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AIAA AVIATION Forum

Experimental Demonstration of Dispersion of Rayleigh Brillouin Scattering from Air with an Atomic Cesium Vapor Prism Cell

Anuj Rekhy, Boris S. Leonov, Amirhossein Abbasszadehrad, Junhwi Bak, James R. Creel, Christopher Limbach, Richard B. Miles

Published Jun 20, 2022• 2 citations
Publisher

Abstract

View Video Presentation: https://doi.org/10.2514/6.2022-3365.vid In this work, we demonstrate a new high throughput, high resolution spectroscopic technique based on dispersion in cesium atomic vapor and use it to image the Rayleigh-Brillouin spectrum from air. This is accomplished by utilizing a prism shaped cesium vapor cell and taking advantage of the steep refractive index in the vicinity of atomic resonances of the cesium D2 hyperfine lines at 852.113 nm. A laser beam from a tunable, continuous, and narrow linewidth Ti:sapphire laser was focused in ambient air and the spectrally dispersed scattering was imaged onto a camera after passing through a 80 mm long cesium prism vapor cell. The strong dispersion enables the measurement of the characteristic thermal and acoustic broadening of the Rayleigh-Brillouin scattering. Mie scattering from particles in the air is also captured and can be separated from molecular Rayleigh-Brillouin scattering by tuning close to the cesium resonance. The trade-off between spectral resolution by dispersion and loss of signal by cesium absorption is analyzed by tuning the laser. This technique shows the feasibility to measure the thermodynamic properties of a gas by reconstruction of the Rayleigh-Brillouin spectrum and could be applicable for advanced LIDAR measurements of atmospheric properties.

