Study On Optical System Of Near-infrared Spatial Heterodyne Raman Spectrometer

Spatial heterodyne Raman spectroscopy is a new type of Raman spectroscopy detection method that combines spatial heterodyne interference spectroscopy and Raman spectroscopy.Compared with dispersive and traditional Fourier transform Raman spectrometers,it has high throughput,high spectral resolution,no moving parts and other characteristics.It is suitable for high-sensitivity detection of weak Raman scattered light signals.Fluorescence is generated while exciting Raman scattered light.The stronger fluorescence will cover the weak Raman scattered light signal.The fluorescence intensity under the excitation light source of different wavelengths is different.The excitation light source in the near-infrared band can effectively reduce the influence of fluorescence on weak Raman scattered light signals and improve the quality of Raman spectra.In this paper,we study on a near-infrared spatial heterodyne Raman spectroscopy technique with low fluorescence background and high sensitivity.It mainly includes the selection and analysis of the wavelength of the excitation light source in the near-infrared band,optical system design and simulation modeling of a wide spectral range and high sensitivity spatial heterodyne Raman spectrometer,theoretical analysis and experimental verification of the relationship between fiber core diameter and system throughput,the development of the experimental prototype and the measurement and analysis of Raman spectroscopy,etc.The specific research content of the article is as follows:1.Outline the basic principles of Raman spectroscopy,compared with the analysis methods of common Raman spectroscopy techniques,and the research status of spatial heterodyne Raman spectroscopy technology is described in detail.The Raman scattered light signal intensity,fluorescence signal intensity and water absorption phenomenon of excitation light sources in different wavelength bands were analyzed.The wavelength of the excitation light source for low fluorescence background Raman spectroscopy detection was determined to be 830 nm.The system structure and basic principle of spatial heterodyne Raman spectroscopy are introduced.The parameters of the instrument,such as spectral resolution,spectral range and spectral resolution ability are analyzed.The methods for widening the spectral range of the spatial heterodyne spectrometer were compared,and the double-grating spatial heterodyne spectrometer was selected to realize the Raman spectrum measurement with a wide spectral range.It can be seen as a simple superposition of two spatial heterodyne spectrometers.Based on the throughput requirement of the spatial heterodyne spectrometer for the Raman probe,the basic relationship between the diameter of the fiber core in the Raman probe and the throughput of the system is deduced.2.In order to realize the detection of Raman spectrum with wide spectral range,high spectral resolution and high sensitivity,a double-grating spatial heterodyne Raman spectrometer system was designed using two sub-grating with grating line densities of150lp/mm and 125lp/mm,respectively.The spectral sampling interval is 3.09cm^-1,and the Raman shift range is 171.71～4128.75cm^-1.According to the design results,the whole system is simulated and modeled.When two sub-grating work in the same field-widened prism,the effects of different incident field-of-view angles on the contrast of the two sub-interferogram are analyzed.At the same time,the simulation results of the polychromatic light source verifies that the system can realize the Raman spectral detection of a wide spectral range while measuring with high spectral resolution.3.Developed a single-channel spatial heterodyne Raman spectrometer prototype for the single grating with a grating density of 150lp/mm,completed the optical design,tolerance analysis and prototype installation of the entire system,and completed the basic index test and Raman spectrum measurement experiment.The measured spectral resolution of the prototype is 5.05cm^-1（trigonometric apodization）,and the detected Raman shift range is 171～2048cm^-1.Using fibers with core diameters of 200μm and1000μm,the relationship between the fiber core diameter and the system throughput was verified.The measured value of the increase in throughput is 7.94%lower than the theoretical value,which is consistent with the theoretical analysis results.It is shown that increasing the diameter of the fiber core can significantly increase the throughput of the spectrometer and further improve the sensitivity of Raman spectroscopy.Samples such as cyclohexane,calcium carbonate,glucose powder,75%alcohol solution were tested,and clear and sharp Raman spectra were obtained.The effects of integration time,excitation light source power,and average number of interferogram merged lines on the signal-to-noise ratio of Raman spectra were analyzed.The experimental results show that the relationship between the three influencing factors and the signal-to-noise ratio is approximately the square root function.The component identification test was carried out using petroleum samples from different regions,and the differences between different samples can be distinguished by comparing the Raman spectrum characteristic peak position,characteristic peak intensity and other information.The quantitative analysis experiments were carried out using glucose solution.The correlation coefficient R² between the characteristic peak area of the high concentration solution and the concentration can reach 0.99,and the correlation coefficient R² between the characteristic peak area of the low concentration solution and the solution is 0.61.