Font Size: a A A

Continuous Wave Ring-cavity Ring-down Spectroscopy Research

Posted on:2017-01-07Degree:DoctorType:Dissertation
Country:ChinaCandidate:L H CuiFull Text:PDF
GTID:1108330482991321Subject:Optical Engineering
Abstract/Summary:PDF Full Text Request
The loss of optical cavity is mainly divided into the following aspects, including the diffraction loss, geometric deflection loss, reflection loss, the absorption loss of the cavity and so on. Among them, the diffraction loss that includes the additional loss caused by differences of mirrors size and misalignment of cavity mirrors; light round-trip propagation in the cavity may deflect out from the side of the cavity, this loss as known as geometric deflection loss, for the ray near axis of stable cavity should be zero; taking use of cavity mirror reflection incomplete loss characteristics, also by detecting resonant cavity emission intensity attenuation, absolute value of the reflectivity is obtained, the characteristics has been widespread concern, will become the only means of ultra high reflectivity value measurement. the technology has an absolute advantage in the field of super-high reflectivity measurement technology; due to the characteristics of the medium absorption in the cavity, and for the "fingerprint" absorption spectrum characteristic line of different gas, according to the Lambert Beer law, to obtain that changes in intensity of the gas through the resonant cavity the gas concentration to be measured and related technology includes: Cavity Enhanced spectroscopy and Ring-Down Cavity Spectrometer etc.. Among them, the Ring-Down Cavity Spectrometer(CRDS) is a symbol of the development and breakthrough of trace analytical instrument technology in recent years. CRDS provides a common gas detection method that other methods can’t be compared with, and can be used in the field of demanding field applications. this paper according to the research status so far, this paper take the continuous wave ring cavity ring down spectroscopy as the focus, and the technique is closely related to the major issues were studied, including mode matching and the alignment of optical axis, the characteristics of the ring cavity structure etc., establishes a complete measurement of spectrum measurement system, and a large number of experimental research and application. Based on the study of the factors affected the measurement precision. The main contents of this paper and conclusions can be summarized as follows:1)Some main problems of the high quality passive cavity, composed of two plane mirrors and a spherical mirror, based on cavity ring down spectroscopy measurement technology are studied, concrete including, Firstly, in order to reduce the impact of misalignment parameter and mismatch parameter on measurement base on optical resonator, the influence of the coupling efficiency of a source laser is stabilized to a fundamental cavity mode as a result of two limiting cases was analyzed and derived by using conversion of Gauss beam, mode coupling theory and coordinate transformation theory, including expression of coupling efficiency of fundamental cavity mode as two limiting cases was emerged simultaneously. Secondly, in order to analyze the characteristic of the close optical axis(conjugate optical axis) and the existence condition of resonator composed of multiple reflective mirrors, the existence condition of conjugate optical axis of the multiple-resonator consisting of many flat mirrors is analyzed and derived from the angle of beam conversion coordinate transformation. The results show that a closed ray axis in resonator with odd number of mirrors can exist only if each mirror is suitably aligned, while a closed ray axis always exists in non-planar resonators with even number of mirrors, and the angle of the cavity conjugate axis direction changes due to the misalignment of different mirrors. Thirdly, from the point of view of the optical multi-pass matrix, the incidence direction of the self-conjugate ray of the resonator consisting of spherical mirrors is analyzed. A detailed analysis of conjugated axis of the resonator consisting of two flat mirrors and one spherical mirror is conducted, and the results show that when different mirrors have angle deviations, the closed conjugated optical axis remains in the cavity, the change of resonator axis occurs, and the position and orientation of new resonance surface are given, thereby indicating that in the case of resonator with spherical mirrors there is a self-conjugate ray irrespective of the other flat mirrors positions. All of these will provide theoretical guidance for achieving the high-accuracy alignment and improving the measurement accuracy of spectral measurement technology based on high-quality optical cavity. Finally, this paper not only take the frequency matching angle into account to consider incident light frequency characteristics, because the simple frequency matching can’t take out the limited analysis of the frequency characteristics of the incident, in order to facilitate the subsequent selection of laser parameters determined, we will from the optical superposition principle, respectively from two angles of time domain and frequency domain analysis, given the optical interference process in the cavity, for the analysis of intra cavity optical field properties provides theoretical basis, to provide a more convenient to analyze the theory of may encounter problems.2) This