Quantum Electronics 26(11) 1014-1017 (1996) ©1996 Kvantovaya Elektronika and Turpion Ltd LASER APPLICATIONS AND OTHER TOPICS IN QUANTUM ELECTRONICS PACS numbers: 42.79.Bh; 42.55.Rz; 42.62.Cf Techniques for fabrication of multilayer dielectric graded-reflectivity mirrors and their use in enhancement of the brightness of the radiation from a multimode Nd 3+ : YAG laser with a stable cavity S G Lukishova, S A Chetkin, N V Mettus, E A Magulariya Abstract. Techniques for fabrication of multilayer dielectric mirrors with the reflectivity varying over their surfaces are described. They were used at the output of the cavity of a high-power industrial Nd 3+ : YAG laser to form a beam with a Gaussian or super-Gaussian intensity distribution. The permissible range of variation of the mirror reflectivity was 40% -90% and it was reproducible from sample to sample to within 5%. A mirror with the reflectivity of 84% was used at the output of a plane-plane cavity of a cw Nd 3+ : YAG laser (output power 12 W) to increase the axial brightness by a factor of 17. 1. Introduction Multilayer dielectric laser mirrors with the reflectivity vary- ing smoothly over their surfaces [1 -7] are used in lasers of various types [1 -14]. In a stable or unstable cavity, such a mirror can perform transverse mode selection when the low- est-loss transverse mode has an amplitude distribution close to the mirror reflectivity profile. When this profile is near- Gaussian (Gaussian mirror, GM) or super-Gaussian (super- Gaussian mirror, SGM), the laser emits radiation which consists of one or few transverse modes, the mode volume in the active medium and the output power decrease, but the axial brightness rises. The use of GMs and SGMs in an unstable cavity is justified for a high-power solid-state laser, because it provides the optimal combination of the cavity output mirror functions in respect of correction and outcoupling of the radiation. An unstable cavity of a high-power Nd 3+ : YAG laser generates of single-mode radiation, but—because of strong thermo-optical distortions in the active crystal—the quality of such radiation is very far from that expected in the diffraction limit. The amplitude losses introduced by GMs or SGMs make it possible to improve the optical quality of the output radiation, i.e. to bring the laser beam parameter M 1 (beam quality factor) [15] closer to unity. In a high-power Nd 3+ : YAG laser with a plane-plane cavity, a thermal lens builds up in the active crystal and makes the cavity stable, so that the output radiation structure becomes multimode. The use of GMs or SGMs can reduce the number of transverse modes participating simultaneously in the lasing process. However, generation of single-mode radiation by means of GMs or SGMs is hardly possible because of the stochastic aspects of the formation of a strong thermal lens in the laser crystal. Nevertheless, a GM or an SGM combined with correction for the thermal lens by an intracavity adaptive optical system [13, 14] can ensure single-mode emission from a high-power Nd 3+ : YAG laser. An alternative to a GM or an SGM in an Nd 3+ : YAG laser cavity is a controlled Fabry-Perot etalon in which one of the semitransparent reflecting surfaces is deformed by a piezoelectric element. The reflectivity profile is governed by the bent surface and the integral reflectivity is determined by the bending amplitude. This combination of the properties of a controlled Fabry- Perot etalon may be useful in a solid- state laser cavity of controlled configuration when the con- trolled parameter is the feedback strength and, therefore, the laser efficiency (the losses introduced by the etalon alter the optimal integral reflectivity that ensures the maximum output radiation power in the cavity) [16]. We shall consider the feasibility of fabricating multilayer dielectric mirrors with a smoothly varying reflectivity profile (graded-reflectivity mirrors) and report experimental results on the use of such mirrors in the cavity of a high-power industrial cw Nd 3+ : YAG laser. 2. Multilayer dielectric graded-reflectivity mirror In a multilayer dielectric GM or SGM the thickness of at least one of the alternate layers (which is I/An, where X is the radiation wavelength and n is the refractive index) varies smoothly over the mirror surface (Fig. 1). Two methods for the deposition of such layers are known: the method of a S G Lukishova Institute of Radio Engineering and Electronics, Russian Academy of Sciences, Moscow; S A Chetkin, N V Mettus Institute of General Physics, Russian Academy of Sciences, Moscow; E A Magulariya Moscow Physicotechnical Institute, Dolgoprudnyi, Moscow Province Received 22 December 1995 Kvantovaya Elektronika 23 (11) 1040-1044 (1996) Translated by A Tybulewicz Figure 1. Schematics of Gaussian and super-Gaussian mirrors: (a) distribution of layers in the form of a sandwich; and (b) all layers of variable thickness.