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AIAA SciTech Forum

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

Robert T. Randolph, Eric Finberg, Christopher Limbach, Richard B. Miles

Published Jan 8, 2026• 0 citations
Publisher

Abstract

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.

DOI
10.2514/6.2026-0882

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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.
Precise wind velocity measurements are of interest for a variety of applications including weather forecasting and detection of clean air turbulence and wind shear. This work investigates a concept known as spectral hole burning to create an actively tunable atomic vapor filter in order to improve the resolution of LIDAR wind velocity profiling. By adjusting the frequency and intensity of a pump beam in an alkali vapor cell, steep slopes in the transmission profile may be used to monitor shifts in frequency of a probe beam. A three-level atomic model to simulate the spectral hole is described and validated experimentally through pump-probe spectroscopy. The model then optimizes parameters including cell length, cell temperature, pump intensity and pump frequency to find the steepest transmission slope obtainable. At this location, frequency resolutions of 0.69 MHz are predicted per 1% change in transmission, corresponding to a 0.27 m/s velocity sensitivity. From this information, the feasibility and limitations of using spectral hole burning as a controllable atomic filter for LIDAR velocimetry measurements are discussed.
Weather modelling, detection of clean air turbulence, detection of wind shear, and other applications benefit from precise wind speed measurements. This work reports on progress towards a direct detection lidar velocimetry method utilizing an adaptable vapor filter. The method uses spectral hole burning, in which a pump beam excites atoms in a vapor cell moving them from the ground state to a trapping state, creating deep “holes” in the inhomogenously transmission profile. A three-level atomic model was created and experimentally validated to characterize the position and magnitude of the hole in the transmission profile, as well as the time required to reach equilibrium, under different pump frequencies and powers. Work towards using the technique for tabletop velocimetry measurements of an under-expanded jet is described.
Weather modelling, detection of clean air turbulence, detection of wind shear, and other applications benefit from precise wind speed measurements. This work reports on progress towards a direct detection lidar velocimetry method utilizing an adaptable vapor filter. The method uses spectral hole burning, in which a pump beam excites atoms in a vapor cell moving them from the ground state to a trapping state, creating deep “holes” in the inhomogenously transmission profile. A three-level atomic model was created and experimentally validated to characterize the position and magnitude of the hole in the transmission profile, as well as the time required to reach equilibrium, under different pump frequencies and powers. Work towards using the technique for tabletop velocimetry measurements of an under-expanded jet is described.
Traditionally, measuring the center-of-mass (c.m.) velocity of an atomic ensemble relies on measuring the Doppler shift of the absorption spectrum of single atoms in the ensemble. Mapping out the velocity distribution of the ensemble is indispensable when determining the c.m. velocity using this technique. As a result, highly sensitive measurements require preparation of an ensemble with a narrow Doppler width. Here, we use a dispersive measurement of light passing through a moving room temperature atomic vapor cell to determine the velocity of the cell in a single shot with a short-term sensitivity of 5.5 μm s^{-1} Hz^{-1/2}. The dispersion of the medium is enhanced by creating quantum interference through an auxiliary transition for the probe light under electromagnetically induced transparency condition. In contrast to measurement of single atoms, this method is based on the collective motion of atoms and can sense the c.m. velocity of an ensemble without knowing its velocity distribution. Our results improve the previous measurements by 3 orders of magnitude and can be used to design a compact motional sensor based on thermal atoms.
The pump-probe spectra in a cell of micrometric thickness containing cesium vapor are reported. The line shape and nonlinear features observed in the case of fluorescence in the direction parallel to the cell windows and the transmission spectra observed along the propagation direction of the probe beam show considerable differences in the spectral profiles. We observed Electromagnetically Induced Transparency (EIT) and enhanced Velocity Selective Optical Pumping (VSOP) signals. Atoms moving nearly parallel to the windows and perpendicular to the collinear pump and probe beams will see much lower Doppler shift of incident frequencies and hence will lead to considerable narrowing of the Doppler background in the fluorescence spectra. The coherence decay rate is also low for such atoms as they do not meet with the cell walls. A theoretical model based on five level optical Bloch equations is used to simulate the spectra. The Doppler convolution includes all possible orientation of atomic velocities with respect to the laser beam direction. The simulated curves reproduce the observed sharp EIT peaks and enhanced broad VSOP signals for the closed probe transition in the fluorescence and absorption spectra. The observed effect of the light intensity and temperature change on the non-linear features is reproduced by the simulation.
Methods to separate and filter Mie, Rayleigh and Raman scattering are required in many atmospheric lidar systems and other laser diagnostic applications. This work reports on progress towards a novel filtering technique to spectrally separate components of a backscattered signal through coherent light generation in an atomic vapor cell. Specifically, this work characterizes 420 nm (blue) light generation in a heated rubidium cell arising from the mixing of 776 nm and 780 nm pump beams. The work finds the optimal pump frequencies and cell temperature and detects a signal using pump beams as low as 0.7 μW. Progress towards using the technique to detect scattered light in a lidar application is described.