paper developed a continuous wave cavity ring down spectroscopy scheme. Carried on the detailed and comprehensive analysis in view of the characteristics of the ring resonator, given out the cycle characteristics of the ring cavity polarization beam, based on the theory analysis results, determine the composed of two plane mirrors and a spherical mirror high-quality passive cavity parameters, then the continuous wave cavity ring down spectroscopy system measuring system overall design, control system, hardware and software implementation are introduced. Among them, the system use the DFB laser as light source and take use of the temperature-tuning and current-tuning characteristics to achieve the tuning of the laser wavelength in order to achieve "cut-off", omitting the optical switch device; the measurement system with triangular ring cavity as resonant cavity, reducing the direct return to the laser optical surface, eliminating the need for the optical isolator; the control system with high precision wavelength meter completed the laser frequency adjusting and locking, under the premise of ensuring the accuracy, omitting the commonly used method that based on phase modulation lock locking device of the principle of optical resonator with a series of lasers and resonant cavity. For each component in the measurement system of performance testing, the laser tuning off time for 2 microseconds, PZT cavity length adjusting range is 2.5nm, adjusting rate is 1k Hz, the wavelength meter at a wavelength of 1000 nm when its absolute accuracy up to ±0.2pm, and can meet the requirements of high precision laser wavelength calibration, the above indexes meet the requirements of measurement system of the theory.3) the characteristics and application of the cavity adjustment ring-cavity CW-CRDS system is deeply studied. On the basis of the theory mentioned above, in consideration of spectral linewidth of light source, light source spectral linewidth of the characteristics of two kinds of alignment calibration scheme: Based on F-P interferometer and the method based on multi quadrant detector detecting resonant cavity out of the light emitted by the regulation loop, build up the cavity adjustable ring cavity ring down spectroscopy system. The system with control ability of calibration of the optical axis to closed. Taking the design of resonant cavity on the optical software for simulation and analysis, we get the relationship between the diffraction loss corresponding to the position of misalignment mirror and relationship between the cavity exit spot characteristics corresponding to the amount of cavity length misadjustment; we get the mirror size limit on the basement mode distribution characteristics, for the resonator alignment to provide guidance. The more important, for resonant cavity of software cannot be obtained that the cavity axis position of the supplementary analysis, we get the theoretical research and numerical simulation of the influence of the mirror misalignment to the closed cavity optical axis position and get the relationship between changes in position of the cavity axis and the change of the position and the direction of incident light, and preliminary experimental study is carried out, to verify the triangular ring resonator parameters design rationality, reduce the triangular ring cavity alignment difficulty.4) The continuous wave cavity ring down spectroscopy error factors affecting the precision of detection is studied. These factors, including the laser scanning speed tuning system, are analyzed, in order to facilitate the description of laser scanning speed limit, this paper introduces two parameters, scanning velocity parameter ? ?cav cavη ?W ?v ? and bandwidth parameters ?, and numerical simulation between the cavity spectra and η, obtained when η >>1, CRDS closer to an e single exponential decay; the influence of the bandwidth characteristics of the laser on the measurement, the cavity, the peak characteristics of stable ray type, making the ring down more smooth linear monochromatic exponential decay; mainly analyzes the influence of pressure on absorption characteristics of gas, take the gas H2 S as an example, presents how to determine the target gas "no interference" band, give out the methods to determine the optimal operating pressure, and provide a reference for the detection of other gases.5) For the cavity length is 200 mm, and the total cavity length is 410 mm, the ring-cavity ring-down spectroscopy system was taken to the experimental study, we build up the continuous wave ring-cavity ring down spectroscopy system and applied the system to measure the super high reflectivity mirrors and method for using high purity nitrogen of cavity absorption spectra measured mirror reflectivity measurements more accurate values are characteristic, in addition to using of standard gas of mirror reflectivity calibration, in order to determine the exact value of the reflectivity of the mirror, and the measuring system is applied to measure the spectral at 6046.9290cm-1 of methane absorption spectrum. And the application of this system in the field of trace gas concentration detection was studied. And the factors affected the measurement precision was analyzed.
Keywords/Search Tags:Optical measurement, Laser Absorption Spectroscopy, Cavity Ring-Down Spectrometer, Passive Optical Cavity, mode match, trace gas concentration, super-reflectivity measurement
PDF Full Text Request
Related items