| This thesis provides a comprehensive introduction of the technical background, characteristics, advantages, and historical progress of cavity ring down spectroscopy (CRDS); moreover, the applications of CRDS in high reflectivity measurement, atomic and molecular absorption spectroscopy, analytic chemistry, mark gas detection, and atmosphere environmental monitoring have been also generally discussed about; furthermore, CRDS technology and its advantages have been briefly introduced. The main content of this thesis include the following aspects:1. CRDS techniques based on the measurement of optical intensity and phase have been systematically introduced; from the technical perspective, the influence of parameter selection of light source, gain amplifier, coupling components, and detector along with the technical features on the measurement resolution and range have been analyzed; and in particular, the absorption cell—ring down cavity is classified into the cavity based on evanescent waves, the cavity with micro-cavity, and the cavity based on fiber gratings. The above three configurations have been summarized in detail, and their respective merits have also been pointed out as well as the disadvantages.2. The evanescent wave model using complex refractive index is presented to calculate the absorption loss of flow, and a four-layer refractive index model has been set up by using the finite element method to acquire the complex refractive index, namely the extinction ratio; furthermore, according to the Beer-Lambert law and evanescent wave principle, the absorption loss and transmission loss of the fiber have also been calculated, based on which the evanescent field leakage and transmission characteristics for the cladding-etched fiber have been investigated as well as the influence of cladding diameter of the HF-acid-etching fiber on its transmission loss has been quantitatively analyzed3. By using HF acid to etch the fiber inside the cavity, absorption cells with partially eradicated cladding are formed, based on which the temperature-immune 980nm fiber CRDS system and 1560nm pico-second pulse fiber CRDS system have been designed and experimentally achieved. With a 5-microliter injector to control the fluid concentration, the refractive index of DMSO solution as a function of the transmission loss has been experimentally calibrated. The high-precision concentration measurement of DMSO solution is achieved with a sensitivity of 1×10-3dB.4. A novel gas sensing technique based on the fiber evanescent sensor coated with carbon nanotubes is presented and experimentally validated. Based on the progress and characteristic introduction of the carbon nanotube materials, the preparation method of carbon nanotubes, the approach to assemble the carbon nanotubes with fibers, and their applications have been summarized. An assembly scheme is presented, which needs to preprocess the carbon nanotube materials with front opening considering the adsorption and de-adsorption characteristics of carbon nanotubes under high temperature. And by using this method, preliminary real-time measurement of gases and VOC is realized.5. A novel low loss measurement method based on the phase-shifted long-period grating (PS-LPG) is presented and experimentally validated. The mode coupling and refractive index sensing characteristics of the long-period grating are analyzed by using the coupled mode theory, and the transmission characteristics of the PS-LPG is analyzed based on the transfer matrix method. According to the theoretical results, appropriate grating pitch and length parameters are selected, and the PS-LPG with low loss ((Loss<-0.74dB) and a resonance wavelength of 1560nm, is inscribed onto a single-mode fiber under exposure of focused high frequency CO2 laser pulses. By introducing this grating into a 1560nm narrow line-width pulse CRDS system, the fragile structure of absorption cells with fiber cladding partially eradicated is avoided, and thus real-time measurement of environmental refractive index and solution concentration is realized.6. A fiber CRDS system based on the phased-shifted ultralong-period grating (ULPG) is proposed and experimentally validated. By exploiting the larger sensitivity of the resonance wavelengths resulting from the mode coupling between the core mode and higher order cladding modes, the low loss PS-ULPG fabricated by high frequency CO2 laser is introduced into a CRDS system as the absorption cell. According to the simulation results based on the coupled mode theory, appropriate grating parameters are selected, and the ULPG and PS-ULPG with resonance wavelengths of 1560nm are written onto single-mode fibers, which have been employed in a 1560nm narrow line-width pulsed fiber CRDS system. Through measuring the ring down time of the liquid with different refractive indices, the real-time measurement of environmental refractive index and solution concentration is achieved.7. Based on the present study on single-parameter refractive sensing of the flow with single constituent, the multi-parameter sensing system with multi-constituent is proposed based on the CRDS technology equipped with dynamic measurement and optimal design of sensing units. The typical scheme is presented and its preliminary application has been attempted as well.
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