Yb:KGd(WO4)2 and Yb:KY(WO4)2 CRYSTALS FOR ULTRAFAST LASERS
The features of the Yb3+ ion in laser crystals are a simple two-level electronic energy structure that eliminates losses due to upconversion and excited-state absorption, a small quantum defect between the pump and the laser wavelengths that greatly reduces heat generation, and a broad emission (gain) bandwidth that allows the laser wavelength to be tuned over 20–100 nm and femtosecond pulses to be generated.
PROPERTIES OF Yb-DOPED MONOCLINIC DOUBLE TUNGSTATES
Yb-doped potassium rare-earth monoclinic double tungstates KGd(WO4)2 (KGW) and KY(WO4)2 (KYW) are attracting attention as laser crystals due to high available doping levels without luminescence quenching and lattice distortion (up to 100 at.% in achieving the stoichiometric structure of KYbW), relatively broad and strong absorption and emission spectral bands allowing efficient diode pumping and ultrashort pulse generation, and a moderate value of thermal conductivity. This results in multi-watt femtosecond laser systems with stable mode-locked operation and excellent beam quality.
The monoclinic double tungstates KGW and KYW are optically biaxial and their optical properties are characterised by an optical indicatrix ellipsoid with three mutually orthogonal axes Np, Nm and Ng (and corresponding principal refractive indices np < nm < ng). Spectroscopic and thermomechanical properties of Yb:KGW are summarised in Table 1. The Yb:KYW crystal shows very similar properties. A remarkable feature of both Yb:KYW and Yb:KGW is that, despite the large bandwidth, the emission cross sections are even slightly larger than those of Yb:YAG. These large cross sections result in a lower gain saturation fluence, which helps to suppress Q-switching instabilities. This is one of the key challenges in passive mode locking of a solid state laser. This Q-switching tendency is strong for gain media with the small laser cross sections typical of Yb-doped materials, especially for many of those with large gain bandwidths.
THERMO-OPTICAL DISTORTIONS IN MONOCLINIC DOUBLE TUNGSTATES
Thermo-optical distortions in tungstate crystals could be tailored by appropriate crystal cutting (athermal crystal orientation). It has been shown that Ng-cut Yb-doped KGW crystal under diodepumping exhibits weak positive thermal lensing with weak astigmatism (compared to Np-cut one) .
The thermal lens in Yb:YAG is at least seven times stronger than that of Ng-cut Yb:KGW. This is related to the lower quantum defect under 980 nm pumping (compared to 940 nm pumped Yb:YAG) and the mutual compensation of the negative influence of the temperature dependence of the refractive index and the positive influence of the thermal expansion effect on the optical power of the thermal lens in Ng-oriented KGW and KYW crystals. A diode-pumped cw laser with an output power of 14 W was demonstrated with an Ng-cut Yb:KGW crystal, showing an impressive slope efficiency of 76% and excellent beam quality (M2 < 1.2) (Fig. 1).
COMPARISON WITH OTHER YB DOPED LASER CRYSTALS
Table 2 compares the laser-related properties of Yb:KGW and Yb:KYW with those of other commercially available Yb-doped laser crystals. Due to high absorption and emission cross sections with a comparatively broad gain bandwidth (~60 nm) and low quantum defect (~5%) between pump and laser photons Yb-doped KGW and KYW enable to obtain efficient and stable operation in diodepumped mode-locked femtosecond oscillators and regenerative amplifiers.
Yb:CALGO with an even wider emission bandwidth is used for similar applications, but has a lower emission cross section. Yb:CaF2 is a good active medium for amplifying femtosecond pulses due to its wide gain bandwidth, comparatively low emission cross section and long emission lifetime, combined with its high thermal conductivity. And Yb:YAG crystal is a well-known laser crystal for high-power CW bulk and thin-disc lasers.
Optogama offers Yb:KGW, Yb:KYW, Yb:CaF2 and other Yb-doped crystals for ultrafast laser applications. For more information please visit www.optogama.com or email [email protected].
References: 1. P.A. Loiko, V.E. Kisel, K.V. Yumashev, N.V. Kuleshov, A.A. Pavlyuk “14 W high-efficient cw Yb:KGd(WO4)2 laser with low thermooptic aberrations”, Optical Materials – 2013.- Vol. 35, №3.- P. 582-585.