Ultra-narrow bandpass filters based on volume Bragg grating technologies

Free space optical communication between movable platforms, especially communication with non-cooperative targets, requires detecting low intensity signals in conditions of multiple sources of contaminating signals. In this paper, we review recent achievements in ultra-narrow bandpass filters based on volume Bragg gratings (VBGs) recorded in the bulk of photo-thermo-refractive glass. The new types of transmission filters show unique characteristics such as high throughput and bandwidths as narrow as a few picometers at any wavelength from 500 to 2700 nm. The first filter type is formed by the incoherent combination of a Fabry-Perot etalon and a VBG that enables tunable ultra-narrow band transmission with a single resonance. The filters demonstrate a bandwidth down to a few picometers at 1064 nm, a transmission exceeding 90%, an ultra-broad rejection band (several hundreds of nanometers), and an extinction ratio better than 30 dB. The second filter type is based on multiplexed frequency shifted VBGs that form a volumetric Moiré Bragg grating. The filter provides a single resonance with transmission higher than 90% in the middle of the reflection lobe of the VBG, a bandwidth down to a few picometers and high mechanical stability. Both types of ultra-narrow bandpass filters can be used for many applications requiring to transmit a single frequency and to reject other adjacent frequencies, e.g., in Lidars, or for selection of longitudinal modes in laser resonators. The new filters provide a significant advantage in terms of stability, tunability and achievable throughput for a given bandwidth.