Fabry-Perot interferometer in a rod-type photonic crystal based on self-collimation

A Low-loss Fabry-Perot interferometer (FPI) constructed in a two-dimensional photonic crystal (2D PhC) is proposed and investigated. The 2D PhC consists of a square-lattice of cylindrical silicon rods in air. It has flat equal frequency contours (EFCs) in the frequency range of 0.187–0.201c/a for TM modes. Two same line defects with spacing of, d = 21√2a, which is the physical length of the FP resonant cavity, are introduced in the PhC to form the FPI. The two line defects have high reflectivity and low transmission. Their transmission is between 20.77% and 40.65% for the self-collimated lights with frequencies from 0.187c/a to 0.201c/a and thus they form the two partial reflectors. Lights propagate in the FPI utilizing self-collimation effect. The transmission spectrum of the FPI has been investigated with the finite-difference time-domain (FDTD) method. The calculation results show that even slight increase of d can cause peaks shift left to lower frequencies. Through changing the configuration of the reflectors which results in transmission between 19.97% and 38.77%, the varieties of the sharpness of peaks and the degree of extinction of the frequencies between the peaks are obviously observed. Free spectral range (FSR) and peaks frequencies of its transmission decrease when d increases. By raising the reflectivity of the reflectors, the full width at half maximum (FWHM) is decreased and quality (Q) factor of peaks is increased.