Beam Propagation And Focusing Properties

As the extensive applications of the laser beam, which has good direction, high coherence and high brightness, beam propagation becomes a hot topic. At present, the propagation of the fully coherent beams has been studied extensively. Recently, considerable interest has been paid to the propagation of the partially coherent beams. In some practical applications, the partially coherent beams have been demonstrated to possess advantages over the fully coherent beams. It has been shown that the partially coherent beams are less affected by the atmospheric turbulence than the fully coherent ones. Moreover, it has been applied in laser fusion for its advantages, such as low sensitivity to speckle and more smooth focused spot etc. Therefore, the investigation on the partially coherent beams and their propagation becomes more and more important.Recently, the propagation of partially coherent beams in turbulent atmosphere becomes a new topic. This dissertation firstly study the properties of polarization and spectrum of vector beams propagating in the turbulent atmosphere theoretically. These researches are greatly significant to the opto-communication in free space and the propagation of high power laser beam in turbulent atmosphere. Moreover the focusing of beams is interesting. It is an effective method to achieve higher power density and decrease the radius of beam spot through focusing, which is of importance in the fields such as laser machining, laser medicine and laser fusion etc. Therefore, it is important to study the focusing of partially coherent beams.In this dissertation, the propagation properties of partially coherent electromagnetic beams in turbulent atmosphere and the focusing of beams are discussed in detail. The research works are mainly listed as follows:1. Based on the extended Huygens-Fresnel principle, the expressions for the polarization state of partially coherent electromagnetic beams in turbulent atmosphere are derived and numerical calculations are performed. The influence of turbulence on the polarization of the electromagnetic Gaussian Schell-model (EGSM, electromagnetic Gaussian Schell model) beam, which is taken as a typical example of partially coherent light, is studied.2. Based on the extended Huygens-Fresnel principle, the expressions for the spectrum of partially coherent electromagnetic beams in turbulent atmosphere are derived and numerical calculations are performed. We study the influence of turbulence on the spectrum of the Electromagnetic Gaussian Schell-model(EGSM, Electromagnetic Gaussian Schell model) beam, which is taken as a typical example of partially coherent light.3. The axial irradiance distribution is studied theoretically in a circular-aperture lens system illuminated by a partially coherent electromagnetic beam. The effective Fresnel number is defined and the simple analytical expression evaluating the relative focal shift is derived, which can be expressed as a function of the effective Fresnel number and is just determined by the effective Fresnel number. Moreover the comparison between this new method and the conventional numerical calculation derived from Collins formula is performed.4. The axial indensity distribution of partially coherent beams focused by a cylindrical lens with aberration is studied. The expression of the relative focal shift is obtained, which is based on the defined effective Fresnel number. The two methods are compared. It is indicated that the method based on the effective Fresnel number is simple and valid to evaluate the relative focal shift. A special case is also discussed, in which the effective Fresnel number turns out to be a pure imaginary number.