Novel Frequency Discrimination Methods for Lidar Using Atomic Vapor Filters

Precise frequency discrimination of photons, often on the scale of GHz or less, is essential for measuring various atmospheric or thermodynamic properties in lidar and other laser diagnostic setups. Alkali vapor cells are often employed as filters in these systems due to the frequency dependent absorption exhibited near the transition lines of the atomic vapor. This work outlines novel frequency discrimination methods utilizing rubidium and cesium atomic vapor cells under development at the Aerospace Laboratory for Lasers, Electromagnetics, and Optics (ALLEMO) at Texas A&M University. The first approach, dubbed a dispersion prism filter, utilizes the frequency dependent refractive index near an absorption line to spatially separate portions of a scattered signal by frequency, allowing measurements of temperature and velocity. Temperature measurements in air between 20°C and 50°C are experimentally demonstrated with this method using a cesium filter and 852 nm scattered signal. The second technique utilizes spectral hole burning in an atomic vapor for velocimetry measurements. Here, a CW pump beam counterpropagating with the signal beam through the vapor cell creates steep drops in transmission at velocity classes near resonance with the pump frequency. These drops, or spectral holes, function as an edge filter, allowing determination of the Doppler shift of the signal and consequently the velocity of the medium by the change in transmission along the edge of the hole. An experimental setup for velocimetry measurements with spectral hole burning in rubidium at 780 nm is described. These methods, which have been demonstrated through tabletop experiments, have the potential to be implemented in an atmospheric lidar system for air temperature and wind measurements.