Demonstration of Doppler Shift Measurements using an Actively Controlled Atomic Vapor Filter

Accurate velocimetry measurements with fine spatial resolution are important for weather forecasting and detection of clean air turbulence and wind shear. This work experimentally demonstrates velocimetry measurements using an actively controlled atomic vapor filter, which has applications as an edge-filter-based direct detection Doppler lidar system. Here, the Doppler shift is determined by measuring the change in absorption of the signal along the edge of a transmission line in an atomic vapor medium. A counter-propagating pump beam resonant with a second ground state absorption line results in deep holes in the transmission spectrum near resonant velocity classes of the vapor. The steeper transmission slope gives an increased sensitivity to the Doppler shift of the probe beam. The location of the slope may be shifted by modifying the pump wavelength and power, essentially creating an actively controllable edge filter. The filter is used to estimate the Doppler shift of light scattered from a rotating disk with tangential speeds between 10 and 45 m/s. Improvements up to 70% in the velocity sensitivity are demonstrated when propagating the pump beam through the cell. Further improvements are anticipated with optimized cell and pump beam parameters. The filter is suggested for future lidar measurements of aerosol velocities and can be applied to other wavelengths resonant with energy level transitions in atomic vapors in the UV, visible, and IR regimes.