CEDAR: Vapor Filters: Developing New Measurement Capabilities for Narrowband Na Lidar

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This project is a two-year effort to initiate the development of vapor filters for lidar measurements in the mesopause region. It proposes two different types of vapor filters whose use will add new measurement capabilities to the narrowband Na fluorescence lidar. First is a narrow band-pass filter based on Faraday dispersion in Na vapor near resonance. This filter has an effective bandwidth of approximately 2 GHz, 85% peak transmission, and wide field-of-view. When used at a Fraunhofer line, it can reject sky background light more effectively than a Fabry-Perot system by more than a factor of 50, making the daytime operation of the Na temperature lidar practical. Second is a temperature stabilized iodine vapor cell to be used as a transmission filter. The transmittance of the I2 vapor cell as a function of frequency varies considerably within a few GHz. Since elastically scattered and Na fluorescence signals occur at different frequencies, a comparison of lidar returns at the two operating frequencies, v2 and vc, with and without passing through the I2 vapor cell will provide enough information for the determination of temperature and quantification of elastic scattering. Therefore, the use of a well characterized iodine vapor filter with a Na temperature lidar in the polar cap region will allow simultaneous monitoring of temperature profiles and noctilucent clouds.