Transmission Characteristics Analysis of a Twin-Waveguide Cavity

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The transmission spectrum of a twin-waveguide cavity is systematically analyzed based on coupled mode theory, using the transfer matrix method (TMM). The results show that the traveling-wave transmission spectra of the twin-waveguide cavity is entirely determined by the coherent coupling effect involving the parameters of the effective refractive indices of the upper and lower waveguides, the coupling length Lc, and the ratio of the cavity length L to the coupling length (L/Lc). Filters with single, double, or triple-notch filtering could be obtained by choosing an appropriate L/Lc value. When the facet reflection is taken into consideration, the traveling-wave transmission spectrum is modified by the Fabry––Perot (FP) resonance, making it a standing-wave transmission spectrum. As a result, resonance splitting has been observed in the transmission spectrum of twin-waveguide resonators with high facet reflectivity. Further analysis shows that such an abnormal resonance phenomenon can be attributed to the destructive interference between the two FP resonance modes of the upper and lower waveguide through coherent coupling. In addition, narrow bandwidth amplification has also been observed through asymmetric facet reflections. Undoubtedly, all these unique spectral characteristics should be beneficial to the twin-waveguide cavity, achieving many more functions and being widely used in photonic integration circuits (PICs).