Laser induced thermal grating spectroscopy thermometry technique

Abstract

In this paper the laser induced thermal grating spectroscopy thermometry technique is investigated. Two coherent, pulsed pump lasers are crossed in NO2/N2 mixture to induce an interference pattern, owing to the resonant absorption and the subsequently quenching effect. The heat released into the bulk gas can modulate the local refractive index (temperature grating). Simultaneously, the sound wave induced by the electric field forms the standing wave (acoustic grating). These two effects mentioned above produce a thermal grating, and a continuous probe laser satisfying the Bragg scattering condition, generates a coherent signal in the crossed region. The spatial and spectral filtering signal is detected with a photomultiplier tube, and displayed with a digital oscilloscope. The signal carries plenty of flow field information. The gas temperature is obtained through frequency analysis. In order to increase the precision of temperature measurement, we calibrate the grating spacing at a known temperature in a pressurized gas cell. Then the temperature in a range of 300-500 K is measured by the laser induced thermal grating spectroscopy technique, and the thermocouple temperatures are recorded at the same detecting point simultaneously. Both of them agree well with each other, though some discrepancies are still existent. The difference is explained according to the heat radiation loss. We also use this technique to measure the gas sound speed directly, which is crucial to studying the gas behaviors at high pressures and the interaction between molecules. In a certain temperature range, the measurement result and the theoretical curve are nearly consistent, which shows the high precision and multi-parameter measurement ability of laser induced thermal grating spectroscopy. The factors influencing the signal waveform are analyzed, too, and the results demonstrate that the signal duration, the signal intensity, and the oscillation peaks increase with pressure increasing. As a consequent, the accuracy of measurement can be improved. Also, other gas dynamic parameters, such as the thermal diffusion rate and the heat conductivity, can also be measured by using this technique. The unique advantage of laser induced thermal grating spectroscopy thermometry technique provides us with a powerful diagnostic tool used in high pressure condition.