Abstract. CW lasing is obtained in Er 3 (5 %) : CaF 2 and Er 3 (5%) : SrF 2 crystals near 2.75 lm with 0.4 and 2 W of output powers, respectively, upon transverse diode laser pumping into the upper 4 I 11=2 laser level of erbium ions at 980 nm. Continuous tuning of the laser wavelength between 2720 and 2760 nm is realised in the Er 3 : SrF 2 crystal. Keywords: IR laser, diode laser pumping, Er 3 : CaF 2 and Er 3 : SrF 2 lasers. 1. Introduction An Er 3 : CaF 2 crystal is the érst laser medium in which, despite the self-terminating nature of the transition, room- temperature lasing has been obtained in the three-micron region upon up-conversion pumping [1, 2]. The Er 3 : CaF 2 and Er 3 : SrF 2 crystals proved to be efécient laser media in the 2.8-mm region both upon direct pumping into the upper 4 I 11=2 laser level by a cw Ti : sapphire laser [3] and up- conversion pumping into the lower 4 I 13=2 laser level [4, 5]. The aim of our paper is to study the possibility of using multisite Er 3 -doped êuoride crystals CaF 2 and SrF 2 to obtain efécient and continuously tunable lasing in the 2.8- mm region upon cw diode pumping into the upper 4 I 11=2 laser level. 2. Crystals under study In CaF 2 ë ErF 3 and SrF 2 ë ErF 3 systems, as in other systems formed by êuorides of rare-earth and alkaline-earth elements MF 2 ë RF 3 , broad regions of heterovalent solid solutions of the êuorite M 1x Er x F 2x structure appear. The limiting molar solubility of ErF 3 in CaF 2 and SrF 2 is 40 % and 43 % (x  0:40 and 0.43), respectively. The parts of phase diagrams of these systems are presented in Figs 1a and 2a [6, 7]. Because the measurement accuracy of the solidus curve of a Sr 1x Er x F 2x solid solution in [7] was insufécient, we reéned it by the method of thermal analysis. One of the main problems in growing solid-solution crystals M 1x R x F 2x is the formation of a concentration inhomogeneity ë the so-called cellular structure. This pheno- menon is caused by the loss of the crystallisation-front stability due to the concentration overcooling appearing because the distribution coefécient of components is differ- ent from unity. The distribution coefécients k 0 of ErF 3 in T.T. Basiev, Yu.V. Orlovskii, M.V. Polyachenkova, P.P. Fedorov, S.V. Kuznetsov, V.A. Konyushkin, V.V. Osiko Laser Materials and Technology Research Center, A.M. Prokhorov General Physics Institute, Russian Academy of Sciences, ul. Vavilova 38, 119991 Moscow, Russia; e-mail: basiev@lst.gpi.ru, orlovski@lst.gpi.ru, marip@lst.gpi.ru, ppf@newmail.ru, tez@rambler.ru, vasil@lst.gpi.ru, osiko@lst.gpi.ru; O.K. Alimov Institute of Nuclear Physics, Uzbekistan Academy of Sciences, Ulugbek, 702132 Tashkent, Uzbekistan; e-mail: olim@lst.gpi.ru; A.Yu. Dergachev Q-Peak, Inc., 135 South Road, Bedford, MA, USA Received 30 March 2006 Kvantovaya Elektronika 36 (7) 591 ë 594 (2006) Translated by M.N. Sapozhnikov PACS numbers: 42.55.Rz; 42.55.Xi; 42.60.Lh DOI:10.1070/QE2006v036n07ABEH013178 Continuously tunable cw lasing near 2.75 l m in diode-pumped Er 3 : SrF 2 and Er 3 : CaF 2 crystals T.T. Basiev, Yu.V. Orlovskii, M.V. Polyachenkova, P.P. Fedorov, S.V. Kuznetsov, V.A. Konyushkin, V.V. Osiko, O.K. Alimov, A.Yu. Dergachev 117/106 ë MB ë 24/viii-06 ë SVERKA ë 4 ÒÑÎÑÔ ÍÑÏÒ. å 1 Quantum Electronics 36 (7) 591 ë 594 (2006) ß2006 Kvantovaya Elektronika and Turpion Ltd 0 0.1 0.2 0.3 0.4 0.5 0.6 x CaF2 ë ErF3 a 1100 1150 1200 1250 1300 1350 1400 T  8C S L S+L 0 0.05 0.10 0.15 0.20 x S 4 12 16 F  K 8 c 0 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 x S b 0.5 0.7 0.9 1.1 1.3 k Figure 1. Part of the phase diagram of the CaF2 ë ErF3 system [6]; () beginning of solidiécation [liquidus (L)], (*) beginning of melting [solidus (S)] (a), concentration dependence of the distribution coefécient of ErF3 upon crystallisation of the solid Ca 1x Er x F 2x solution from a melt; () initial calculation points, (&) experimental data for the phase diagram, the solid curve is the approximation by the expression k  1:34 0:91x 0:33 S (b), and the stability of the crystallisation front of solid solutions as a function of the concentration overcooling for the Ca 1x Er x F 2x system (c).