Research On High Sensitive Spectral Measurements And Instruments Design Based On Optical Waveguide

Spectroscopy is an important analytical tool for material analysis and composition analysis,and is widely used in various fields.This paper mainly focused on the theory and key technology in spectral measurement,as well as the method of instruments design.The aim of the study was to explore the high-sensitive detection method based on optical waveguide and realize the sensitive detection of trace substances.This paper made full use of the characteristics of the double-sided metal cladding waveguide,and realized highsensitive spectral detection methods and high-performance spectral instruments,which includes the enhancement of the weak optical signal realized by the field enhancement effect of the double metal-cladding waveguide,the trace detection of material based on the sensitive response of attenuated total reflection(ATR)curve to the structure parameters of double metal-cladding waveguide,the fast accuracy wavelength calibration method based on the wavelength selection of double metal-cladding waveguide,and a high resolution micro-spectrometer based on the wavelength dispersion ability of optical waveguide structure.In order to assist in the measurement,a large numerical aperture high-throughput folded transmission spectrometer was also built.The main work of this paper is as follows:(1)The possibility of enhancing the weak light signal of Raman scattering or fluorescence by using double metal-cladding waveguide was studied.The mechanism of the electromagnetic field enhancement in the metal clad waveguide was theoretically analyzed,including the enhancement caused by the resonance field in waveguide when guided mode is excited and the enhancement of radiation field of point source when it is placed in the guiding layer of waveguide.Based on this theory,the resonance field distribution of guided mode and the radiation field distribution in waveguide were numeric calculated.The maximum enhancement coefficient in different modes was calculated,the influence caused by thickness of the metal coupling layer and guiding layer was studied as well.Then an experiment was carried out to show the enhancement of Raman scattering by using a double metal cladding waveguide.The enhancement in this experiment was about 20 times,which agreed to the theoretical analysis.Finally,the influence of surface roughness of metal coupling layer in waveguide on energy coupling efficiency was studied,including the derivation of an approximate expression of second order scattering field caused by rough surface and a contrast experiment of Raman enhancement in waveguides with different surface roughness.According to the theory and experiment,it was found that the second order scattering field can make an improvement of coupling efficiency,and the top surface of metal coupling layer with appropriate roughness may make a further enhancement of Raman scattering signal.The method can effectively solve the problem of low detection depth and low repeatability stability in the surface enhancing technology.(2)An ultrahigh sensitivity concentration measuring method by using double metalcladding waveguide was proposed.The method works on the basis of high sensitive response of ATR curve to the changes of structure parameter of waveguide.The sensitivity analysis was studied theoretically.It is found that a resonance field sensor will be more sensitive to the change of dielectric coefficient than evanescent field sensors by a contrast numeric simulation of different sensor structures in ATR method.A metal cladding waveguide sensor was designed,and a rotating platform for ATR detection was built,including circuit design and software coding.The linear relationship between the imaginary part of the dielectric constant and the concentration is deduced.According to the relationship,a concentration measuring experiment was carried out,and get an ultrahigh sensitivity.In the experiment a detection limit of hexavalent chromium reached 0.064/7)7).The method has a compact system structure and a small sample demand.The sensitivity is close to that of some traditional large instruments.(3)A wavelength calibration method based on optical waveguide was put forward.First,the reflection spectrum of optical waveguide was deduced and numeric calculated.The spectrum contains a uniform distribution spectral lines,and is easy to expand.Benefit to these properties,optical waveguide can be applied to high accuracy wavelength calibration.Then the spectral properties and design procedure of the waveguide calibrator was deduced and carried out.Finally,two calibration experiments were presented: straight calibration based on standard spectrometer and auto-corrective calibration by aligned with standard atoms lines.Both methods are with high accuracy and able to reduce the influence of the wavelength shift caused by the perturbation of waveguide structure.This calibration method can effectively solve the problem caused by the limited number and the uneven distribution of standard spectral lines used in the traditional calibration method.The problem will result in a difference calibration accuracy distribution in the whole spectral range of spectrometer.The calibration device is compact,the optical structure is simple,and the calibration process takes a short time.(4)A concept of miniaturized high resolution spectrometer was proposed.The spectrometer contains a recursive light path based on mirror and Fabry-Perot cavity.The relationship between the spectral resolution,wavelength range,number of wavelength channels and measured spectrum geometric assignment were also deduced.Then a high reflectivity film and a Fabry-Perot cavity film stack were designed to build the spectrometer.On the basis of above researches,a multi-channel Fabry-Perot cavity array layout was designed to expand the wavelength range of spectrometer.Finally,a wavelength calibration method for this type of spectrometer was introduced.The method can improve the love optical energy transfer efficiency in the spectrometers using the Fabry-Perot cavity.(5)In order to improve the throughput and spectral resolution,a design based on folded transmission path layout was presented.A three-lens objective for such an optical path was also designed and optimized.According to the performance test,it can be proved that the spectrometer has high-sensitivity and high spectral resolution.In order to make the measurement of Raman spectroscopy more convenient,an autofocusing platform for a portable Raman probe was developed.Three kinds of focusing evaluation factors were selected to be applied in a fast autofocus algorithm.Through the experiments,the accuracy of these focusing evaluation factors in the algorithm was analyzed.Finally,the hardware circuits and software for spectrometer controlling and spectral data processing was designed.This designed can effectively improve the throughput of the spectrometer,and maintain a high spectral resolution.