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Ultrafast Nonlinear Imaging and Spectroscopy

Low wavenumber Raman spectroscopy using atomic filters (Conference Presentation)

Xiaobo Xue, Corey T. Janisch, Yizhu Chen, Zhiwen Liu, Jingbiao Chen

Published Nov 2, 2016• 0 citations
Publisher

Abstract

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.

DOI
10.1117/12.2238338

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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.
A Faraday anomalous dispersion optical filter (FADOF) and an atomic resonant absorption filter are used in tandem to demonstrate a low-frequency shift Raman measurement down to few cm−1. The FADOF, with an ultralow bandwidth of 0.08 cm−1 at 780 nm, serves as a bandpass filter, while the rubidium atomic cell acts as a notch filter which has a bandwidth of 0.3 cm−1. A proof-of-concept study to measure a Raman signal generated from a silica optical fiber is performed, demonstrating a low-frequency measurement of both the Stokes and the anti-Stokes shift down to 3 cm−1 at an equivalent signal level. These results indicate the prospect for gigahertz–terahertz low-energy Raman spectroscopy based on atomic filters.
Phase-modulator-generated Raman laser is widely used in atom gravimeters to simplify system and improve robustness. However, the induced additional sidebands (ASBs) lead to systematic errors in gravity measurement. This work presents a novel method to generate optical single-sideband (OSSB) Raman laser based on Faraday anomalous dispersion atomic filter (FADOF). The experimental result indicates that it can reduce the additional sidebands to lower than -50 dB, and the phase shift caused by ASBs is demonstrated to be effectively suppressed from 358.8 mrad to 2.2 mrad. Furthermore, this system has already applied on atom gravimeter to achieve primary gravity measurement. It shows that FADOF-based Raman laser is a new scheme for a compact atom absolute gravimeter.
Phase-modulator-generated laser for Raman transition is widely used in atom gravimeters to simplify a system and improve robustness. However, the induced additional sidebands (ASBs) lead to systematic errors in gravity measurement. This work presents a novel, to our knowledge, method to generate an optical single-sideband (OSSB) laser for Raman transition through phase modulation based on a Faraday anomalous dispersion atomic filter (FADOF). The experimental result indicates that it can reduce the additional sidebands with a signal-to-noise ratio (SNR, the intensity ratio of carrier and unwanted sidebands) of better than 50 dB, and the phase shift caused by ASBs is demonstrated to be effectively suppressed from 358.8 to 2.2 mrad. Furthermore, this system has already been applied on an atom gravimeter to achieve a primary gravity measurement. It shows that the FADOF-based Raman laser system is a new scheme for a compact atom absolute gravimeter.
Phase-modulator-generated Raman laser is widely used in atom gravimeters to simplify system and improve robustness. However, the induced additional sidebands (ASBs) lead to systematic errors in gravity measurement. This work presents a novel method to generate optical single-sideband (OSSB) Raman laser based on Faraday anomalous dispersion atomic filter (FADOF). The experimental result indicates that it can reduce the additional sidebands to lower than -50 dB, and the phase shift caused by ASBs is demonstrated to be effectively suppressed from 358.8 mrad to 2.2 mrad. Furthermore, this system has already applied on atom gravimeter to achieve primary gravity measurement. It shows that FADOF-based Raman laser is a new scheme for a compact atom absolute gravimeter.
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.
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.