Temperature retrieval from Rayleigh-Brillouin scattering profiles measured in air

Development of a Rayleigh-Brillouin scattering spectrometer for fast high-gas-temperature measurements

We proposed a Rayleigh-Brillouin scattering (RBS) spectrometer based on a virtually imaged phased array (VIPA) for fast measurements of high-gas temperature. We measured the RBS spectra of air in the temperature range of 374 to 1073 K with an acquisition time of 7 s. We used the Tenti S6 model to fit the spectra and retrieve the absolute temperature values. The root mean square errors of spectra fit residual were less than 3.05%, and the absolute error of the retrieved temperature was less than 39 K. This study demonstrated the ability of the RBS spectrometer to realize fast high-temperature measurement and its potential for combustion control applications.

Detection of molecular backscattering with a tapered fiber amplifier based coherent heterodyne lidar

Fiber based coherent heterodyne lidars are highly valued and robust tools especially in sensing of wind speed and turbulence in the atmosphere. The magnitude of aerosol backscattering is also possible to be analysed from the data. However, the aerosol backscattering values cannot be calibrated without the data of molecular backscattering reference, which has not been available earlier due to power and bandwidth limitations. We present the detection of aerosol and molecular backscattering simultaneously with a fiber based coherent lidar instrument utilising a tapered fiber amplifier that yields to a pulse peak power of 1.9 kW at the wavelength of 1053 nm. Further, our receiver bandwidth of 1.5 GHz enables the spectral analysis of aerosol and molecular scattering spectra, which are recorded and analysed for multiple altitudes up to 1 km. The results demonstrate the potential of coherent heterodyne lidars to extend their capabilities toward backscattering and extinction analysis.

Demonstration of a Rayleigh-Brillouin scattering spectrometer with a high spectral resolution for rapid gas temperature detection

A novel, to the best of our knowledge, Rayleigh-Brillouin scattering (RBS) spectrometer based on a virtually imaged phased array (VIPA) with a high spectral resolution is proposed for rapid gas temperature detection. CO2 RBS spectra at gas pressure of 0.5-4 bar were acquired with a spectrum acquisition time of 10 s, and temperature inversion analysis was performed using TENTI S6 model. The root-mean-square error (RMSE) of the RBS profile fitting is less than 2.95%, and the maximum absolute error of temperature inversion is less than 2.45 K. Compared with traditional methods, this method has low RBS signal loss and short acquisition time without the frequency scanning process, which is more conducive to real-time detection applications.

Uncertainty Estimation for the Brillouin Frequency Shift Measurement Using a Scanning Tandem Fabry-Pérot Interferometer

The expanded uncertainty of the measured Brillouin scattering shift frequencies is essential in assessing the measurements of parameters of various materials. We describe the general operation principles of a Brillouin light scattering (BLS) spectrometer with a high-power laser and a scanning tandem Fabry-Pérot interferometer (TFPI) for material characterization. Various uncertainty components have been analyzed for the BLS spectrometer following the Guide to the Expression of Uncertainty in Measurement (GUM). The expanded relative uncertainty in the measured Brillouin frequency shift of 15.70 GHz for polymethyl methacrylate (PMMA) was estimated to be 0.26%. The calculated Brillouin frequency shift (based on material properties of PMMA) was determined to be 15.44 GHz with expanded relative uncertainty of 2.13%. It was shown that the measured and calculated Brillouin frequency shifts for PMMA agree within their expanded uncertainties. The TFPI-based BLS spectrometer can be used to measure the longitudinal modulus of materials with an expanded uncertainty of 1.9%, which is smaller than that of the ultrasonic velocity-based method (estimated to be 2.9%).

Coherent high-spectral-resolution lidar for the measurement of the atmospheric Mie-Rayleigh-Brillouin backscatter spectrum

In this study, a 1550 nm coherent high-spectral-resolution lidar (CHSRL) is developed to measure the optical properties of aerosols and atmospheric wind profiles in the atmospheric boundary layer. To determine the optical properties, a coherent frequency discriminator based on the fast Fourier transform is designed in the CHSRL to separate the Mie and the Rayleigh-Brillouin backscatter spectra to fulfill the needs of high-spectral measurements. The atmospheric wind velocity is retrieved using the simultaneously measured Doppler shift. This non-optical frequency discriminator is a feasible and low-cost solution compared to a narrow-bandwidth optical filter, such as a Fabry-Perot interferometer or an atomic filter. However, shot, amplifier spontaneous emission, and detector noise interfere with the Rayleigh-Brillouin spectrum. Therefore, a spectrum correction algorithm is proposed to recover the interfered Rayleigh-Brillouin spectrum, and the measurement results of the spectral line agree well with those modeled with Tenti S6 at different central frequencies. Finally, field observations for comparison are conducted with the co-located CHSRL, Raman lidar, and coherent Doppler wind lidar. The comparison results indicate that the correlation coefficient of the aerosol backscatter coefficient is 0.84. The correlation coefficient and standard deviation of wind velocity are 0.98 and 0.13 m · s^-1, respectively.

