Effects Of Spherical Aberration And Spatial Coherence On Laser Beam Self-focusing From Orbit To The Ground In The Atmosphere

Energy and environment issues are two serious problems which the human being will face in the future.There is abundant of solar energy outside of the earth.It is a new global concept of renewable energy sources to utilize laser radiation to transport converted solar energy to the ground.Until now,only a concept is proposed,i.e.,to exploit a self-focusing effect in the atmosphere to assist delivering powerful laser beams from orbit to the ground.However,for such high power regimes,effects of beam distortions and spatial coherence on the beam quality are much more significant than in low power situations.In this thesis,the effects of spherical aberration and spatial coherence on laser beam self-focusing in the atmosphere from orbit to the ground are studied in detail.The main research results are summarized as follows:1.The effect of spherical aberration on the laser beam self-focusing in the atmosphere to assist delivering powerful laser beams from orbit to the ground is studied.A computer code of the propagation of laser beams from orbit to the ground in the atmosphere is designed,and the effect of spherical aberration on laser beam self-focusing in the atmosphere is studied in detail by using the numerical simulation method.It is shown that the spot size on the ground and the threshold critical power depends on not only the orbital mirror radius R0 but also the beam spherical aberration coefficient.The compressed beam width at the ground decreases as R0 increases.However,the dependence of the compressed beam width at the ground on spherical aberration is relative to R0.For the large R0,a Gaussian beam without spherical aberration is more strongly compressed as a whole than those with spherical aberration,and so spherical aberration should be avoided in practice in this case.However,for the small R0,a Gaussian beam with negative spherical aberration mightbe more strongly compressed without filamentation than others,and its spot size on the ground can be reduced well below the diffraction limit.Thus,negative spherical aberration may be exploited in practice in this case.Finally,it is worth noting that the threshold critical power is inversely proportion to spot size.Therefore,optimal beam will necessarily involve a trade-off between the spot size on the ground and the threshold critical power.The results obtained in this thesis are very useful to assist delivering powerful laser beams from orbit to the ground.2.The effect of spatial coherence on laser beam self-focusing in the atmosphere to assist delivering powerful laser beams from orbit to the ground is studied.A computer code of the propagation of partially coherent beams(PCB)from orbit to the ground in the atmosphere is designed,and the effect of spatial coherence on laser beam self-focusing in the atmosphere is studied in detail by using the numerical simulation method.It is found that a fully coherent beam is more strongly compressed on the ground than a PCB,even so,for a PCB the compressed spot size on the ground may be reduced below the diffraction limit due to self-focusing effect.In particular,a PCB has higher threshold critical power than a fully coherent beam,which is useful to assist delivering more powerful laser beams from orbit to the ground.3.An effective design rule for maximal compression without beam splitting of the transported PCB from orbit to the ground is presented.The effective design rule shows the relationship between the beam correlation radius,the initial beam power,and the orbital mirror radius,which is confirmed based on the numerical simulation result.The result is very interesting for delivering powerful laser beams from orbit to the ground.