Influence Of Hard Aperture And Chirp On The Beam Quality Of High-power Laser Beams Propagating Upwards In The Atmosphere

Human beings have explored outer space for 65 years since the former Soviet Union launched the first artificial earth satellite in 1957.Due to the attention of every country,space exploration is becoming more and more frequent,and the competition for space resources is becoming more and more fierce.So space technology has become one of the symbols to measure the technological level of each country.However,due to the increase in the number of human space activities,the number of space debris has increased significantly,which will cause serious harm.Using ground-based laser to clear space debris is one of the ways to deal with the challenge by space debris.Therefore,the long-distance propagation of high-power laser in the atmosphere must be considered,such as the influence of nonlinear self-focusing and linear group velocity dispersion（GVD）effect on the beam.It is worth noting that the hard-aperture effect is usually encountered because a laser beam emitted from an optical system is more or less diffracted by a hard aperture in practice.It is meaningful to study the hard-aperture effect on self-focusing and beam quality of high-power laser beams propagating in the atmosphere.Furthermore,the high-power laser for ground-based laser space debris removal can be obtained by chirped pulse amplification（CPA）,so it is necessary to study the propagation characteristics of chirped pulsed in the atmosphere and the influence of chirp on the self-focusing effect of high-power laser.The specific work of this thesis includes:1.The hard-aperture effect on self-focusing and beam quality of high-power laser beams propagating from the ground through the atmosphere to space orbits is studied.It is shown that the spot size on the target increases as the truncation parameter decreases and the beam power increases.However,as the truncation parameter decreases,the apertured laser beam is less affected by self-focusing.In addition,the focal shift caused by both the hard-aperture effect and nonlinear self-focusing effect are also studied in this thesis.On the other hand,two effective design rules are presented to optimize the intensity on the target,i.e.,the optimal power and the optimal focal length.We have fitted the calibration formulas of the optimal transmission power and the optimal focal length of the lens to optimize the light intensity on the target.It is proven that the self-focusing effect and the hard-aperture effect both can be compensated by applying the optimal initial beam defocusing.It is also found that the higher maximum intensity on the target will occur by applying the optimal focal length rather than the optimal power.2.The GVD and self-focusing effects and the beam quality on the target are studied,when high-power chirped pulsed laser beams propagate from the ground through the atmosphere to space orbits.A simplified model of the GVD coefficient in the inhomogeneous atmosphere is proposed,and analytical formula of chirp parameter caused by GVD is derived,which is confirmed.Furthermore,the analytical formulae of chirp caused by GVD effect and selffocusing effect are also derived,respectively.It is shown that the chirp caused by GVD effect or self-focusing effect increases as the propagation distance increases.The chirp caused by GVD effect is related to the initial chirp,while the chirp caused by self-focusing effect is related to the beam power.On the other hand,the influence of initial chirp on the pulse width and the self-focusing is also investigated.It is shown that,as the initial chirp increases,the GVD effect becomes stronger,and the change of the pulse width also becomes stronger.As the propagation distance increases,the change of pulse width for the negative initial chirp parameter is different from that for the positive one.In addition,the self-focusing effect for the negative initial chirp parameter is stronger than that for the positive one.In particular,it is found that there is an optimal pre-chirp to maximize the intensity on the target,and the pre-chirp is negative chirp.The optimal pre-chirp method proposed in this thesis can greatly improve the intensity on the target.In addition,it is shown that the optimal pre-chirp method is more suitable for picosecond pulses than that for femtosecond pulses in the application of ground-based laser space debris removal.