Development And Application Of Multi-Parameter Sensing System On Optical Fiber Cavity Ring-down Spectrosocy

This thesis provides a comprehensive introduction of the historical progress,features, and merits of the cavity ring-down spectroscopy （CRDS） technology aswell as its distinguished performances in spectral absorption measurement andsensing detection; the principle of CRDS technology has been briefly introducedwhile various CRDS schemes have been compared in detail from the aspects of lightsource and cavity structure; moreover, a good variety of CRDS configurations andtheir applications have been introduced followed by a future prospect of thistechnology. The main content of this thesis includes the following aspects:1. The principle and system configuration of CRDS technology has beensystematically described, and the influence of light source, gain module, coupler,and optoelectronic detector on the sensing accuracy has been analyzed fromtechnological perspective; the relationship between ring-down time and theabsorption loss of intracavity sample and sensing element has been investigatedin quantity, furthermore a few fiber-loop-based CRDS sensing system has beenalso analyzed in detail to point out their respective merits and disadvantages.2. Gauss-like fiber microcavity was fabricated in single-mode fibers by using thehigh frequency carbon dioxide （CO₂） laser, and makes it as a kind of new sensingunit access to the1060nm cavity ring-down sensor system for the first time. Theexperimental measurement for temperature and strain double parameters wasrealized. On the basis of it, a three parameter optical fiber sensing system wasdeveloped successfully. The temperature, the refractive index and the strain weremeasured at the same time.3. A CRDS microcavity temperature sensing system has been proposed anddeveloped. This system employs a fiber microcavity fabricated by the highfrequency CO₂laser to serve as the temperature sensing element, and hightemperature sensitivity has been achieved by measuring the ring-down time underdifferent temperature. Furthermore, temperature sensing experiments were conducted in the fiber microcavity with different depth to establish the influenceof the cavity depth on temperautre sensitivity.4. Based on the analysis of optical-time-domain-multiplexing （OTDM） principle andthe system configuration, a multi-channel CRDS sensing scheme has beenproposed，A phase modulation is introduced in different optical ring-downcavity，so that the sensing result of different parameters output in time sequence,the sensor detection of the multiple parameters is realized at the same time.5. Module method is used for the design of the sensing interrogation system. Adetailed envelope detection and peak keeping circuit scheme was presented. Theacquisition of the peak voltage and the envelope demodulation for the CRDSsensing output signal was achieved. Based on the procedures written for the A/Dconverter of the single-chip micro-computer （SCM）, and the C programming forexponential fitting by least square method is realized, and hardware module andsoftware processing was harmoniously combined.