Design, fabrication and characterization of narrowband angularly-insensitive resonant cavity filter

We describe the design, fabrication, and characterization of resonant filters having concurrently a wide range of acceptance angles and a narrow wavelength passband, based on one-dimensional photonic crystals (PC) with defects. A filter having these characteristics would be extremely useful, for example, in a free-space optical communication system among mobile nodes, where the relative bearing of the transmitter is unknown. However, the design of such a structure requires careful design, as these two characteristics are not readily compatible. In the following, we discuss our design approach to implement the wide-field-of-view narrowband resonant filter, describe the fabrication procedure in the GaAs/AlAs material system including oxidation to achieve the high refractive index contrast needed for good performance, and present the structural and optical characterization results for the finished device. The objective is to design a wide-field-of-view narrow band wavelength filter using a multilayer 1-D PC structure with one or more defects. The primary defect in the resonant cavity is realized with the highest possible index of refraction, to minimize the variation of the cavity phase with changes in the incidence angle. Since the angular dependence of the incidence angle cannot be completely eliminated in an isotropic 1-D structure, the ideal filter performance characteristics would be a perfectly square bandpass filter with a bandwidth that can be specified to match the specific application. To approximate this type of performance, we use an asymmetric secondary resonant PC cavity in place of each mirror surrounding the primary cavity, resulting in a resonant 1-D PC multilayer structure having three defects. This approach permits control of the spectral reflectivity and phase of each mirror, facilitating control of the transmission spectrum of the device as a whole.