Study on Innovative Flexible Design Method for Thin Film Narrow Band-Pass Filters

Abstract

The thin film narrow band-pass filters are widely used in precision optical systems such as gas detection, fiber optic communication and optical analysis instruments due to their unique optical characteristics. However, as the applications become more widespread, the problems of regulating the half bandwidth and cut-off bandwidth of the thin film narrow band-pass filters based on the Fabry-Perot (F-P) principle are becoming more apparent. In this work, we took the thin film narrow band-pass filter with a center wavelength of 450 nm as the design target, and comprehensively optimized the film system based on standard F-P principle to obtain the film systems with different half bandwidths and their systematic adjustment methods. The half bandwidth can be adjusted effectively by using the conventional half bandwidth control methods such as increasing the interference orders and the number of mirror layers or the cavities, but it is quite difficult to accurately obtain the filters with various half bandwidths since the adjustment range is too large. Based on the principle of equivalent refractive index, the optical thicknesses of the high and low refractive index film layers of the mirror layers were modified in an unconventional way, and thus the half bandwidth can be flexibly adjusted. Moreover, the standard F-P narrow band-pass filter film system was also modified by the principle of induced transmission film system to achieve a significant broadening of the cut-off bandwidth.