To increase efficiency of continuous 1.5 μm laser radiation crystal laser media should be used instead of the glass ones. Generation efficiency in major laser crystals with ytterbium and erbium is at a disadvantage in relation to phosphate glasses. A cause of this phenomenon is a reverse energy transfer. Could be found crystals comparable in efficiency with Yb-Er phosphate glasses? At present a number of investigations on the use of ions relaxators Се 3+ in crystals Ca2Al2Si07 with ytterbium and erbium have been carried out to solve the problem of the reverse energy transfer in crystals [1,2]. It has been determined [1-3] that high content of cerium is needed to depress the reverse energy transfer. But cerium and ytterbium are in different ends of a lanthanide series, so silicate crystals with more isomorphic capacity such as Yb,Er,Ce:CaGdSiO (CGCS) have been proposed [3]. A sufficient solubility of Се 3+ ions in these crystals gives fast excitation relaxation on a laser level and guards against the reverse energy transfer from Er3+ to donor ions. Furthermore, a number of crystal media on the base of borate crystals with a developed phonon spectrum have been proposed [4,5]. They lack the reverse energy transfer, fast multiphonon nonradiative relaxation shunts the 4I11/2 – 4I13/2 Er3+ transfer. New crystals on the base of anhydrous borates – calcium-yttrium oxyorthoborates - Ca4YO(BO3)3 (YCOB) with ytterbium appeared recently [6], which showed high efficiency of generation, as well as calcium-barium fluor-orthoborates YbEr:Ca BaFBO3 (CBFB), in which ytterbium ions formed centers of luminescence with excellent characteristics [6]. Unfortunately, erbium ions in YCOB are characterized by high three level parameter [6], therefore the efficiency of generation Er,Yb:YCOB is not high. It has been determined recently that erbium ions in crystals CBFB have outstanding spectroscopic parameters, besides, between ytterbium and erbium ions fast energy transfer of electronic excitation takes place [7]. Since the efficiency of a sensitized laser medium depends on joint characteristics of the Yb and Er centres and the efficiency of their interactions, then available data allow considering crystals CBFB as a considerably promising matrix for an effective 1.5 μm laser. In order to obtain the most promising laser crystals and to answer the question cited above, it should be made their comparative analysis on basic spectroscopic and generative parameters. One of the important parameters related directly to the efficiency of the laser medium is an energy density of the generation threshold. Given spectroscopic parameters of active centers in crystals, the generation threshold of the one-activated laser medium (in an idealized model without passive losses) with a tree-level or quasi-four-level scheme of generation is specified by properties of active centers and can be easily determined by a simple formula [8]. The results obtained by this method are often used for determination of limiting parameters of the three-level laser media, the ytterbium media, for instance, and their comparative analysis [8]. There are known calculations of sensitized media generation parameters, among them the ytterbium-erbium glasses with a tube pumping [9], which although can be used for a diode pumping, nonetheless they are too unwieldy for simple estimation and comparative analysis. Hence there is a demand for simple analytical expressions such as [8] to calculate the lower limit of generation of the sensitized two-activated laser media with a diode pumping. The work reports the growing of single crystals CBFB and YCOB doped with erbium and ytterbium with erbium, and single crystals Yb,Er,Ce:CGS as well. We have made a comparative analysis of an ytterbium-erbium media for 1.5 μm lasers with a diode pumping on the base of simple expressions for limiting generation thresholds of the idealized sensitized media with a three-level scheme of generation, obtained in the approximation of balance equations without considering nonlinear and cumulative processes.
Energy transfer in iron-chromium co-doped ZnSe middle-infrared laser crystals