Light Transportation Characteristics In High Diffuse Cubic Cavity

Sensitive gas sensing is of great importance in the areas of environmental monitoring, industral production, food safety and medical care. Among the various gas sensing techniques, absorption spectroscopic techniques have become the main methods for gas sensing, due to its high selectivity, high sensitivity, anti-interference of other gases, non-invasive, and in-situ monitoring advantages. To improve sensitivity of absorption spectroscopy, the most efficient and convinient way is to increase the absorption path length of the sample gas. In this thesis, we developed high diffuse cubic shaped cavities and studied light transportation characteristics in the cavity. This kind of gas cells overcome interference fringes effect, which can not be ignored in multipass cells and cavity enhanced techniques. It also solved difficulties for scattering porous media methods, such as low signal-to-noise ratio, low system consistancy. Compared to an integrating sphere, a diffuse cubic cavity is easy to be manufactured, has low cost, and more convinient to be set in an optical system. We studied effective optical path length characteristics for a cubic cavity and tandem cubic cavities based on Tunable Diode Laser Absorption Spectroscopy(TDLAS) technique measuring oxygen. A novel diffuse reflectivity measurement method using a diffuse cubic cavity was developed.First of all, high diffuse reflectivity as gas absorption cell was studied. A series of diffuse cubic cavities coated with high diffuse anti-water materials, and of different sizes were manufactured. Theoretical formula of Effective Optical Path Length(EOPL) for a cubic cavity in relation with the diffuse reflectivity of the coating r, port fraction f, and single path average path lengthaveL were developed based on radiation transformation theory. The EOPL for a cubic cavity was given by measuring calibration curve between optical parameters and oxygen absorption path length in the open air.Second, EOPL characteristics for a diffuse cubic cavity was studied. First, it was proved by continuous measurement of EOPL in a cubic cavity that light field i n the cubic cavity was stable and uniform. We measured EOPL variation with added port fraction, and three parameters of r, f, andaveLwere obtained by curve fitting based on a theoretical formula. An additional path length was added to the traditional formula of EOPL based on launch and delaunch conditions. Experimental method to obtain the additional path length was developed by measuring aveL variation with simulated additional path length for cavities of different diffuse reflectivity. Revisions for the temporal responds based on launch and delaunch conditions were made.Third, a method for measuring diffuse reflectivity using cubic cavity based on the variable port fraction method was developed. The method can be used to measure the diffuse reflectivity of liquid and solid materials. We measured the diffuse reflectivity of a cubic cavity with scattering coatings of different thickness. The error of diffuse reflectivity was reduced from 0.5% to 0.3% when the diffuse reflectivity increased from 0.867(4) to 0.9887(3). A simulation result manifests that the error of diffuse reflectivity has the potential to be further reduced at higher diffuse reflectivity. To measure materials of low diffuse reflectivity, we can put the sample on a small area on the cavity wall. An average diffuse reflectivity was contributed by the sample and cavity wall of known diffuse reflectivity. Thus the diffuse reflectivity for the sample can be calculated. This method has prosperous application prospects.At last, theoretical and experimental studies for tandem cubic cavities were developed. It was proved by theoretical deduction that the EOPL for a tandem cubic cavity can be seen as the sum of EOPL of the two single cubic cavities when the connection aperture for the tandem cavities was small. Experimental results showed that when port fraction from the connection aperture was smaller than 0.01, EOPL for a tandem cubic cavity can be seen as the sum of EOPL of the two single cubic cavities. Otherwise, the simulation result is not correct.