Design Of Optical Filter Based On Photonic Crystals

Photonic crystal (PC) structure, whose dielectric constant varies periodically in space, possesses a photonic band gap (PBG) in which electromagnetic wave cannot propagate. Great attention was paid to research on PC-based optical devices due to their compactness and the potential application in photonic integrated circuits and all-optical communication network. Optical filters are key components to access signals on a particular channel or multiple channels in wavelength division multiplexed optical communication system. Therefore, research on PC-based filter is also critical. In this paper, researches are given on the design of optical filter based on PC waveguide gratings. They are summerized in the following.In this paper, according to the analyses on PC waveguide and PC waveguide gratings, photonic crystal waveguide sampled gratings are proposed to realize multi-channel optical filter in a PC waveguide. Coupled-mode theory (CMT) and Transfer-matrix method (TMM) are used to analyze the transmission characteristics and the results are compared with the numerical results calculated by two-dimensional (2-D) finite-difference time-domain (FDTD) method. The reflection spectrum of the filter exhibits several reflection peaks in the PBG. Most of the reflection peaks are distributed with equal spacing and their reflection efficiencies are larger than 90%. From the theoretical analyses, it indicates that this device leads a potential way for the optical filter based on PC using in the dense wavelength division multiplexing (DWDM) optical communication systems with channel spacing of 200GHz and a flat bandwidth of 100GHz.Standard photonic crystal (PC) waveguide gratings show a stopband only in reflection. By incorporating single phase-shift region into the PC waveguide gratings, an extremely narrow transmission peak appears in the center of the stopband. Using coupled-mode theory together with transfer-matrix formalism, it is shown that by inserting several phase-shift regions into the PC waveguide gratings and properly choosing their locations and magnitudes, the transmission spectrum can be tailored into a nearly rectangular line shape with flap top response and sharp roll-off. The performance of the newly designed filter is numerically calculated using the two-dimensional (2-D) finite-difference time-domain (FDTD) method. As a specific application, the designed filter based on the phase-shifted photonic crystal waveguide gratings can be used as a bandpass filter in dense wavelength-division-multiplexed (DWDM) optical communication systems with 100-GHz channel spacing and a 50-GHz bandwidth.