Studies On Raman Gas Analysis Systems With A Parabolic Reflective Cell

Raman gas analysis is a kind of sensing technology based on spontaneous Raman scattering,it becomes more and more popular because of its real-time response and no-invasion for samples.However,the small Raman cross section and low molecular number density of gases lead to weak singals,which hamper its ability of detecting trace gases.In order to conquer this problem and get a stronger Raman singal,researchers have been dedicating to seek approaches of signal enhancement.In our works,the traget is seeking the balance between singal enhancement and background suppression for Raman system to realize the improvement of limits of detection.The collecting system for weak Raman singal normally has largeétendue,which is also needed for the applied spectrometer.So,a Raman spectrometer with large NA was designed with transmitting volume phase holographic gratings and imaging lens.By the test with a spectral calibration lamp,it was found that the Raman shift range of our spectrometer is 220-4228 cm^-1 when the wavelength of exciting laser is 532 nm,and the averaged resolution is5.17 cm^-1.The Raman systems based on lens and metal-lined capillary were designed and studied.Though the Raman signal levels acquired by the lens system with different focal length are nearlly the same,the len with short focal length is needed to improve the spatial resolution and suppress the signal background of Raman system.The signal level with a capillary of length 250 mm is three folds of that from the lens system.However,due to the strong background,the SBR(signal to background ratio)of the capillary system is decreased by12%compared with lens system.Due to the low signal enhancement of lens system and strong background of capillary system,a parabolic reflective cell constructed by a parabolic reflector and a planar reflector was designed and studied.Compared to lens,the parabolic cell has larger collecting solid angle,which means higher collecting efficiency.Besides,the parabolic cell has longer distance between the collecting area and the cell wall than capillary,so its background level is lower.The experimental results show that,the SNR(signal to noise ratio)of the signal acquired by parabolic cell is increased by 203%and 22%respectively compared to the lens system and the capillary system,and the SBR is increased by 528%and 595%.The parabolic cell shows good balance between signal enhancement and background intensity.In order to improve the performance of parabolic cell further,the collecting efficiency of parabolic cells with different pairing of p(semi-latus rectum of parabolic cell)and d(offset of collecting aperture to symmetry axis of parabolic cell)were studied by theoretical analysis and Zemax calculation.The experimental data are consistent with the theoretical predictions and show that,the parabolic cell with p=10 mm&d=10 mm has the highest SBR,which demonstrates that it has the best balance between the signal enhancement and background suppression compared to other cells.The limit of detection of 21 ppm·bar for C-H gases was acquired by it.It was shown by the experimental results that,the parabolic cell with smaller p has stronger collecting ability,and when d=p,the collecting area for forward and backward scattering radiation and the optical path of laser are symmetrical,the irregular scattering from the unsymmtrical optical path can be avoided,the cell has relative low background and temperate collecting ability.