A narrow bandpass filter based on two-dimensional photonic crystals with two reflectors

Abstract

With the rapid development of wavelength division multiplexing technology, narrow bandpass filters have drawn widespread public attention. In this paper, a compact narrow bandpass filter based on two-dimensional photonic crystals is proposed. The transfer characteristics of the filter with a single mode resonator and two reflectors are analyzed by using coupled mode theory. Research results show that the bandwidth of the filter can be controlled by adjusting the distance between the resonator and the two reflectors, which can be applied to the realization of narrow bandpass filters, and even ultra-narrow bandpass filters. Based on the theoretical model mentioned above, we design a narrow bandpass filter based on two-dimensional photonic crystals, which is composed of silicon rods with square lattice in air. Two single mode waveguides are formed by removing two rows of rods. Meanwhile, a point cavity is formed by removing a dielectric column. In order to precisely control the phase change between the resonator and the two reflectors, a phase adjustment region is introduced. We study the transmission spectrum of the structure by the finite-difference time-domain (FDTD) method. We find that the bandwidth of the filter can be narrowed when the phase change between the resonator and the two reflectors satisfies the specific conditions, and the transmission ratio is still high as well. These are consistent with the theoretical analyses. But it is worth noting that there is a difference between the simulation result and theoretical result. This is because in the theoretical analysis, we consider that the propagation constants of the frequencies close to the central frequency are the same. In fact, the propagation constant increases with the increase of frequency, however, this does not affect the central frequency nor its transmission. The performance of the designed filter is analyzed by FDTD, showing that the working frequency is close to 193.40 THz, the bandwidth is smaller than 5.9 GHz, the peak transmittance is up to 94%, and the length of the working area is only 9 m. Compared with the conventional photonic crystal filters, the designed narrow bandpass filer is very compact, and the performance is suitable for dense wavelength division multiplexed communication systems.