Light Propagating Characteristics In High Difuse Cubic Cavity And Measurement Of Low Concentration Oxygen

Sensitive gas monitoring of low concentration is significant in the fields of environmental protection, medical care and industrial production. Tunable Diode Laser Absorption Spectroscopy(TDLAS) is becoming a mainstream technique in spectral detection for the advantages of high sensitivity, high resolution and fast response. Gas cells with longer absorption path length is very beneficial to reduce the minimum detection limit and increase system sensitivity. In this thesis, we studied light propagating characteristics in high diffuse cubic cavity and achieved highly sensitive measurement of low concentration oxygen with a high diffuse cubic cavity as the TDLAS gas cell.The TDLAS system with a high diffuse cubic cavity as the absorption cell was built. Sealed cubic cavities of different sizes and sprayed with Avian-D organic coatings were manufactured. The calibration curve of Effective Optical Path Length(EOPL) was obtained by measuring the second harmonic signals of oxygen in the open air at different absorption distance. It was found the cubic cavities extent the optical path length by 17 times.Research on light transportation characteristics in diffuse cubic cavity was developed. Light propagating characteristics in high diffuse cavity were derived from the Beer-Lambert law in the case of weak absorption. EOPL of the cubic cavity was determined and the port fraction was measured through EOPL fitting experiment. The influences of gas concentration and EOPL on light transmission characteristics were studied and it’s observed that light in cubic cavity doesn’t propagate according to Beer-Lambert law unless the absorbance was smaller than0.011.Research on sensitive detection of low concentration oxygen was developed.LabVIEW was introduced in TDLAS system where a high diffuse cubic cavity was used as the gas cell. The performance of TDLAS system was greatly improved by optimizing the modulation parameters, tuning parameters and lock-in amplifier parameters. Using 35 cm high diffuse cubic cavity as the gas cell in TDLAS system,the oxygen detection limit was reduced to 350 ppm and the system fluctuation was0.9%.