Effects Of Spatial Coherence On Laser Beam Self-focusing In Nonlinear Media

The self-focusing effect takes place when a powerful laser beam propagates in nonlinear media,and the propagation characteristics and beam quality are affected by the self-focusing effect.In practice,a partially coherent beam is often encountered,and the self-focusing effect depends on the beam spatial coherence.Therefore,it is necessary to study the self-focusing effect of partially coherent beam in nonlinear media.In addition,the optical damage occurs due to the self-focusing effect.Until now,these studies only concerned about the quasi-steady-state self-focusing of fully coherent light pulses.Therefore,it is very interesting to study the quasi-steady-state self-focusing of partially coherent light pulses in nonlinear media.On the other hand,the detrimental effect of beam diffraction and self-focusing will inevitably be encountered in the space-debris removal facilitated by a ground-based laser system,and these two effects depend on the beam spatial coherence.Therefore,it is very important to study the effect of beam spatial coherence on ground-based laser space-debris removal in the inhomogeneous atmosphere.In this thesis,the effects of spatial coherence on laser beam self-focusing in nonlinear media in detail.The main research results are summarized as follows:1.The propagation formulae and characteristics of partially coherent laser beams in nonlinear media are studied.Analytical propagation formulae of partially coherent beams?PCBs?in nonlinear media are derived,including not only the beam width and the spatial coherence width,but also the curvature radius,the inversion coherence distance,the self-trapping coherence width,the Rayleigh range,the far-field divergence angle and the M ²-factor,etc.It is shown that in self-focusing media,the spatial coherence for the beam spreading case is better than that for the beam self-focusing case when the propagation distance is longer than the inversion coherence distance.An optimal beam will necessarily involve a trade-off between collimation and spatial coherence when PCBs propagate in self-focusing media.It is proven that the ABCD law is valid if a new complex beam parameter is adopted,which presents a simple method to study the propagation of PCBs through an optical system in nonlinear media.2.The theory of the quasi-steady-state?QSS?self-focusing of partially coherent light pulses?PCLPs?in nonlinear media is studied.The analytical formulas of the QSS self-focusing of PCLPs in nonlinear media?e.g.,the beam width,spatial coherence width,and focal length?are presented.The effect of spatial coherence on the focal length and focus moving is investigated in detail.In particular,it is found that a PCLP has more advantages to avoid the optical damage of materials than a fully coherent light pulse,which can be applied in applications.3.The effect of spatial coherence on laser space-debris removal in the inhomogeneous atmosphere is studied.The analytical formulae of the beam width,the spatial coherence width and the curvature radius of partially coherent beams propagating from the ground through the atmosphere to the orbital debris are derived.The focal shift takes place because of beam diffraction and Kerr nonlinearity effects.It is found that the actual focus can be moved to the debris target by initial beam defocusing only when the initial spatial coherence width is larger than a critical value.We derive the analytical formula of the modified focal length F_mod of the lens to move the actual focus of PCBs to the debris target.The F_mod increases as the initial spatial coherence width decreases.It is demonstrated that the detrimental diffraction and Kerr nonlinearity effects can be compensated effectively only when the F_mod is less than a maximum.The results obtained in this thesis are very useful for the ground-based laser space-debris removal.