Optical Path-length Retardation Of Cubic Cavity Structure

In recent years, monitoring of trace gases concentration becomes a hot topicwhich is concerned by all the country. It refers to the atmospheric pollution, industrialproduction, medical health and many other aspects. It has important significance tohuman life and environment protection. The TDLAS technology has been widely usedby the field due to the advantages of good selectivity, high sensitivity and fastresponse time. According to the Beer-Lambert law, increasing the optical path lengthfor gas absorption is the most direct and effective way to monitoring gas of lowconcentration. The technology of traditionally increasing the optical path length wasreplaced by the integrating sphere technique which developed in recent years.However, the production process of integrating sphere complicated and it wasexpensive. Therefore we developed a high diffuse reflection cavity, it was convenientand easy to adjust parameters. In the thesis, we measured the absorption signal ofoxygen in the air based on TDLAS technology, using a absorption cavity as themulti-pass cell to realize the growth of gas absorption effective optical path length. Itfocused on researching the laws of growing the effective optical path length by cavity.We studied the monitoring of O2using spectroscopic technique. The spectralabsorption line of760nm was selected as the studied range based on the study of theabsorption spectrum of O2. According to the absorption spectrum of O2and theBeer-Lambert law, we established a method to measure oxygen using TDLAStechnology, and set up the experimental system and device. We measured the directabsorption signal of oxygen in the air at different distance, and obtained the linearrelationship between the optical absorption path length of oxygen absorption underdifferent distance and optical parameter.We developed the research on the laws of rising gas optical path length using thecubic cavity. In theory, by the analysis of light transportation theory in the cavity, weverified the applicability of the theoretical formula for the effective optical path length.In experiments, we established the oxygen measurement system of cavity using cavitybased on TDLAS Technology. The effective optical path variation of a cubic cavity atdifferent incident location and detection position was detected. It proved that theuniform and stable light field was formed in the cavity, EOPL as a function of thestructure parameters such as the port fraction, the side length, the reflectivity of thecoating in cubic cavity was developed. The results show that the relation between EOPL and the port fraction in the cubic cavity was regularly inversely proportional.Add the EOPL is proportional with the side length the cubic cavity and the reflectivityof the scatter layer. The EOPL of20times the side length of the cubic cavity wasobtained when the reflectivity of the inner coating was97%and the port fraction was0.006. Higher precision of the measurement can be achieved by increasing EOPL,which can be obtained by decreasing the port fraction, increasing the side length ofthe cubic cavity and the reflectivity of the coating simultaneously.We developed the research on the laws of EOPL of a double cubic, by changing theposition and diameter of the aperture on the connection baffle. It shows that positionof the aperture, does on influence the EOPL of the double cubic cavity. However, withthe change of the aperture diameter from small to big, the internal surface area ofdouble cubic cavity was decreased continuously. EOPL showed a slow downwardtrend. Although the EOPL of a double cubic cavity was smaller than the sum of twosingle cubic cavity, it was greater than EOPL of the rectangular cavity with the samevolume.