Study On The Propagation And Transform Of Laser Beams Through A Relay Mirror System

With the use of a relay mirror system, the effects of atmosphere on the propagation of laser beams can be significantly reduced. Meanwhile, the obscurations at the propagation path, such as buildings, mountains can be avoided. In military,a relay mirror system can be not only used as a ballistic missile defense system to attack the air and ground targets, but also used for the space surveillance and space control. In optical communication, the propagation distance and cover area of laser beams can be significantly enhanced with the help of a relay mirror system.There are many factors that will effect on the propagation of a laser beam through a relay mirror. For example, diffraction, turbulent atmosphere and thermal blooming will induce the spread of laser beam; molecule, aerosol, rain and frog will absorb and scatter the light of the laser beam; laser beam concentrating on the central obscuration or reaching beyond out the aperture of the receiver telescope will cause the loss of light. All of these will limit the application of a relay mirror system.In this paper, based on the previous researches on relay mirror system, I have studied the propagation and transform of laser beams passing through a relay mirror system. My motivation is to understand the advantages and limitation by using a relay mirror system comparing with the directing propagation of laser beams in military. These studies include many aspects.The first, the structure of a relay mirror system and the links between each section are studied and illuminated. The functions of each parts and its work principle are investigated.The second, from the structure and function of a relay mirror system, the propagations and transform of laser beams through a relay mirror in vacuum are studied in details, such as the effects of obscuration ratio and size of telescopes on the loss of energy and the intensity distribution at target. The relation between the maxium peak intensity and the propagation distance is obtained.The third, after studying the propagation and transform of laser beams in vacuum, the properties of laser beam through a relay mirror in atmosphere are also studied. With the help of extended Huygens-Fresnel principle, the propagation of mult-Gaussian beams is studied. The analytical expression for average intensity at receiver is obtained. Based on the studies on the propagation of dark hollow beams in turbulent atmosphere in a slant path, the coupling efficiency between two Cassegrain telescopes are analyzed and discussed. To understand the reasons that cause the beam spread, the variations of coherence length of laser beams through a relay mirror in turbulent atmosphere is studied. Meanwhile, with the use of adaptive optics, the spreading due to diffraction, turbulence and thermal blooming of high energy laser in atmosphere is studied. These researches above show that the compensation by the relay mirror system on the aberration of the reveive laser beam is important.The forth, the relations between the aberration and the intensity at focal plane are analyzed. The influences of the top tenth aberration on the on-axis intensity are studied. The relations between the on-axis intensity Strehl ratio or encircled power Strehl ratio and the turbulent aberration are obtained.At the end of the paper, based on the wave optics computer code, the propagations of high power laser through a relay mirror system are simulated. If the criterion of power density is 1kW/cm2, from the simulation and comparison with the directed propagation, we can see that the area under 6km from the ground can not be defended for 3.8μm, 1m diameter telescope and 600kW power laser. But for 3.8μm, 1.8m diameter telescope and 1MW power laser, the area that can be defended is large (above 2km from the ground). If a relay mirror is used as a platform for further up propagation (the criterion of power density is 10W/cm2), the most range is 300km for 3.8μm, 1m diameter telescope and 600kW power laser, 600km for 3.8μm, 1.8m diameter telescope and 1MW power laser. The simulation also shows that if the wavelength of laser beam is 1.06μm, the defence area is much large than that for 3.8μm in the same conditions.