Research On Characterization Of Mid-Infrared And Terahertz Spectral Imaging And Detection System

The electromagnetic waves in the mid-infrared and terahertz bands have unique spectral characteristics,especially the wavelength range of 5-11 μm and 30-3000 μm.The vibrational and rotational energy levels of many biological macromolecules have intrinsic absorption peaks in this band.Using the fingerprint spectrum characteristics of this band,different substance molecules can be detected,identified and quantified.Therefore,the mid-infrared and terahertz spectral imaging characterization systems have important application value in disease diagnosis and biomedical research.In the experiment,we have built a mid-infrared multispectral confocal microscope,a fiber-type terahertz time-domain spectroscopy system,and a terahertz time-domain spectroscopy system based on pump light field regulation.Theoretically,the focusing characteristics of off-axis parabolic mirror are calculated by vector diffraction theory,the effects of different aberrations and beam offset on the focusing spot shape are analyzed.The main work of this paper is as follows:1.Off-axis parabolic mirrors are widely used in mid-infrared and terahertz optical path systems due to their non-chromatic aberration and wide wavelength range.However,they require high adjustment accuracy,and they are sensitive to aberrations and beam offsets.Therefore,it is necessary to study the light field focusing distribution of off-axis parabolic mirror when building the imaging system.According to the vector diffraction theory of electromagnetic field,focusing on the effects of parallel offset,defocus,and beam tilt on the spot distribution at the focal point.Theoretical calculation results show that under the condition of oblique incidence of the beam,the spot shape on the non-focal plane will be greatly distorted and result in an elliptical point spread function,while the defocus and beam parallel offset have little effect on the spot shape.These results provide guidance for the optical path construction of mid-infrared and terahertz systems.2.According to the focusing characteristics of the off-axis parabolic mirrors,we have built and optimized the mid-infrared multispectral confocal microscope.The light source is a wavelength tunable quantum cascade laser with higher brightness,and the detection adopts a broad-spectrum mercury cadmium telluride detector with high sensitivity.In addition,the system integrates scanning,control and data acquisition modules.Using this system to perform a multispectral imaging characterization test on the mid-infrared vibrational absorption peak of the epiretinal membranes,which provides the reference for studying the pathogenesis of related diseased tissues.3.We have built a compact fiber terahertz time-domain spectroscopy system,we use a grating pulse width stretcher to compensate for the nonlinear effect of the optical fiber,the spectrum range of the experimental test is 0.1-4 THz.In terms of imaging,we perform hyperspectral imaging characterization of biological tissues,combined with the K-means clustering segmentation algorithm of the image,to calculate the absorption coefficients of different regions,and realize the qualitative analysis of different regions.4.We have built a light field modulated terahertz time-domain spectroscopy system for the first time.The 780 nm spatial light modulator is used to modulate the phase of the laser,the modulated structure light precisely matches the interdigital array antenna to generate terahertz.The pump light utilization and emission power of the photoconductive array antenna are improved,and the binary modulation of terahertz signal phase is realized.In addition,the combine degree,detection power increase multiple,signal-to-noise ratio of different numbers of modulated light fields are analyzed in detail,these results provide a reference for the research of power array combine in terahertz band.