Generation Of Partially Coherent Beams And Their Application In Reducing Speckle

As one of the inventions of the twentieth century that had a great impact on human life,the invention of lasers strongly promoted the rapid development of optical engineering and related disciplines.The laser beams emitting from the lasers have the advantages of high brightness,high coherence and high directivity,and are widely applied in many technical fields.However,with the deepening of research,some researchers have found that in some areas the advantages of the coherent laser itself sometimes cause adverse effects.For example,in the atmospheric optical communication,the atmospheric turbulence will cause greater interference to the laser beam,and seriously affect the quality of optical communication.The studies conducted by many researchers have shown that the usage of partially coherent beams can reduce the influence of atmospheric turbulence,so it is very important to investigate the generation,transmission and related properties of partially coherent beams.This thesis mainly includes two aspects: On the one hand,the generation and coherence characteristics of partially coherent beams are studied.On the other hand,the application of partially coherent beams in speckle spots is explored by speckle theory.The main work of this thesis includes:First,a new partially coherent beam with special coherence distribution was experimentally formed by letting the laser beam pass through a rotating special ground glass and the coherence characteristics of the beam was studied.The interference fringes produced by the double-hole interference experiments with different hole spaces were recorded and the corresponding coherence degree was calculated.The coherence distributions of the beam at different transmission distances were obtained.The experimental results show that the coherence distribution of the new partially coherent beam is different from that of the traditional Gaussian-Schell model beam.Secondly,partially coherent beams are usually produced by the use of a laser beam through a rotating ground glass whose rotational speed can be controlled.However,the influence of the rotational speed of the ground glass on the coherence of the partially coherent beams has not been studied systematically.Therefore,we have achieved precise control of the rotational speed of the ground glass by improving the rotating system and then the laser is incident on the rotating glass.The spatial coherence of the beam is measured by the interference fringes produced by this beam passing through the double-holes.It is found that the coherence degree of partially coherent beams decreases with the increment of the rotational speed of the ground glass,but the rotational speed has little effect on the coherence when the rotational speed of the glass is fast enough.Finally,we have experimentally explored the characteristics of speckle produced by the radial polarized vortex beam through the scattering medium and the application of the partially coherent beam in the elimination of speckle.Based on the beam propagation theory and the theory of probability and statistics,the speckle produced by the radial polarized vortex beam through the scattering medium was numerically simulated.At the same time,by introducing the radial polarization converters and spiral phase plates,the radially polarized vortex beams were generated and irradiated onto the scattering plates to produce speckle,and the related parameters of speckle were measured.The results of simulation and experimental results show that the size of the speckle particles decreases with the increment of the number of topological charges of radially polarized vortex beam,and the size of the speckle particles increases with the particle size of the scattering medium.We also explored the effect of partially coherent beams on speckle.By comparing the speckle contrast produced by the coherent laser beam with the speckle contrast produced by the partially coherent beam of different coherence degrees,we find that the partially coherent beam can reduce the contrast of the speckle and the lower the coherence of the partially coherent beam,the lower the speckle contrast.Therefore,according to the speckle theory,we find that partially coherent beams have special application value in speckle elimination.