Coherent anti-Stokes Raman Spectroscopy (CARS) is applied to thermometry for high temperature gases. A broad-band CARS spectrometer is constructed for shot by shot measurement of a full CARS spectrum of nitrogen. It consists of a frequency doubled Nd:Yag laser, broad-band modeless laser, and optical multi-channel analyzer installed in a double grating monochromator.
The slit function of the detection system is measured and described by an analytical functional form to enhance the fitting goodness of spectra and the accuracy of CARS temperature. The accuracy of CARS temperature is evaluated by comparison with the standard thermometers in a sodium heat-pipe furnace from 300 K to 1300 K and a graphite-tube furnace from 1300 K to 2300 K. A pre-calibrated R-type thermocouple is used to 1300 K and a radiation pyrometer to 2300 K as temperature standards. The accuracy of CARS temperature is $\sim\,2$ \% in the temperature range from 300 K to 2300 K.
To evaluate the CARS spectrometer as an instantaneous thermometer, the intrinsic temperature probability distribution functions (PDF) are measured at steady states of temperatures. The PDF is even narrower than the normal distribution with the variance reduced from the shot by shot spectra. The mean is coincide with the standard temperature within the variance. The intrinsic PDF could act as a basis of diagnosis for turbulent flames.
The spectrometer is converted to a scanning type one to verify usefulness of CARS for diagnosis of the low density plasma. The rotational and vibrational temperature of the neutral nitrogen are measured in the glow discharge. The severe disagreement between two temperatures shows a non-equilibrium property of the glow discharge.
Effect of the erroneous functional forms of the slit functions on the best-fit temperature in CARS thermometry was investigated. When the shape of the slit function was described as a Lorentzian or a Gaussian function and its width was adjusted with the room temperature CARS spectrum, the maximum systematic errors due to the slit functions reached $1.5\%\,\sim\,2.5\%$ in temperature range from 300 K to 2100 K. Noises of CARS signal and non-linearity of optical multi-channel analyzer are discussed to understand the uncertainty of CARS temperature.
A fast fitting algorithm exponentially interpolating the CARS spectrum at an intermediate temperature with two pre-calculated spectra is proposed and its validity was assessed by calculating the error due to the fitting algorithm.