#FULL LATTICE CRYSTAL FILTER DESIGN FREE#
So the out-of-plane losses may become a big issue since the guided modes will couple vertically with the radiating modes in free space. However, that filter is based on two-dimensional (2D) PhC with triangular lattice of air holes in dielectric medium, where the operating wavelength is in the light cone. The wavelength-selective operation of this type of filter has been demonstrated. Although the optical filter based on contra-directional PhC waveguide coupler is not as compact as other filtering devices in photonic crystals, it is not so sensitive to loss and fabrication errors. Due to the periodicity, PhCs waveguide couplers are natural grating-assisted waveguide couplers, so contra-directional coupling between two PhC nearby waveguides can be easily achieved. Line defects in otherwise perfect PhCs can be utilized as waveguides for light. Photonic crystals (PhCs) open new possibilities for ultra compact, highly wavelength selective optical devices, owing to their photonic band gaps (PBGs). However, the coupler-type filters usually require length in the range of several millimeters, which is far from the demands of compact integrated photonic devices. Among these designs, waveguide-grating-based coupler-type filters are of great interest, due to their advantages of simple structure and nearly ideal spectral filtering characteristics. Various designs of optical filters have been proposed. In today’s dense wavelength division multiplexing (DWDM) system, optical filters are building blocks to multiplex and de-multiplex wavelengths. The results show that a filtering bandwidth of 4 nm can be achieved in the range of 1500∼1600 nm, and over 83% drop coefficient is obtained. The transmission properties are analyzed using coupled modes theory (CMT) and simulated using the finite-difference time-domain (FDTD) method. The filter has a broad operable bandwidth due to the absence of mini stop bands. The reduced-index and increased-index waveguides of filter have dispersion curves with opposite slopes to realize contra-directional coupling, and the point of anti-crossing is designed below the light line to avoid vertical radiation.