Design and performance analysis of multi-band reflective Rugate filters

Notch filter serves as a vital optical filter, selectively blocking specific wavelength bands while transmitting both shorter and longer wavelengths. Traditional notch filter, designed by alternating layers of high and low refractive index materials, often suffer from undesired higher-order reflection bands. To address this problem, rugate filters with a sinusoidal variation in refractive index eliminate this issue. The refractive index distribution will affect the performance of the notch film. Therefore, the impact of refractive index distribution on the sidelobes of rugate filters was investigated using a developed software. The factors influencing the bandwidth and reflectance of rugate filters were also investigated. It was found that the sinusoidal index distribution modulation function could be more effective in sidelobe suppression. Furthermore, we found that index distribution modulation function decreases the reflectance of the reflection band, and the bandwidth was determined by maximum refractive index contrast. Additionally, we proposed a method for designing arbitrary multi-band rugate filters.