Molecular simulation of Rayleigh-Brillouin scattering in binary gas mixtures and extraction of the rotational relaxation numbers

Rayleigh-Brillouin scattering (RBS) in gases has received considerable attention due to its applications in LIDAR (light detection and ranging) remote sensing and gas property measurements. In most cases, the RBS spectra in the kinetic regime are calculated based on kinetic model equations, which are difficult to be applied to complex gas mixtures. In this work, we employ two widely used molecular simulation methods, i.e., direct simulation Monte Carlo (DSMC) and molecular dynamics (MD), to calculate the spontaneous RBS spectra of binary gas mixtures. We validate these two methods by comparing the simulation results for mixtures of argon and helium with the experimental results. Then we extend the RBS calculations to gas mixtures involving polyatomic gases. The rotational relaxation numbers specific to each species pair in DSMC are determined by fitting the DSMC spectra to the MD spectra. Our results show that all the rotational relaxation numbers for air composed of N_{2} and O_{2} increase with temperature in the range of 300-750 K. We further calculate the RBS spectra for binary mixtures composed of N_{2} and one noble monatomic gas, and the simulation results show that the rotational relaxation of N_{2} is greatly affected by the mass of the noble gas atoms. This work demonstrates that RBS is a promising and alternative way to study the rotational relaxation process in gas mixtures.

Airborne temperature profiling in the troposphere during daytime by lidar utilizing Rayleigh-Brillouin scattering

The airborne measurement of a temperature profile from 10.5 km down towards ground (≈1.4km above sea level) during daytime by means of a lidar utilizing Rayleigh-Brillouin (RB) scattering is demonstrated for the first time, to our knowledge. The spectra of the scattered light were measured by tuning the laser (λ=354.9nm) over a 11 GHz frequency range with a step size of 250 MHz while using a Fabry-Perot interferometer as a spectral filter. The measurement took 14 min and was conducted over a remote area in Iceland with the ALADIN Airborne Demonstrator on-board the DLR Falcon aircraft. The temperature profile was derived by applying an analytical RB line shape model to the backscatter spectra, which were measured at different altitudes with a vertical resolution of 630 m. A comparison with temperature profiles from radiosonde observations and model temperatures shows reasonable agreement with biases of less than ±2K. Based on Poisson statistics, the random error of the derived temperatures is estimated to vary between 0.1 K and 0.4 K. The work provides insight into the possible realization of airborne lidar temperature profilers based on RB scattering.

High-spectral-resolution lidar for measuring tropospheric temperature profiles by means of Rayleigh-Brillouin scattering

A novel high-spectral-resolution lidar receiver based on a Fizeau interferometer and a photomultiplier tube array for tropospheric temperature profiling is introduced. Compared to other temperature lidars, an imaging approach is used to resolve the entire Rayleigh-Brillouin (RB) spectrum without applying frequency scanning techniques. The functionality of the system is demonstrated by means of a nighttime measurement. Atmospheric temperature is retrieved from 4.0 km to 9.2 km by analyzing the measured RB spectra with the Tenti S6 line shape model. The systematic error of the retrieved temperatures is determined to be smaller than 3 K, and the corresponding random error varies between 1.7 K (4.0 km) and 2.3 K (9.2 km) for an observation time of 5 min and a vertical resolution of 0.3 km. Considering the short averaging time and the stable arrangement of the system, the suggested approach is also attractive for future airborne applications.

Improved method for gas temperature and pressure retrieval in Brillouin lidar remote sensing

The Rayleigh-Brillouin scattered spectrum is an important tool for analyzing the temperature and pressure of gas in Brillouin lidar remote sensing. The Tenti-S6 model has been widely used to retrieve atmospheric temperatures. However, the retrieval accuracy of this method is unsatisfactory. We analyzed the influence of several factors on the retrieval accuracy of this method and developed an improved method for temperature and pressure retrieval. First, the Rayleigh-Brillouin spectral baseline was corrected using a new fitting procedure, and an experimental spectrum that is of high coincidence with the line shape of the S6 model could subsequently be obtained. Second, the influence of the Airy function on the retrieval accuracy was analyzed, and the retrieval error could be decreased using the Tenti-S6 model without the Airy function. We found that the gas parameters could be precisely detected under low-pressure conditions. Compared with the traditional method, our improved method could effectively reduce the temperature and pressure retrieval errors. The experimental results of nitrogen scattering in the laboratory and air scattering demonstrate the effectiveness, universality, and viability of the proposed improved method.

Heterodyne high-spectral-resolution lidar

In this work, a novel lidar technique to perform high-spectral-resolution measurements of the atmospheric backscatter is discussed and the first results are presented. The proposed method, which relies on a heterodyne detection receiver, allows us not only to separate the molecular and the aerosol component of the atmospheric backscatter, but also to investigate the spectral shape of the Rayleigh-Brillouin line. As in the case of the direct-detection high-spectral-resolution lidars, the separation of the different scattering processes would allow an independent system calibration and aerosol extinction measurements. The proposed retrieval technique was successfully tested on the Deutsches Zentrum für Luft- und Raumfahrt airborne Doppler wind lidar system with measurements conducted during different measurement campaigns and under different atmospheric conditions. In light of these results, further ideas for the implementation of a dedicated heterodyne high-spectral-resolution lidar are discussed.

Temperature Dependence of the Rayleigh Brillouin Spectrum Linewidth in Air and Nitrogen

The relation between spontaneous Rayleigh Brillouin (SRB) spectrum linewidth, gas temperature, and pressure are analyzed at the temperature range from 220 to 340 K and the pressure range from 0.1 to 1 bar, covering the stratosphere and troposphere relevant for the Earth's atmosphere and for atmospheric Lidar missions. Based on the analysis, a model retrieving gas temperature from directly measured linewidth is established and the accuracy limitations are estimated. Furthermore, some experimental data of air and nitrogen are used to verify the accuracy of the model. As the results show, the retrieved temperature shows good agreement with the reference temperature, and the absolute difference is less than 3 K, which indicates that this method provides a fruitful tool in satellite retrieval to extract the gaseous properties of atmospheres on-line by directly measuring the SRB spectrum linewidth.