Atomic Line Filter for SDI Applications

Abstract : When optical signals are embedded in background noise, narrowband optical filters are needed for receivers to achieve quantum limited performance. Such is typically the case with daytime optical communications and lidar. Narrowband filtered detectors. with acceptance bands of just a few picometers (pm) and high out of band rejection are well suited for these applications. The main thrust of this program has been the development of two means for narrowband detection: the Fast Atomic Line Filter/ Field Ion Detector (FALF/FID) and the Faraday magneto-optic filter. The FALF/FIDs transduce optical radiation within a narrow band directly into electrical signals. They are laser pumped filters based on the spectroscopy of atomic vapors. We designed, built and tested an infrared FALF/FID with an rise time of under 10 ns and a quantum efficiency of >25%. Faraday filters are imaging magneto-optic filters with passbands of a few pm. Faraday filters with passbands in the visable and the near infra-red were constructed and tested. Transmission spectra were measured, and found to agree closely with theoretical predictions. Both of these narrowband detection schemes must be used in conjunction with a tuned narrowband laser source. We developed a frequency locked alexandrite laser designed to match the acceptance band of a rubidium based filter. Finally, a preliminary demonstration of a laser interrogator using a tuned infrared diode laser in conjunction with a filtered receiver was demonstrated. Atomic line filter, Alkali metal vapor, Faraday filter, Active optical filter, Laser pumped filter, Magneto-optic filter, Laser communications.