Borate crystals with the general formula RAl₃(BO₃)₄ (R=Y; Pr–Lu) have good mechanical, thermal properties, and their possibility of wide isomorphous substitutions in R-positions is favorable as one of the most ideal laser host materials. Recently, flux growth technology has been developed for single crystals YAl₃(BO₃)₄ (YAB) co-doped with Er³⁺ and Yb³⁺ as highly efficient CW, Q-switched and ultra-fast diode-pumped solid-state and wave-guide lasers emitting in the eye-save 1.5-1.6 mm spectral range [1,2]. Their power levels demonstrate several times higher that those of lasers developed earlier for the spectral NIR range around 1.5 mm. These eye-save crystalline laser materials are found to be of particular importance for applications in ultra-high speed data-communications as well as in medicine, environmental sensing and range-finding. Spectroscopic properties and efficient laser operation at 1.5–1.6 m of (Yb³⁺,Er³⁺):GdAl₃(BO₃)₄ (Er:Yb:GdAB) crystal are also reported [3]. In this paper, recent investigations of crystal growth conditions, spectroscopic and laser properties of both these materials has been discussed.

Phase relationships and Er:Yb_xGd_1-xAl₃(BO₃)₄ solubility in the K₂Mo₃O₁₀-B₂O₃-(Gd,Er,Yb)₂O₃ complex fluxes have been refined in the temperature range of 1150-800^оС. In this case,  the х value was varying from 0 to 0.2, but the Er admixture  was up to 1 at.%. Additions of ytterbium and erbium to the system leads to increasing saturation temperature of high-temherature solutions (about 5-10°С, depending on the flux composition).  Solubility diagrams of Yb_xGd_1-xAl₃(BO₃)₄ were studied in the range of x = 0.05–0.15. The main attention is focused on comparison of primary fields of YAB and GdAB crystallization, and their solubilities depending on the flux composition. Since the regions of single-phase YAB and GdAB crystallization do not exist at their concentration above 20 wt % in the temperature range of 1150-1050°C, these crystals can be grown only at lower nutrient concentrations. As a result, (Er,Yb):GdAB single crystals with optical quality and the size up to 20х10х10 mm³ have been grown on the seeds. Also, an attempt is made to compare the data obtained with earlier results on crystal growth of other RAl₃(BO₃)₄. The  segregation coefficients of the Er and Yb are close to unit as a consequence of minor difference in the sizes of R-cations.

The absorption spectra of (Er,Yb):GdAB crystal were measured using Cary-500 spectrophotometer with spectral resolution 0.4 nm. An optical parametric oscillator LOTIS LT-2214OPO pumped by Nd:YAG laser was used as an excitation source for lifetime measurements.

The polarized absorption spectra of (Er,Yb):GdAB crystal around 980 nm at room-temperature are shown in Fig. 1. A strong absorption band corresponding to the transition          ²F_7/2→²F_5/2 of Yb³⁺ ions is centered at 976 nm in σ polarization, with a maximum absorption cross-section of about 2.5×10^-20 cm² and bandwidth of 18 nm (FWHM).  The absorption and emission spectra of  (Er,Yb):GdAB and (Er,Yb):YAB are similar.

The decay curve of ⁴I_13/2 level of Er³⁺ was single exponential, and the decay time was measured to be about 350 μs. Luminescence decay from the ²F_5/2 level of Yb³⁺ was measured in Yb single-doped GdAB as well as in Er,Yb:GdAB. To prevent radiation trapping (reabsorption) caused by significant overlap of the absorption and emission bands, a fine powder of crystals immerced in glycerin was used [4]. The lifetime of Yb³⁺ ion in Yb(1 at.%):GdAB crystal was measured to be 450 μs, whereas in Er(1.5 at.%),Yb(11 at.%):GdAB it shortens to 75 μs. The energy transfer efficiency was obtained to be of about 83%.

The CW laser experiments were carried out with the four-mirrors cavity. As a pump source a 7 W fiber-coupled (Ø=105 μm, NA=0.22) laser diode emitting near 976 nm was used. The 1.5-mm-thick Er(1.5at.%),Yb(11at.%):GdAB crystal was kept at 18 °C by the thermoelectrically-cooled heatsink. The cavity-mode diameter at the active element was close to the pump beam waist. CW diode-pumped Er,Yb:GdAB laser was realized for the first time to our knowledge. The laser threshold was measured to be about 1 W of absorbed pump power. The maximum CW output power of 750 mW with slope efficiency near 26 % was obtained at 1531 nm at about 4 W of absorbed pump power.

 This research was supported in part by the RFBR grants 12-05-90010-Bel_а, 12-05-00912_аnd BRFFR 12P-183.

1. N. I. Leonyuk, Crystallography Reports 53 (2008) 511.

2. N.A. Tolstik, S.V.Kurilchik, V.E. Kisel,  et al., Optics Letters 32 (2007) 3222.

3. Y. Chen, Y. Lin, X. Gong, et al., IEEE J. Quantum Electron. 43 (2007) 950.

4. V.E. Kisel, A.E. Troshin, N.A. Tolstik, et. al., Optics Letters 29 (2004) 2491.

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