A filtering system is disclosed for receiving and bandpass filtering free space optical signals. The filtering system includes a plurality of optical fibers for accepting an optical signal. The fibers are implanted in an embedding material, having an index of refraction which is less than the optical fiber, for collimating the optical signal. The signal is further passed through a narrow, bandpass filter before detection by a photodetector device. In an alternative embodiment, the optical fibers are enclosed in a cladding, having a surface roughness at the interface of the cladding and the fiber, for enhancing the collimation of the optical signal.