The Effects Of Self-focusing And Turbulence On Ground-based Laser Space-debris Cleaning

A great number of space debris seriously threaten the safety of spacecrafts.The space debris removal using a high-power ground-based laser is one of the main methods for this problem.Furthermore,it is important to study the effects of self-focusing of laser propagating in the atmosphere for ground-based laser spacedebris cleaning,because the laser power is well above the critical power for selffocusing in air.On the one hand,uniform irradiation on the target is important and interesting for the laser space-debris clearing.Therefore,it is worth to study the problem of uniform irradiation on the space debris target.On the other hand,as we all known,the laser beam quality will also be affected by atmospheric turbulence or the beam coherence.So,it is very important to study the characteristics of high-power partially coherent laser beams propagating upwards in the turbulent atmosphere.In practice,the Cassegrain telescope is often used in high-power laser systems,which results in the annular beam profile in the space domain because of the second mirror of Cassegrain telescopes blocking out the laser beam.Therefore,it is important to study the characteristics and beam quality of high-power annular-pulsed laser beams propagating upward the atmosphere.The main research results in this thesis are summarized as follows:1.The uniform irradiation on the space debris target is studied.It is found that the uniform irradiation may appear on propagation of an initial flat-topped beam from the ground to space orbits because of the phase modulation caused by self-focusing in the inhomogeneous atmosphere.To achieve the uniform irradiation on the debris target,we present the fitting formula of the modified focal length.In addition,the influence of the beam order on the beam quality due to self-focusing is also investigated.It is shown that the good beam quality on the debris target may achieve for an initial flat-topped beam even if the beam power is very high,but it is unavailable for an initial Gaussian beam(i.e.,the beam distortion occurs).A way to achieve the uniform irradiation is offered,i.e.,the uniform irradiation may appear when flat-topped beams propagating through a nonlinear inhomogeneous media.2.The characteristics of high-power partially coherent laser beams propagating upwards in the turbulent atmosphere are studied.Based on the “thin window” model,the analytical propagation formulae(e.g.intensity and beam width)are derived by using the quadratic approximation of nonlinear phase shift.It is found that the turbulence effect plays an important role in beam propagation characteristics.But the turbulence and self-focusing effects can be suppressed by increasing the laser elevation.In particular,the optimal focal length and optimal wavelength are proposed to decrease the beam spot size on the target.The analytical results obtained from this work greatly simplify the optimization of the pre-focusing and modeling.3.The characteristics and beam quality of high-power annular pulsed laser beams propagating upward the atmosphere is studied.It is found that because of the interplay of self-focusing and group velocity dispersion(GVD)effects,the temporal pulse splitting and self-healing may take place on propagation,and the temporal pulse splitting may take place more than once.On the other hand,due to self-focusing,there exist an optimal beam power make the intensity on the target arrive in the maximum value(i.e.,the intensity on the target will decrease as the beam power increases further when the beam power is higher than the optimal beam power).The GVD effect plays a main role when the beam power is low,while the self-focusing plays a main role when the beam power is high.Therefore,the beam quality of femtosecond pulsed beams becomes worse than that of nanosecond and picosecond pulsed beams when the beam power is low,while the beam quality of nanosecond and picosecond pulsed beams becomes worse than that of femtosecond pulsed beams when the beam power is high.In addition,the B integral and the Strehl ratio are also adopted to study the beam quality on the target.