Ho<sup>3+</sup> lasing at 2060 nm in Co-doped Tm, Ho:KLu(WO<inf>4</inf>)<inf>2</inf>

Abstract

Summary form only given. Many crystalline laser hosts have been studied with Tm-Ho co-doping where the trivalent Tm ion serves as sensitizer (conveniently pumped near 800 nm by laser diodes) and the trivalent Ho ion, excited by energy transfer, lases on its 2-μνα 5I7→5I8 transition. Little attention has been paid so far, however, to the monoclinic double tungstate crystals although it is known that they exhibit maximum absorption and emission cross sections and minimum concentration quenching. Continuous-wave (CW) lasing of Ho3+ with Ti: sapphire laser pumping was achieved for the first time in Tm, Ho:KY(W04)2 [1] and these results were later extended to mode-locking. Almost simultaneously we grew and investigated the spectroscopic properties of the related Tm, Ho:KLu(W04)2 (Tm, Ho:KLuW) [2]. Under Ti:sapphire laser pumping, however, few different dopant combinations and output couplers indicated dual wavelength operation (simultaneous lasing of Tm and Ho) and the maximum output power of the Ho laser reached only 156 mW [3]. Here we demonstrate an improvement by a factor of >2, both in terms of slope efficiency and output power of the CW Tm, Ho:KLuW laser, oscillating solely at 2060 nm.In the present experiment we employed a 0.5 at. % Ho-, 5 at.% Tm-doped (higher than the actual Tm-doping of 3.5 at. % in [3]) sample with a thickness of 2.75 mm along the Ng optical axis. Transversal dimensions were 2.71 mm (along Nm) and 2.95 mm (along Np). The sample was placed under Brewster angle between two 10 cm radius of curvature focusing mirrors in a 4-mirror X-type astigmatically compensated cavity with a total length of ~62 cm. All mirrors, including the plane rear reflector and the output couplers (OC) supported oscillation both in the Tm and Ho spectral ranges. Both pump and laser were polarized along Nm. The pump beam at 802 nm was focused by a 7-cm lens. The sample was mounted in a Cu-holder whose temperature was maintained at 16°C.Maximum output power of 378 mW was obtained with TOC=1.7% at an absorbed power of 1.73 W (Fig. 1a). The maximum slope efficiency with respect to the absorbed power η (25.5%) was achieved with the same OC for which the threshold pump power amounted to ~220 mW. The laser wavelength was 2060 nm with all OCs. Tuning was studied at maximum pump power introducing a 3-mm-thick quartz plate (optical axis at 60° to surface) close to the OC. With the 1.7% OC a range of almost 160 nm could be covered (Fig. 1b). Certain structure within the 5I7AE5I8 transition of Ho3+ is seen, around the maximum at 2060 nm, the upper wavelength limit is set by the spectral extension of the highly reflecting cavity mirrors, while the short wave wing (up to 1960-1970 nm) is most probably related to Tm-lasing. However, the overall tuning performance is substantially different from the one observed in [3] where at maximum pump level, Tm-lasing prevailed. In conclusion, substantial improvement in the performance of the Tm,Ho:KLuW laser has been achieved, attributed basically to optimized co-doping levels and reduction of the overall parasitic losses (reduction of the steady-state Ho-ion inversion level). The dynamics of this laser turns out to be rather complex but obviously, under certain conditions, stable operation of the Ho-ion only can be achieved without observable thermal effects (no roll over in the input-output characteristics and no improvement under quasi-CW pumping). However, further optimization seems still necessary in particular of the balance between small signal absorption (Tmdoping level and sample thickness) and co-doping ratio (energy transfer mechanisms).