Narrowband Optical Filtering for Background-limited Photon-Counting Free-Space Optical Communications

For free-space optical (FSO) communications through the turbulent atmospheric channel, multi-spatial-mode photon-counting detectors can provide an attractive high-sensitivity receiver solution. However, multi-mode detection also increases optical background noise, which can degrade the overall system performance. Narrow band optical filtering becomes an important background rejection tool that can enable good performance in background-limited conditions. Here, we report the development of low-insertion-loss high-contrast-ratio optical filters at 1.55 mu m that are well suited for FSO communications. The filter has less than 0.7 dB insertion loss, 46 dB contrast ratio, 2.9 GHz 3-dB bandwidth, and 3.5 GHz noise-equivalent bandwidth, which is well matched to 1 GHz return-to-zero (RZ) pulsed waveforms. Other factors that influence filter performance and stability such as angular and temperature dependencies are characterized. Additional theoretical analysis and experimental results highlighting the trade-offs between spectral bandwidth, spatial bandwidth, filter insertion loss, and temperature-induced performance changes are presented. When coupled with photon-counting detection, these filtering techniques can enable sensitive FSO receiver operation, even in the presence of significant background.