Effects Of Self-focusing In The Atmosphere And Thermal Blooming In The Seawater On The Propagation Property And The Beam Quality Of Laser Beams

A large number of space debris poses great threat to human space activities.The laser space-debris cleaning is an important and effective method.However,the problem of high-power laser beam propagation through the atmosphere will be encountered in laser space-debris cleaning.Furthermore,the effects of self-focusing and the group velocity dispersion on the beam quality are very significant.Therefore,it is important to study the effects of self-focusing and the group velocity dispersion of laser pulses propagating in the atmosphere for the ground-based laser space-debris cleaning.On the other hand,the laser radiation is utilized for transport of converted solar energy to the ground is one of the assumption of green energy sources.The telescope that create an annular beam is often used as the transmitter in high-power laser systems.Therefore,it is very important to study the self-focusing effect of annular beams propagating in the atmosphere to assist delivering powerful laser beams from orbit to the ground.In addition,since recently the interest in underwater laser application,it has become more important to deeply understand how the thermal blooming affects light beam propagation in the seawater.In this paper,the effects of self-focusing and group-velocity dispersion in the atmosphere and thermal blooming in the seawater on propagation properties of laser beams are studied in detail.The main research results are summarized as follows:1.For the application of the ground-based laser space-debris cleaning,the effects of self-focusing and group-velocity dispersion on laser pulses propagating in the atmosphere from the ground to the orbit are studied.A computer code of the propagation of laser pulses from the ground to the orbit is designed,and the influence of self-focusing and group-velocity dispersion on propagation property and the beam quality of laser pulses is studied in detail by using the numerical simulation method.It is found that changes of the pulse duration and the beam spot size with the propagation distance are noticeable due to the interplay of the GVD effect and the self-focusing effect,which is quite different from the behavior in the linear case.It is shown that the temporal pulse splitting may appear on the space debris,and the spatial side lobe usually appears together with the temporal pulse splitting.As compared with linear case,the beam width and the pulse width on the debris target increase.On the other hand,crucial formulae of the modified focal length and the M ~2-factor for laser debris removal are also derived.It is found that the beam quality on the debris target becomes better if our modified focal length is adopted,and the beam quality on the debris target will be good if the value of M ~2-factor is less than 1.6.The results obtained in this paper are very useful for the ground-based laser space-debris removal.2.For the application of utilizing laser radiation to transport converted solar energy to the ground,the self-focusing effect of annular beams propagating 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 influence of self-focusing on propagation properties of annular beams propagating in the atmosphere is studied in detail by using the numerical simulation method.It is found the annular beam is compressed more strongly due to the self-focusing effect as the beam obscure ratio increases.On the other hand,the self-focusing effect on the propagation property and the beam quality between annular,flat-topped and Gaussian beams is compared.It is shown that the self-focusing effect on the annular beam is stronger than that on flat-topped and Gaussian beams.However,the self-focusing threshold critical power values of annular,flat-topped and Gaussian beams should be in sequence from small to large.Furthermore,the expression of the B integral of annular beams propagating from orbit to the ground in the atmosphere is derived,and the fitting equation related to the B integral of annular beams for maximal compression without filamentation is presented.The results obtained in this paper are very useful to assist delivering powerful laser beams from orbit to the ground.3.The thermal blooming effect of laser beams propagating through seawater is studied.The influence of the salinity and the depth of seawater and the wavelength on the thermal blooming is investigated in detail by using the four-dimensional computer code of the time-dependent propagation of high-power laser beams through the seawater,where the absorption coefficient of seawater is considered to be changed with the wavelength.It is found that an increase of the salinity in the seawater causes the more severe thermal blooming.As compared with the wavelength,the absorption coefficient is the main factor that dominates the thermal blooming because the absorption coefficient is very high in the seawater.In the seawater the thermal blooming becomes more severe for the wavelength corresponding to the higher absorption coefficient.Furthermore,both the behavior of the thermal blooming effect and the main factor dominating the thermal blooming effect in the shallow sea region are different from those in the deep sea region.In the shallow sea region,the dependence of the thermal blooming on the depth is not monotonic as the time increases.However,in the deep sea region,the thermal blooming effect becomes more severe monotonously as the depth increases.The results obtained in this paper will be very useful for applications of laser beams in the seawater.