Double critical coupled ring resonator-based add-drop filters

In this paper, we introduced double-critical coupling condition (DCCC) for lossy mode couplers of the add-drop resonator to achieve the desirable and tunable signals required for optical communication and photonics sensors applications. The performance of add-drop resonator under double-critical condition is simulated and analyzed. Some equations were derived for optical parameters including full width at half maximum (FWHM), the out-of-band rejection ratio (OBRR), quality factor and the crosstalk of the add-drop resonator, and the output signals were examined as a function of DCCC. Based on double-critical condition, the OBRR values larger than 40 dB, the crosstalk larger than 50 dB, highest quality factor of 9000 and the FWHM as small as 0.17 nm were realized in silicon add-drop resonator, which are quite compatible with the reported experimental data. Double-critical condition shows that the lossless coupling is not enough condition to acquire high-quality filtered signal with a large crosstalk and in presence of coupling losses, the DCCC can determine the optimum relation between the strength of coupling coefficients and the coupling losses.