Study Of The Phenomenon Of Focal Switch In Beams Focusing

Beam focusing is one of the most interesting subjects in the field of high-power-laser technology. Recently, a lot of literatures have studied focusing properties of beams particularly. Hereinto, the concept of focal shift advanced by Wolf and Li is the important amendment of the classical theory. The existent investigations indicate that when laser beams passing through some optical system, simultaneously with the focal shift, an effective permutation of the focal point would appear, this kind of novel effect is referred to as focal switch. Consequently, the existence of focal switch makes the actual focusing work more complex and more significant. There are some domestic and external researches based on focal switch, generally considering, the emergence of this novel phenomenon is somewhat related to diffraction. And that, most investigations are based on perfect beams and optical systems, but in the actual work, owing to diversified imperfectness, there exist aberrations effect in beams or optic systems more or less. Further studies of beam focusing are of great significance for lasers application and design of optical system. Abased on the available theory, the present thesis is devoted to studying peculiarities and the conditions for its emergence. The main results achieved in this thesis can be summarized as follows:1. Focal switch in Laguerre-Gaussian(LG) beams passing through an aperture-lens separated optical system is studied by using the expanded form of complex Gaussian function. It was advanced for the first time that owing to the existence of aperture, there is a prerequisite permits the focal switch: Only when truncation parameterδ changed between the region δ_c.min < δ < δ_c.max the focalswitch can take place (δ_c,max, δ_c,min denote critical maximum and minimumtruncation parameters respectively). And the minimum truncation parameter δ_c,min has the direct relationship with the dimension of aperture , thecorresponding physical interpretation has been tried.2. Starting from the generalized Huygens-Fresnel diffraction integral (Collinsformula), the focal switch in spherically aberrated LG beams passing through an optical system with the aperture lens separated is studied. Influences of spherical aberration coefficient on relative focal shift, the turning point where the focal switch can take place, and the relative transition height is analyzed.3. Expressions for the axial irradiance of focused astigmatic Hermite-Cosine-Gaussian (HCG) beams passing through a system with the aperture and lens separated are derived. And the arisen focal switch is investigated in detail. It is shown that the influence of astigmatism on the behavior of the focal switch is the relative transition height but not the turning point, where the focal switch can take place. Additionally, it is similar to L-G beams the emergence of focal switch in astigmatic H-G beams is related to the dimension of aperture (truncation parameter).4. Based on the Collins formula, the propagation expression of astigmatic Cosh-Gaussian beams passing through a bifocal lens system is derived. Axial intensity distribution and focal shift is analyzed. It is suggested that the focal switch may appear twice. It is also indicated that the focal switch can come forth even when beams passing through non-aperture system.5. The propagation expression of astigmatic Gaussian beams focused by a bifocal lens system is derived. The focusing properties of Gaussian beam are discussed. It is proved that the focal switch in Gaussian beam can emerge under certain conditions.