Research On Pseudothermal Light Ghost Imaging Based On The Two-photon Geometry Optical Mechanism

Ghost imaging is a new technique that forms an object image or its diffraction pattern through measuring two correlated optical fields.The first demonstration of ghost imaging utilized a two-photon entangled source,and the result was interpreted as a quantum phenomenon owing to the entanglement of the source photons.The theoretical and experimental results later demonstrated that ghost imaging could be achieved with the pseudo-thermal light.However,the spatial correlation properties of quantum entangled source and a thermal light source are different.Two-photon geometric optics pointed out that the imaging in coincidence counts of an aperture placed in one of the down-conversion beams is found to be the analog of a simple spherical mirror system.In the classical case,the classical thermal source looks like a phase-conjugate mirror in correlated imaging.Moreover,some results have proved that the two light beams in the object and reference arms can have different wavelengths for vacuum propagation.Due to the novel physical peculiarities of ghost imaging,more and more attention has been paid to its potential applications in practice.For an imaging system through atmospheric turbulence,the influence of turbulence can be weakened by the second-order intensity correlation.For ghost imaging in scattering media,by applying the second order correlation of light,the effect of scattering on the imaging quality may be decreased.Based on the principle of two photon geometric optics,the main work is summarized as follows:(1)The point-spread function is fundamental importance in estimating the imaging resolution in optical imaging systems.By using the Collins formula,a analytical imaging formula for ghost imaging system is obtained.The intensity fluctuation correlation function can be viewed as the convolution of the original object and a point-spread function.The imaging resolution is determined by the width of point-spread function.Based on the optical transfer matrix theory,we present the analytical formula describing the width of the point-spread function,by which one can estimate imaging resolution of a new-designed imaging scheme when compared with that of the existing imaging system.Several typical ghost imaging systems are chosen to verify experimentally our theoretical results.We find that imaging resolution increases when a lens inserted into the test arm,the lens in the referencearm has no effect on imaging resolution,and it only changes the magnification factor of ghost-image.Finally,we discuss the combined effects of a test lens and a reference lens,and the imaging scheme may be helpful for high-resolution far-distance ghost imaging.Moreover,the correlated imaging can be plotted by the graphics of ray optics,by taking into account the correlation of rays emitted from the source.In general,the thermal source acts as a phase-conjugate mirror and hence the image is reflected to itself.According to our research,two photon geometric optics theory agree well with the point-spread function theory.(2)We give a proof-of-principle experimental demonstration of homogeneous medium ghost imaging.Compared with direct imaging,ghost imaging can reconstruct high-resolution imaging in a certain range.The results may be helpful for underwater imaging.We discuss the effect from the depth of water on underwater ghost imaging.Ghost imaging through different transparent liquid is also studied.The analytical expression describing the effects of the refractive index of liquid on the point spread function of ghost imaging is derived,and the optimal resolution condition is analyzed.An increase of the refractive index of liquid can result in the raise of imaging resolution.From the two-photon geometric optics theory aspect,the experimental results agree well with the theoretical predictions.(3)We discuss the geometrical optics in Fourier-transform ghost diffraction systems with thermal light,and demonstrate that a incoherent thermal source looks like a phase conjugate mirror.It is shown that the criteria of lensless ghost diffraction of two arms with the same length is just a unique case,which can be extended to more simple terms by modulating of the distance from object to the test detector and ghost imaging plane to the reference detector,both the two distances satisfied in the Fraunhofer region.Moreover,a lens based ghost diffraction system is also studied.