A Study On Optical Properties Of Caustic Beams

In this thesis,we mainly research on the construction and optical properties of caustic beams.In this paper,an experimental scheme for generating partially coherent caustic beams using two different light sources,laser and quasi-monochromatic LED,is designed.Meanwhile the effect of the coherence length on the optical properties of the caustic beam and the spot variation trend of the caustic beams after different transmission distances are discussed.In addition,the self-reconstruction properties of caustic beams are studied systematically.The main contents of this research are:1.This paper proposes an experimental scheme of using laser as a light source to produce partially coherent caustic beams.The research range of the caustic beam is extended from coherent beam to partially coherent beam.Based on the cross-spectral density function and Fresnel-Kirchhoff diffraction integral theory,the analytic expression of partially coherent caustic beam is derived.Numerical simulation with MathCAD software and the influence of astigmatism introduced by tilted axicon at various inclination angles on partially coherent caustic beam with respect to the propagation axis was studied theoretically and experimentally.Furthermore,we also invested the partially coherent caustics beam that is effected by the field coherence on the partially coherent caustic beam.Results show that the intensity contrast ratio between light focus and the surrounding dark regions can be reduced with the decrease of the spatial coherent lengths.2.The quasi-monochromatic red LED is used as the light source,and the quasimonochromatic LED light is converted into partially coherent light by means of small aperture and long distance transmission,and then the incident axicon is obliquely inclined to generate a partially coherent caustic beam of the LED.A method of using cross-spectral density function and superposition of multi-spectral is proposed to calculate the intensity distribution of the LED partially coherent caustic beam through the axicon.The influence of parameters such as dispersion is theoretically and experimentally demonstrated.The influence of the transmission distance on the LED partially coherent caustic beam is further studied.It is found that the outer contour of the caustic beam becomes larger and the central light spots symmetrically split into light spot array as the transmission distance increases.Meanwhile,the central intensity decreases correspondingly,and the contrast of the light spot array decreases gradually.It will evolve into an approximately hollow beam eventually.This research results enrich the connotation of caustic beam and broaden its research field.3.The paper systematically studies the self-reconstruction characteristics of the caustic beam.Based on Fresnel diffraction integral theory and Babinet principle,the general expression of the intensity distribution of caustic beams passing through a circular obstacle is derived.The self-reconstruction process of caustic beam is simulated based on the analytical expression,and relative experiments are verified.The experimental phenomena are in good agreement with the theoretical prediction.The results show that the reconstruction of the zero-order caustic beams will occur after passing the on-axis and off-axis obstacles.And as the transmission distance increases,the outer contour size of the caustic beams become larger,the number of central spot arrays increases,and the complete beam is gradually reconstructed.The beam is observed during the reconstruction process.The reconstruction speeds of the horizontal and vertical directions are different,and there is a certain speed difference.Finally,the high-order caustic beam is generated by the spiral phase plate,and it is verified that the high-order caustic beam has the same self-reconstruction characteristics after being shielded by obstacles.The research results broaden the research range of caustic beam,and provide a theoretical basis and reference value for the study of caustic beam.