DOI
10.2514/6.2022-3365

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View Video Presentation: https://doi.org/10.2514/6.2022-3365.vid In this work, we demonstrate a new high throughput, high resolution spectroscopic technique based on dispersion in cesium atomic vapor and use it to image the Rayleigh-Brillouin spectrum from air. This is accomplished by utilizing a prism shaped cesium vapor cell and taking advantage of the steep refractive index in the vicinity of atomic resonances of the cesium D2 hyperfine lines at 852.113 nm. A laser beam from a tunable, continuous, and narrow linewidth Ti:sapphire laser was focused in ambient air and the spectrally dispersed scattering was imaged onto a camera after passing through a 80 mm long cesium prism vapor cell. The strong dispersion enables the measurement of the characteristic thermal and acoustic broadening of the Rayleigh-Brillouin scattering. Mie scattering from particles in the air is also captured and can be separated from molecular Rayleigh-Brillouin scattering by tuning close to the cesium resonance. The trade-off between spectral resolution by dispersion and loss of signal by cesium absorption is analyzed by tuning the laser. This technique shows the feasibility to measure the thermodynamic properties of a gas by reconstruction of the Rayleigh-Brillouin spectrum and could be applicable for advanced LIDAR measurements of atmospheric properties.
We report a new spectroscopy method achieved by combined refraction and propagation delay of light pulses passing through a prism cell filled with an atomic vapor. The atomic vapor medium has strong refractive index gradients near the atomic resonant absorption features, and these features enable the separation in time and space of different spectral features of a light pulse propagating through the vapor cell. The gradient of the refractive index leads to a slowing of the light pulse, while simultaneously, the magnitude of the refractive index and the prismatic windows or internal prism elements of the atomic vapor cell spatially separate the frequencies due to dispersion. The combination of these two effects leads to high-resolution spectral selectivity and strong suppression of out-of-band light. Applications are achieved for Rayleigh, Raman, and Thomson spectroscopy for which the wavelength of the scattering spectra tracks the wavelength of the excitation laser source. The laser is tuned such that the spectral line of interest falls near the resonance of the atomic vapor. Applications for rotational Raman spectroscopy are of particular interest since specific Raman lines can be selected while other Raman lines as well as background Rayleigh and other scattering are strongly rejected.
View Video Presentation: https://doi.org/10.2514/6.2023-1861.vid In this article, the authors discuss an experimental demonstration of a seeded Optical Parametric Oscillator as a light source for Slow Light Imaging Spectroscopy applications. An advantage of such a system, namely narrow linewidth of less than 400 MHz, allows for precise profiling of slow light features of atomic rubidium vapor, produced by high refractive index gradients in the vicinity of resonant transitions. Time delays of up to 48 ns were observed with a clear definition of the delayed pulse from the parasitic Mie and Rayleigh scattering. A comparison between the experimental data and numerical simulation over a frequency detuning range from -1 GHz to 4 GHz shows a good agreement for both qualitative and quantitative results.
We report the preliminary results of experimental studies of a dense cesium vapor in all-sapphire sealed-off Cs cell (temperature up to 500 °C, atomic and molecular densities up to 1018 and 3.2x1016 cm-3, respectively) in selective reflection (SR) configuration. Probing the vapor by a 780 nm laser radiation scanned in 20 GHz range, we have observed a spectral structure, which appears in the vapor temperature range of 250 - 400 °C. Though the cell is filled with pure cesium, starting from 300 °C we have revealed a noticeable SR contribution of rubidium atomic D2 line transitions that fall in the laser tuning range. The spectral features observed in present work can be attributed not only to cesium dimers (Cs2) having absorption band in this spectral region, but also to RbCs molecules. Relevant spectroscopic properties of these molecules are also addressed. We believe this is the first observation of selective reflection from the interface of dielectric and molecular vapor. More experiments and theoretical treatment is required to clarify the mechanisms involved and identify the origin of each spectral component, the further work program is outlined.
In this work we investigate the transmission in an optical cell containing an atomic vapor, when a laser is tuned to the wings of the Doppler line, presenting inhomogeneous broadening. The experiment is designed to detect the transmission of a laser beam through a an atomic cesium vapor within a quartz cell. Due to the high atomic density in the 1 mm cell, the laser beam is totally absorbed at resonance. On the other hand, when the detuning by few GHz from the center of the line an oscillatory signal is observed as a function of the laser frequency. The amplitude of the transmission signal varies linearly with the laser intensity. These oscillations exhibit diferent behavior as we change some parameters of the system, and the rate of the oscillations vary with laser detuning and on the atomic density. The phase of the transmited field is also sensitive to the angle of incidence of the laser beam in the cell, which allows to vary the lineshape of the signal without changing the parameters of the laser (intensity, frequency, ...) or atomic vapor. Out of resonance with the atomic vapor, the parallel windows of commercial optical cell cavity works as a low finesse Fabry-Pérot with a free spectral range of 150 GHz. However, in resonance, the optical path of the cavity changes varies with frequency due to the dispersive response of the atomic vapor. This study experimentally characterized the transmission signal through a cavity containing resonant vapor as a function of intensity and frequency of the laser, as well as its dependence on the angle of incidence of the beam. A numerical model that allows to determine the collisional broadening of the atomic vapor adjusting the spectral curves of the transmission signal was developed. As an application, exploring the oscillations in the wings of the resonance, we can lock the laser frequency at a few GHz off the center of the atomic resonance. The lock was performed in the frequency range between 0,8 GHz and 2,4 GHz from the center of the D2 line of cesium. To characterize the locking stability we measure the optical beat signal between two similar experiments, using the same locking technique.
Strong enhancement of group refractive index in a dense buffered atomic vapor is recorded using a technique of reflection from a low-finesse Fabry–Perot cavity filled with dense atomic vapor allowing the retrieval of a dispersion curve for the hyperfine structure of Rb D2 line buffered by a high-density Cs vapor. Oscillations of the recorded signal resulting from interference of beams reflected from the front and rear windows of the cell appearing with the laser frequency scanning across the resonance allow easy reconstruction of the dispersion curve. Contribution from concomitant interconnected processes, in particular, the determinative role of radiation channeling in enhancement of the resonator Q-factor, is analyzed.
Improvements in spectral resolution and sensitivity of a cesium based resonance fluorescence imaging monochromator (RFIM) are described. Alignment of the two lasers used in the excitation scheme in a co-propagating geometry have resulted in a detector spectral bandpass (270 MHz) that is narrower than the Doppler broadened profile (380 MHz) of cesium vapor at room temperature. Improvements in the experimental design, namely the addition of an image intensifier and cooled charge-coupled device (CCD) camera, have resulted in increased sensitivity; we report a limit of detection of 10 8 photons per pixel limited by image intensifier noise. Images of the cesium D 2 line emission (852.12 nm) from a cesium hollow cathode lamp were compared to an argon emission line (4Sº 1/2 → 4P 3/2 ) at 852.14 nm from a silver/argon hollow cathode lamp as a demonstration of the spectral resolution. The ability of the RFIM to discriminate between photons exciting the F = 4 vs. F = 3 ground state hyperfine level (a frequency difference of 9.2 GHz) of the cesium 6 2 S 1/2 state was also investigated to further demonstrate the spectral selectivity. The potential use of the described RFIM for chemical imaging applications is discussed.