We are concerned with the similarity and scaling law of the thermal effects of windows subjected to laser propagation and influences on laser beam quality. Using characteristic physical quantities and dimensionless equations, appropriate similarity relations are derived, independent of the specific properties of the materials and beams. As an example, a full-scale and a half-scale window are numerically analyzed to verify the relations. It is concluded that the phase aberration resulting from thermal deformation and thermo-optic effects comply completely with similarity-based scaling, while the phase aberration resulting from elasto-optic effects scales approximately. From the results of a model window, the performances of a full-scale window can be obtained using the similarity relations.
High-energy laser systems include an optical train and a gas medium that must be capable of transporting and directing the beam. The optical train is termed a laser beam tube. Because of the finite absorption of laser energy, the thermal effects of the beam tube make the beam diffuse and cause degradation of the laser beam's quality. We study systematically a beam tube consisting of a laser window made from white bijou (i.e., Al2O3) or fused silica and a tube of nonflowing nitrogen or helium gas. The results show that the thermal effects of the window and the gas on the beam neutralize each other; in particular, in some cases they compensate for each other completely, such that the beam tube has no effect on the beam quality in spite of the fact that separately each has a severe effect. We explain how to acquire the specific cases.
The influence of atmospheric turbulence on partially coherent flat-topped Gaussian beam is investigated in this paper.Partially coherent flat-topped Gaussian beam is expanded as a superposition of independent Hermite-Gaussian modes.Propagation of laser beam through the turbulent atmosphere is numerically studied using phase screen approximation.We deduce two expressions to fit the results ofnumerical simulation. One describes the dependence ofthe rms width of the beam on the order number N, the spreading distance and the intensity of turbulence. The other describes the dependence ofthe coherent length ofthe beam on that ofit's initial time and that of turbulence.In many applications oflaser, flat-topped beam is often used. Among various descriptions for this kind ofbeam, the flat topped Gaussian beam is a very useful model, which was put forward by Gori in l994'.In the propagation of laser beaim through the atmosphere, the wave front ofthe beams will be stochastically fluctuated by turbulence, which will induce beam wobbles, intensity fluctuations and the variations of coherence, etc. Studies of recent years demonstrate that partially coherent beams are less sensitive than fully coherent ones to the same turbulence [3][4]• In 2003, Tomohiro Shirai et al.15 studied the spreading of partially coherent Gaussian beams through atmospheric turbulence using coherent mode representation, and they gave some analytical solution about the physical quality of the beams. However, for partially coherent flat-topped Gaussian beams propagating through turbulent atmosphere, the analytical solution has not been given so far due to it's complexity in calculation. In this paper our purpose is to study the influence of atmospheric turbulence on partially coherent flat-topped Gaussian beam by means of numerical simulation. . PARTIALLY COHERENT FLAT-TOPPED GAUSSIAN BEAMThe coherent of laser are both in time and in space. In this paper we will study the space-coherence of laser. It is well known that many real lasers oscillate on a superposition of transverse modes of the cavity. Such modes oscillate inside the cavity at slightly different frequencies when no degeneration occurs, and often they can be assumed to be totally uncorrelated. Thus the beams that emitted from the multimode laser cavity can be regard as the partially coherent beams and can be expanded as the superposition of independent Hermite-Gaussian modes or Laguerre -G aussian modes.
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