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Optics Express

Thermometry of liquid water through slow light imaging spectroscopy

Boris S. Leonov, Robert T. Randolph, Anuj Rekhy, Arthur Dogariu, Christopher Limbach, Richard B. Miles

Published Dec 21, 2022• 13 citations
Publisher

Abstract

This work presents the first, to the best of our knowledge, experimental demonstration of slow light imaging spectroscopy for thermometry of liquid water. This novel technique for measuring temperature relies on detecting the spectral shift of Brillouin peaks in water using the temporal delay through a cell containing an atomic vapor. Stand-off sensing capabilities are achieved by time-domain measurements of Brillouin scattering tuned to be near a rubidium atomic resonance and passed through a cell filled with rubidium vapor. An injection seeded optical parametric oscillator (OPO) is demonstrated to be a versatile light source for slow light imaging spectroscopy applications. The narrow OPO pulse spectrum allows for a precise profiling of slow light features of rubidium and accurate tracking of the temperature dependence of Brillouin scattering spectral shift. A comparison between the experimental data and numerical simulation over a temperature range of 20 to 99 degrees Celsius shows a good agreement for both qualitative and quantitative results.

DOI
10.1364/oe.478604

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This work presents the first, to the best of our knowledge, experimental demonstration of slow light imaging spectroscopy for thermometry of liquid water. This novel technique for measuring temperature relies on detecting the spectral shift of Brillouin peaks in water using the temporal delay through a cell containing an atomic vapor. Stand-off sensing capabilities are achieved by time-domain measurements of Brillouin scattering tuned to be near a rubidium atomic resonance and passed through a cell filled with rubidium vapor. An injection seeded optical parametric oscillator (OPO) is demonstrated to be a versatile light source for slow light imaging spectroscopy applications. The narrow OPO pulse spectrum allows for a precise profiling of slow light features of rubidium and accurate tracking of the temperature dependence of Brillouin scattering spectral shift. A comparison between the experimental data and numerical simulation over a temperature range of 20 to 99 degrees Celsius shows a good agreement for both qualitative and quantitative results.
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.
In this study, we investigate the effect of the optical thickness of an atomic vapor cell and the delay of the gated camera on the instrument function in Slow Light Imaging Spectroscopy. In an atomic vapor medium, the slow light effect happens at certain frequency windows where there is a large slope of the index of refraction versus frequency. The extent of these windows depends on the important parameters of the vapor cell (i.e., the optical thickness). Furthermore, the gate delay determines the minimum amount of delay that a signal needs to experience before it is detected. These two parameters determine the shape of the instrument function of the spectrometer in slow light imaging spectroscopy. Simulation and experimental results show that when absorption lines of cesium vapor at 852.1 nm are used as a delay medium, the instrument function at low temperature consists of four peaks corresponding to the steep refractive index gradients at either side of the two hyperfine absorption lines. By increasing the temperature, these peaks move away from the absorption lines. When the two peaks between the two absorption lines move away from their corresponding absorption lines, they move closer to each other until they blend, forming a strong peak. By increasing the gate delay, the four peaks of the instrument function reappear.
We report an experimental realization of the delayed images in a hot rubidium atomic vapor. With a rubidium atomic vapor cell as slow light medium, the image quality of the experiment could be improved greatly, compared with the results without a slow light medium. By analyzing the results about the image visibility of the slow light imaging system under three different conditions, the image visibility becomes better with the increment of the temperature, during the time that the wavelength of the laser is within dispersion range.
Summary form only given. Slow light has been explored for building quantum networks, with particular interest in slowing the group velocity of single photons [1], and more recently exploited to enhance the measurement of small phase shifts. Generally, slow-light effects have been characterized as the net effect of a pulse propagating through the slow-light medium, i.e., as a pulse delay time Δt measured with a fast photodiode at the output of the medium [2]. In this work, we use a single-photon imaging camera to observe slow light in situ, and thus provide a direct measurement of spatial pulse compression and temporal dispersion as the pulse travels through the slow light medium, in this case a hyperfine absorption doublet in hot Rb vapor. Our method combines light-in-flight imaging techniques with a camera comprised of an array of single-photon avalanche diodes (SPAD camera) [3] to image the photons scattered by the Rb vapor in the direction of the camera as shown in Fig. 1(a). In addition, the single photon nature of the SPAD detector allows us to obtain a measurement of the single photon group velocity. As shown in Fig. 1(c) and (d), we observe a significant delay, on the order of nanoseconds, in the detection of the photons scattered when the pulse first enters the slow-light medium. This lag in scattered-photon arrival time is a direct visualization of the slowing down of the single-photon group velocity. The pulses used here had a temporal full width at half maximum (FWHM) of τ ~1 ns, with measured group velocities as low as vg~ 0.006c. At these low group velocities we observe a full fractional pulse delay of up to F D = Δt/τ ~ 40 over 7 cm of propagation, and F D ~ 5 for the scattered single photons, which propagate through ~ 1 cm of Rb vapor prior to exiting the cell en-route to the camera.
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.