability of longitudinal mode discrimination. A coupled-waveguide Fabry-Perot structure made of two parallel waveguides with reflecting mirrors at the ends is proposed for applications as mode selective resonator in single-mode diode lasers and as narrow-band wavelength filters. The interference of counter propagating waves from reflection by end mirrors and the coupling of waves between the two parallel waveguides contribute to the operation of this resonator structure. Thus, the device exhibits the attributes of both Fabry-Perot resonator and directional coupler. The coupled-mode theory of parallel waveguides is employed to analyze the structure. Both cases of identical and unidentical waveguides are examined. Resonance conditions and spectral characteristics are determined. It is shown that the coupled-waveguide Fabry-Perot resonator provides significant improvement in mode discrimination capability and longitudinal mode spacing over the conventional Fabry-Perot resonator.