Design And Implementation Of Cavity Ring Down Spectroscopy System

Cavity ring down spectroscopy (CRDS) technology,a new indirect absorption spectroscopic technique, has been widely used in atmospheric chemistry, kinetics of chemical reactions, trace gas monitoring, plasma species diagnosis and monitoring of pollutants. After nearly three decades of development, there have appeared commercial instruments based on CRDS in America, but most of the domestic research stll remains at the stage of laboratory. Since the introduction and maintaining of instruments costs a lot, CRDS technology research has positive implications for CRDS instrument localization.Therefore, the thesis focuses on the design and implementation of CRDS, introducing the principle and development of CRDS, system design and construction, circuit test as well as signal analysis.Based on the wavelength scanning method, this thesis builds a micro CRDS system using a near-infrared DFB diode laser as the light source,designes a small chamber absorption cell with a resonant cavity consists of two pieces of 99.99% high mirror, builds a kind of symmetrical optical structure, realizes the AOM trigger circuit used to switch laser, prepares acquisition program for dealing with ring-down signal and describes the workflow of the CRDS system.Circuit testing of the system is completed. The OPA2365-555 monostable trigger circuit and the amplifier circuit are integrated with PIC16F87 auxiliary adjustment after comparing work characteristics of different circuit designed in the thesis through circuit simulation and optical system real signal test. The circuit response time is controlled within 200 ns, thus making laser can be quickly turn off.Software debugging of the system is completed. The ring-down time is obtained by nonlinear fitting of the ring-down curve based on the L-M algorithm. By discussing the effect of the time window and average number of times on the fitting time, the fitting starting point and the average number of acquisition are determined,making the signal-to-noise ratio improved. The absorption spectra of H2O at 6626.47 cm-1, 6626.52 cm-1, 6627.7 cm-1 and 6628.25 cm-1 were obtained, which indicates that the CRDS system set up in this thesis has the ability to detect absorption spectrum of the sample with high resolution.