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| Research on Spatial Resolution Adjustable USEDCARS Technology |
| ZHANG Zhen-rong, LI Guo-hua, HU Zhi-yun, WANG Sheng, YE Jing-feng, TAO Bo, SHAO Jun, FANG Bo-lang |
| Northwest Institute of Nuclear Technology, State Key Laboratory of Laser Interaction With Matter, Xi’an 710024, China |
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Abstract Coherent anti-Stokes Raman scattering (CARS) is a very important combustion diagnostic technique, for its high accuracy and strong anti-interference ability in combustion field parameter measurement. But when there is a temperature gradient in the detected spatial region, spectra superposition of low and high temperature CARS signal result in spatial average effects which will cause CARS spectral distortion. As a result, it is difficult to analyze the CARS spectrum, and the combustion field parameters cannot be retrieved from the CARS spectrum. In Unstable-resonator Spatially Enhanced Detection CARS(USEDCARS), the lower spatial resolution has limited the application of CARS in flow with a high temperature gradient. Various factors affecting the spatial resolution of the technology were analyzed. The circular beam shaping of the pump laser was carried out by using the axicon group, and ring-shaped pump beam with different diameters was obtained by adjusting the distance between the axicons. Then a USEDCARS diagnostic system with the adjustable spatial resolution was established, and the spatial resolution of the USEDCARS was measured, the distribution of CARS signal strength with the spatial position was obtained. Taking spatial region over 95% of the total CARS intensity to represent the longitudinal spatial resolution, the spatial resolution of the system can be adjusted continuously from 1.7 to 6.5 mm. At the highest spatial resolution state, the spatial resolution achieves the level of BOXCARS technology. The temperature of an alcohol/air premixed flame was measured using the established USEDCARS system. Single-pulse CARS spectra with different longitudinal resolutions were obtained. The high-quality CAR spectrum was achieved when the measured spatial resolution was 1.7 mm, and the flame temperature was indicated by the spectral fitting. When the spatial measurement resolution was 4.9 or 6.5 mm, the CARS signal intensity was very strong, but the CARS spectra were distorted. The results show, the spatial resolution has a large impact on the intensity and spatial average effects of CARS signals, the spatial averaging effect can be reduced by improving the spatial resolution, the continuously adjustable spatial resolution enables the USEDCARS system to better adapt to various combustion flow fields.
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Received: 2019-11-17
Accepted: 2020-04-09
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