Intracavity Laser Beam Control and Formation


1. Introduction

Adaptive optics is known to be invented as a tool for compensation of different wavefront distortions of the light beam penetrated through some turbulent media and usually is used for various astronomical applications1. But starting from late 70th several types of experiments showing the efficiency of the use of adaptive systems to improve the laser beam quality were carried out2. Almost all these experiments were made on the CO2 high power lasers (up to MWatt) and the obtained results were really promising. Here we should mention the works of Oughuston3,4 (theoretical calculations), and R.Freeman et. al.5,6,7 (mostly experiments). But then there was some sort of a gap in the interest towards the application of active optical elements in laser resonators. The main reason for this probably was the cost of the key elements of any adaptive system - deformable mirrors, wavefront sensors and systems of electronic control with the high speed computers. But later the situation changed together with the progress in laser technology and new problems were to be solved with the help of lasers. The interest in active laser beam control appeared again as well as the price of the elements of adaptive systems significantly dropped.

The main tasks that could be resolved by methods and technique of adaptive optics are:

  1. Stabilisation and optimisation of different laser radiation parameters.
  2. Formation and maintenance of the given intensity distribution of laser beam on the given surface.

Possible new fields of application of adaptive systems are laser microtechnology, laser etching, laser heating technology as well as laser ophtalmosurgery and laser dermatology.

The problem of controlling the parameters of laser radiation has become particularly topical in view of the widespread use of lasers in technological processes and medicine. The comparatively high radiation power, the relatively high efficiency and the fairly small dimensions of different types of lasers make them suitable for a wide range of applications. The expensive optomechanical systems are employed to achieve the required intensity distribution on the surface of a laser-treated sample in industrial systems and high-quality (having a homogeneous refractive index) active elements (crystals) are used to fabricate these such lasers. Under real conditions, one of five or six industrially grown crystals is selected as the active element of laser which raises the cost of the whole technological system still more. Normally stable resonators are used in CW solid state lasers having rather low gain and small active volume. Such resonators have very poor discrimination factor between transverse modes and so poor output beam structure. To improve beam quality in practice the hard-edged apertures are used which, however, also produce diffraction rings in the near field and significantly reduce output beam power. The optimum resonator for such laser would consist in an open cavity having a lowest order modes of a stable (for a low loss) and all the other modes for unstable type (for a good discrimination)8. The design of such type of laser was proposed in Ref. 9 where the near-axial part of cavity is stable and other part unstable. In contrast, it was proposed10, 11 to use a mirror with a convex central part and concave rim. Another way of improving laser beam quality - the use of graded-phase mirror resonators12. The technology for fabricating such mirrors is now well developed13, and many commercial laser companies have adopted them. But one of the shortcoming of such mirrors - their stable surface profile. This narrow the spheres of application of such mirrors in lasers because every active element needs its own unique graded-phase mirror.

In modern laser technology it is often necessary to form the given intensity distribution at the surface being treated. This has become particularly topical because of the widespread use of component etching systems in mechanical engineering. This problem was solved previously by using compensators, kinoforms, spatial filters, etc14, 15. These optical elements were developed for beams having a given structure (mostly Gaussian). When the illumination conditions vary, the efficiency of such elements deteriorates appreciably. Another method of forming a given intensity distribution at the laser exit involves the use of an intracavity controlled mirror which not only can distort the profile of the laser-emitted mode, but can also create conditions for successful generation of some and suppression of other mode structures.

The results of some experiments with excimer, copper-vapor, CO2, and YAG:Nd3+ industrial lasers and intracavity deformable bimorph mirrors concerning the possibility to control and form mode structures are presented in this paper.



Back