Research On The Scattering Of Partially Coherent Beams Upon Random Media

Light wave scattering refers to the case that when the beam is incident into a transparent material,the scattering transmission medium makes the beam deviate from its original direction and scatter to other different directions.The phenomenon mentioned above is called the scattering of beams.Since the 1980 s,the theory of light wave scattering has made great progress.From the perspective of beam waves,the researchers have expanded types of incident beam,extending the plane light waves that are often studied in early years to more complex and more application-oriented light waves,such as pulsed or partially coherent beams.From the perspective of scattering medium,the scope of research has been broadened from classical deterministic medium to random medium,hollow or multi-particle medium,and so on.One of the characteristics of scattering medium is that the refractive index can significantly affect the far-field optical properties.The generous changes make light scattering widely used in optical imaging,control,communication and other fields.The scattering properties will seriously affected by discontinuity of the refractive index in the scattering medium because the process is the interaction between beams and the transmission medium.Quasi-homogeneous random medium is an earlier scattering medium model,and its previous research mainly focused on soft boundary medium,which means a Gaussian-distribution boundary.Under this research background,based on the first-order Born approximation,we aim to explore the influence of the refractive index of mediums on the scattering characteristics,and research on the statistical characteristics of quasi-homogeneous medium with different partially coherent beams passing through semi-soft and semi-hard boundaries.Main contents of this dissertation are as follows:(1)The scattering of partially coherent anomalous hollow beams in semi-soft boundary quasi-homogeneous media is studied for the first time.The analytical expression of the angular spectrum function in the scattered field is derived,and the influence of the incident beam and scattering medium parameters on the far-zone scattered field is studied in detail.The numerical simulation results show that the spectral density distribution of the scattering field is significantly affected by the transverse coherence width of the incident beam,the eccentricity of the incident beam and the correlation length of the scatterer.The coherence of the scattering spectrum is affected by the effective radius,correlation length and edge softness of the scatterer.It is worth noting that the far scattered field is only coherence in a small part of the beam center,and the degree of coherence is circularly distributed.In addition,the center of the scattering spectrum intensity distribution does not have a special hollow distribution after being scattered by the quasi-homogeneous medium,but becomes an adjustable ordinary dark hollow distribution.This is different from the propagation characteristics of anomalous hollow beams in other media.(2)The far-zone scattered field distribution of partially coherent radially polarized vortex beams passing through a semi-hard boundary random medium is studied under the precise condition of first-order Born approximation.In this paper,the cross-spectral density(CSD)matrix in the scattered field is derived,and the spectral intensity,the polarization degree,the polarization angle,and the formula of the ellipticity in the scattered field are simulated.The influence of the parameters of the partially coherent radially polarized vortex and the semi-hard boundary quasi-homogeneous medium parameters on the far-field scattering characteristics above,and verify the existence of the phase singularity in the scattered field are discussed in detail.The results show that the scattered field in the far-zone is circularly polarized with a Gaussian distribution,and the focusing of the scattered field is controlled by the coherence of the incident beam.The scattered field is fully polarized in the center,and the degree of polarization will fluctuate with the change of the coherence length of the incident beam,the correlation length of the scatterer and the summation index at the edge of the scattered field.(3)The far-field characteristics of pulses scattered upon a quasi-homogeneous medium with a semi-hard boundary are studied.The temporal mutual coherence function,spectral intensity,and temporal coherence degree expressions of the scattered field of a partially coherent Gaussian pulse scattered upon a medium are derived.The spectral intensity distribution and temporal coherence degree distribution in spatial and temporal domains are simulated.The results show that the spectral intensity distribution of scattered light in the spatial domain converges with the increasing spatial coherence.In the time domain,the normalized spectral intensity distribution is only affected by the initial pulse width.The spectral distribution range in the time domain increases as the initial pulse width increases.At the same time,the temporal coherence of the scattered field is only related to the temporal parameters of the incident pulse.As the initial pulse width of the incident pulse increases and the coherence time extends,the coherence time of the scattered field becomes longer and the attenuation becomes slower.The spectral intensity distribution,spectral coherence characteristics and spectral polarization characteristics can further enrich the scattering theory,and can be applied in the fields of particle capture,medical diagnosis and obtaining the structural information of quasi-homogeneous medium.