A Grüneisen relationship is defined for gases, following the formulation of the original microscopic Grüneisen ratio γ = (dlnω)/(dlnV ) for solids. In the case of gases acoustic excitations represent the modes at frequency ω to be considered. By comparing to measured Brillouin shifts in various gases (SF6, N2O, and CO2) and under various conditions of pressure and temperature, a specific value of the defined ratio γ0 = 0.064±0.004 is found to provide a universal description of the active modes in a gas. This finding of such universal gas law may find application in extrapolation of properties of ideal gases to regimes where those cannot be measured easily, like the acoustics and shocks at extremely high temperatures.
Brillouin spectroscopy is a powerful tool to measure the water temperature and salinity profiles of seawater. Considering the insufficiency of the current spectral measurement methods in real-time, spectral integrity, continuity, and stability, we developed a new lidar system for spectrum measurement on an airborne platform that is based on a Fizeau interferometer and multichannel photomultiplier tube. In this approach, the lidar system uses time-of-flight information to measure the depth and relies on Brillouin spectroscopy as the temperature and salinity indicator. In this study, the system parameters were first optimized and analyzed. Based on the analysis results, the performance of the system in terms of detection depth and accuracy was evaluated. The results showed that this method has strong anti-interference ability, and under a temperature measurement accuracy of 0.5 °C and a salinity measurement accuracy of 1‰, the effective detection depth exceeds 40.51 m. Therefore, the proposed method performs well and will be a good choice for achieving Brillouin lidar application in seawater remote sensing.
In this study, a lidar system based on the Fizeau interferometer and photomultiplier tube array is designed to analyze the entire Rayleigh‐Brillouin spectrum aiming at obtaining the atmospheric temperature profiles. The laser wavelength emitted by the lidar is 355 nm with the pulse energy of 20 mJ, and the diameter of the receiving telescope is 0.6 m. We analyze the influence of the cumulative average of different pulse numbers on the accuracy of temperature inversion. The temperature inversion results in different height ranges are calculated and the results show that if the integration time is 100 s, the temperature deviation is less than 3 K in the altitude range below 24 km.
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