Active Optics For High Power Lasers

Abstract

ABSTRACT The performance of high power laser systems depends strongly on achievable beam quality, which determines focus intensity on the work-piece at given laser power. To minimize wavefront aberration limiting the beam quality, active optics are a very promising tool. A survey of active optics systems and technology is given with special attention to high power lasers. Sources and properties of beam distortions typical high lasers are described. Various basic concepts for measuring and correcting optical aberrations in real time are discussed. Related instrumentation is described, in particular wavefront correction devices like adaptive mirrors and wavefront sensors. Examples of active for a high-power CO^-laser system and their technical features are presented. Finally, performance and limitations of active optics systems in high power laser systems are discussed. 1. INTRODUCTION Active optics are employed in nature since millions of years. Looking at something of interest to you, your eye lenses are deformed by muscles, performing best imaging on the retina. All characteristic elements of an active or adaptive optical system are represented:The eye lens as active optical element, the retina as sensor and the brain as control computer closing the servo loop.In technical optical instruments active control systems have found little application until the last decade. Correction of static optical aberration in classical optics is achieved by combining sophisticated mirrors and lenses, whose surfaces must be figured and remain stable to a fraction of the wavelength light. In astronomy, however, actual resolution obtained with large telescopes is not limited by diameter or quality of the optics. It is influenced severely by the amount of variation in refractive index of the air between telescope and sky. To obtain a seeing that is diffraction-limited by telescope aperture Babcock proposed in 1953 the use of a closed-loop feedback system to measure and compensate wavefront distortions in real-time. First successful operation of^a real-time atmospheric compensation (RTAC) system was obtained in 1973 by Hardy et.al. . Pearson reported in 1975 the use of coherent optical adaptive techniques (COAT) to achieve diffraction-limited focusing of a laser beam transmitted through turbulent atmosphere.For the last ten to fifteen years, considerable efforts have been made to develop active optics in high energy laser (HEL) systems mainly in USA and in Europe. Using these high power lasers, it was becoming evident that adaptive optics can be applied not only for compensating atmospheric turbulence, but also to minimize distortions within the beam- guiding optics, including the laser itself.To implement active optics in a special laser system, at first the sources and charac­ teristics of aberrations, that one encounters there, have to be examined.