The Research Of Spatial Heterodyne Raman Spectroscopy

Raman scattering is an inelasticity light scattering phenomenon,which reflects the vibration,rotation or other low frequency modes of molecules or crystal lattices.Compared with infrared absorption spectrum or fluorescence spectrum,the characteristic bands of Raman spectrum are much sharper.The standoff and safe detection of poisonous chemicals,dangerous explosives,minerals and biomarkers has high scientific values in national defense security,Earth and planetary exploration.Most of today’s Raman systems are in situ analysis system,which cannot satisfy the requirements mentioned on those targets.A new type of standoff Raman spectroscopy will provide a safe,efficient and accurate method for those targets detection.The research upsurge has experienced prism or grating spectrometer with single pixel(1920-1980),Fourier transform spectrometer(1980-1990),grating spectrometer with multichannel(1990-2010)and the spatial heterodyne Raman spectrometer(2010-today)discussed in the thesis in the history of Raman spectroscopy.Fourier transform spectrometers cannot be used together with time-resolved technique because of their moving mirrors.Limited by their narrow slits,dispersive grating spectrometers cannot achieve high throughput.Since 2011,SHRS has become one of the research hot spots in the fields of Raman detection and Raman application due to its characteristics of wide field of view,high spectral resolution,high throughput,compact and small package,no moving parts and suitable for standoff detection.The research targets of the thesis are to verify the detection ability of SHRS and realize the breadboard and prototype of SHRS with the advantages of high spectral resolution,wide passband and standoff ability.The main parts are consisted of the mechanization analysis of SHRS,one dimensional(ID)detection research,two-dimensional(2D)detection and the design of small-sized prototype.The main parts of the thesis are as follows:1.The detection principle of SHRS.The basic detection principle of 1D SHRS,bandpass broadening method based on the 2D interference are analyzed.The processing methods for 1D and 2D interference data are built.The traditional 1D flatfielding,zero filling and Fourier transform are expanded to 2D.For raw Raman spectrum,the processing method of spike removal,white noise reduction and baseline removal are built.Three bottleneck problems in the design of small-sized SHRS are proposed.The relationship between the grating order overlap and the design difficulty of the imaging lens are given in theory.The reason why spectral calibration model for narrow band SHS is not suitable for SHRS is given.A new model is built by expanding the first order spectral model to the second order.2.1D SHRS detection experiments.The possibility of in situ detection is proved based on the 1D SHRS experimental breadboard.The Raman spectra of pure samples and natural rocks are acquired.The realized spectral resolution is better than 5 cm-1.The 1D interferogram data processing method is tested with the achieved results.After processing,the recovered spectrum is smoother than the spectrum recovered with the raw interferogram.The signal to noise ratio(SNR)can be improved by 30%.The standoff detection experiment is realized by adding a telescope.The Raman bands of natural rocks,chemical warfare agents are achieved at a distance of 10 m.The SNR is better than 5 and can satisfy the requirement of quantitative analysis.3.2D SHRS detection experiments.The passband broadening with 2D interference is proved by the SHRS breadboard.The realized Raman shift range is better than 3000 cm-1 at the spectral resolution better than 4 cm-1.The 2D processing method is tested with the achieved results.The SNR can be improved by 20%.The feasibility of Stokes and anti-Stokes Raman spectra simultaneous detection is analyzed in theory.The simultaneous detection of Raman shifts lower than 500 cm-1 can be realized.4.The optimization design of small-sized spectrometer and performance evaluation.An integrated SHRS prototype has been designed.The main issues of diffraction order overlap,optical design of imaging lens and wide bandpass spectral calibration are resolved.A small interferometer,a collimation lens and an imaging lens are designed.The performance is evaluated for the three main parts of the prototype.After assembling,the spectral resolution calibration and experimental research are carried out.Pure and natural samples are all tested.The achieved Raman shift range is about 50-3990 cm-1,and the spectral resolution is about 4.68 cm-1.When the integration time is set to 2 s and the laser power is set to 300 mW,the SNRs of main peaks of cyclohexane and ammonium nitrate is better than 150.The standoff detection ability of the prototype is also proved.Raman bands of some minerals,shells and chemicals are achieved at a distance of 5 m with the integration time of 8 s.The estimated SNR can be higher than 50.