Quantum Electronics 24 (2) 117-119 (1994) © 1994 Kvantovaya Elektronika and Turpion Ltd PACS numbers: 42.79.Ag; 42.55.Rz Investigation of a soft aperture formed by photooxidation of a rare-earth impurity in fluorite and used as an intracavity component in a YAG : Er 3+ laser S G Lukishova, N R Minhuey Mendez, T V Tulaikova Abstract. Intracavity investigations were made of a soft (apodised) aperture made by photooxidation of Pr 2 " 1 " in y-ray-coloured CaF2 : Pr under the influence of argon laser radiation. This aperture reduced the divergence and increased the output energy of single-mode YAG : Er 3 + radiation, compared with the case when a hard aperture was used. 1. Introduction Photooxidation of a rare-earth impurity in fluorite [1-3] has been used [4-7] in the fabrication of soft (apodised) apertures preventing the formation of Fresnel diffraction rings in amplifying modules of power lasers. Crystals of CaF 2 : Pr have been used for this purpose in lasers emitting near-infrared radiation. We shall report the results of our investigation of a soft aperture made by photooxidation of CaF2: Pr and placed inside the resonator cavity of a YAG: Er 3 + laser. The aim was to determine the feasibility of improving the output characteristics of this laser, which emits single-mode radiation, compared with the case of a hard intracavity aperture. Much experimental and theoretical work has been done already (see, for example, [8- 12]) on the improvement of the output characteristics by various intracavity components [8], particularly apodised components with optical characteristics that vary over their cross section [9 - 12]. Among apodised components the most popular are multilayer dielectric mirrors with a smooth variation of the reflecting coefficient over the cross section (graded reflectance mirrors) [13-15]. Among soft apertures for the near infrared, whose operation is based on the absorption as in the case of our apertures made by photooxidation, those used inside a resonator include apertures made by photoconversion of colour centres directly in the cavity by the action of the laser radiation itself on an LiF crystals [16,17], as well as those formed by the diffusion of metal ions into glasses [18] and by photothermochemical reactions in additively coloured fluorite [19]. An improvement in the angular divergence and in the degree of filling of the cross S G Lukishova Institute of Radio Engineering and Electronics, Russian Academy of Sciences, Moscow; N R Minhuey Mendez, T V Tulaikova Institute of General Physics, Russian Academy of Sciences, Moscow Received 22 June 1993 Kvantovaya Elektronika 21 (2) 126-128 (1994) Translated by A Tybulewicz section by a beam have been reported [16]. Unfortunately, we do not know the results of investigations of apertures [18,19] inside laser resonator cavities. Reports of the use of liquid crystal mirrors have appeared recently [20-22]. 2. Experimental methods and results The soft aperture used in the present study was made by a method [4-7] involving illumination for 3 h of y-ray- coloured CaF2 : Pr 3+ crystals (with Pr molar fraction 0.2%) by radiation (radiation dose 10 9 R) from a 2 W cw argon laser made by Coherent Radiation (emission wavelength X = 0.488 um). The Pr 2+ ions, formed by y-ray colouration [23,24] (their maximum fraction was ~10% of the initial total number of Pr 3+ ions before ionising irradiation [1]) and characterised by wide near-infrared absorption bands, were converted by the photooxidation process [1-3] again to the trivalent state. This process bleached the sample in the zone of interaction with the argon laser radiation, so that an aperture with smoothed-out edges was formed as a result of a smooth reduction in the intensity in the incident argon laser beam along its radius. The transmission profile of the soft aperture at X = 2.94 um, formed in a crystal 7 mm thick, was bell- shaped (Fig. 1). The transmission at the centre of the aperture was 87% at this wavelength. 0 2 4 r/mm Figure 1. Transmission profile T(r) of a soft aperture, made by photooxidation, recorded at the wavelength of 2.94 um (r is the radial distance from the centre of the aperture). The semiconfocal resonator cavity of a YAG : Er 3 + laser containing such a soft aperture 4 (Fig. 2) was formed by a concave copper mirror 1, with a radius of curvature of + 3 m and a reflection coefficient close to 100%, and a plane mirror 2 in the form of a silicon plate ~ 300 um thick with a reflection coefficient of one of the surfaces amounting to ~30%. (In the multimode regime a second spot appeared alongside the main one in the output radiation of this YAG laser. This additional spot disappeared under conditions close to single-mode emission.) The resonator length was