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APPLICATION OF ARGON PRESSURE BROADENED RUBIDIUM VAPOR CELLS AS ULTRA-NARROW NOTCH FILTERS

Sam Groetsch

Published Jan 1, 2022• 0 citations
Publisher

Abstract

Raman spectroscopy is a widely used, highly versatile, and non-destructive method for characterizing the molecular makeup of unknown samples. In nearly all applications the vibrational modes of the constituent molecules provide the mechanism by which the laser light is inelastically scattered and consequently identified. However, the rotational modes of these molecules provide much greater relative rates of inelastic scattering than the vibrational modes. Unfortunately, due to the nature of current filters used in Raman spectroscopy when the laser light is filtered out so is the light scattered by the pure rotational modes. In this research, we propose the use of a Rubidium vapor cell filled with a buffer gas of Argon as a viable ultra-narrow notch filter. This filter would provide a robust laser rejection without removing the light scattered by the pure rotational modes of molecules. Measurement of the pure rotational spectrum of gases can have numerous applications such as precision measurement of exhaled gases for medical diagnostics, highly accurate remote measurement of atmospheric temperatures, and extremely sensitive detection of dangerous air contamination in dusty environments.

DOI
10.37099/mtu.dc.etdr/1368

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Raman spectroscopy is a widely used, highly versatile, and non-destructive method for characterizing the molecular makeup of unknown samples. In nearly all applications the vibrational modes of the constituent molecules provide the mechanism by which the laser light is inelastically scattered and consequently identified. However, the rotational modes of these molecules provide much greater relative rates of inelastic scattering than the vibrational modes. Unfortunately, due to the nature of current filters used in Raman spectroscopy when the laser light is filtered out so is the light scattered by the pure rotational modes. In this research, we propose the use of a Rubidium vapor cell filled with a buffer gas of Argon as a viable ultra-narrow notch filter. This filter would provide a robust laser rejection without removing the light scattered by the pure rotational modes of molecules. Measurement of the pure rotational spectrum of gases can have numerous applications such as precision measurement of exhaled gases for medical diagnostics, highly accurate remote measurement of atmospheric temperatures, and extremely sensitive detection of dangerous air contamination in dusty environments.
We introduce new methods for capturing rotational Raman scattering that simultaneously suppress background scattering and have exceedingly high spectral dispersion at low wavenumbers. These methods are based on resonant absorption and refractive index dispersion of atomic vapor, and take advantage of the availability of tunable lasers. Three different configurations are presented. The first is an atomic notch transmission filter which is based on refluorescence from an optically thick atomic vapor cell. This configuration is demonstrated in air using mercury vapor at 254 nm. The second configuration is an atomic resonance prism cell which is demonstrated in CO2 using mercury vapor, again at 254 nm. The third configuration is a density gradient atomic vapor cell which uses rubidium vapor at 780 nm. In the last two configurations, the atomic cell simultaneously blocks the elastic scattering while spatially dispersing the rotational Raman lines. In each case, a tunable Ti : sapphire laser was employed as an illuminating source. Copyright © 2000 John Wiley & Sons, Ltd.
The comprehensive monitoring of the food chain, i.e. from growth to consumer, without gaps is of outstanding importance to consumer health and the food industry alike. Yet, due to the lack of suitable sensing technologies at adequate costs, only selective, critical spots can currently be monitored. While the particular food to be monitored may take any shape or form during processing, gases are always present and their composition is uniquely suited to learn about the state of the system. In this contribution, a universal quantum sensor concept to identify and quantify the components of complex gas matrices is presented. The sensing approach is based on detecting Raman scattered photons from inside a gas sample filled optical cavity. Because of the typically small Raman scattering cross-sections on the order of 10-30 cm2 a Fabry-Perot cavity with a Finesse of about 2,000 is used to enhance the available laser power of an extended cavity diode laser running at 780 nm. The scattered light is funneled onto a spectrometer after Rayleigh scattered photons have been suppressed using a notch filter. To allow in-situ determination of arbitrary gas matrices, the sensor concept relies on the diffusion of ambient air into the cavity. The setup is currently able to simultaneously detect gases with Raman spectra up to 3000 cm-1, which includes important indicators for food packaging, including nitrogen, oxygen, and carbon dioxide.
Low-wavenumber Raman spectroscopy has long been demonstrated as a method of optical characterization in a variety of applications, such as thermal detection and semiconductor analysis. However, accessing low-wavenumber Raman shifts remains a challenge, usually requiring an expensive and complex multi-stage spectrographic system to measure several cm-1 Raman shifts. In this work, we demonstrate a method to measure low-wavenumber Raman shifts down to 1 cm-1 using atomic filters. By using a narrow-band Faraday anomalous dispersion optical filter to remove spontaneous emission noise from the laser cavity and a heated atomic cell as a notch filter to remove the excitation laser, the system is able to measure low Raman shifts (down to 1 cm-1). To demonstrate the capabilities, we measure the broadband Raman spectrum from a silica optical fiber with approximately 0.3 cm-1 resolution, detecting both Stokes and Anti-Stokes Raman shift as low as 0.7 cm-1.
The nature of atomic vapors, their natural alignment with interatomic transitions, and their ease of use make them highly suited for spectrally narrow-banded optical filters. Atomic filters come in two flavors: a filter based on the absorption of light by the Doppler broadened atomic vapor, i.e. a notch filter, and a bandpass filter based on the transmission of resonant light caused by the Faraday effect. The notch filter uses the absorption of resonant photons to filter out a small spectral band around the atomic transition. The off-resonant part of the spectrum is fully transmitted. Atomic vapors based on the Faraday effect allow for suppression of the detuned spectral fraction. Transmission of light originates from the magnetically induced rotation of linear polarized light close to an atomic resonance. This filter constellation allows selective acceptance of specific light frequencies. In this manuscript, we discuss these two types of filters and elucidate the specialties of atomic line filters. We also present a practical guide on building such filter setups from scratch and discuss an approach to achieve an almost perfect atomic spectrum backed by theoretical calculations.
Abstract The nature of atomic vapors, their natural alignment with interatomic transitions, and their ease of use make them highly suited for spectrally narrow-banded optical filters. Atomic filters come in two flavors: a filter based on the absorption of light by the Doppler broadened atomic vapor, i.e. a notch filter, and a bandpass filter based on the transmission of resonant light caused by the Faraday effect. The notch filter uses the absorption of resonant photons to filter out a small spectral band around the atomic transition. The off-resonant part of the spectrum is fully transmitted. Atomic vapors based on the Faraday effect allow for suppression of the detuned spectral fraction. Transmission of light originates from the magnetically induced rotation of linear polarized light close to an atomic resonance. This filter constellation allows selective acceptance of specific light frequencies. In this manuscript, we discuss these two types of filters and elucidate the specialties of atomic line filters. We also present a practical guide on building such filter setups from scratch and discuss an approach to achieve an almost perfect atomic spectrum backed by theoretical calculations.