14 W high-efficiency diode-pumped cw Yb:KGd(WO4)2 laser with low thermo-optic aberrations

A comparative experimental study of thermal lensing effect was performed for diode-pumped Yb3+-doped Y3Al5O12 and Ng-cut KGd(WO4)2 crystals under cw (continuous wavelength) operation mode. For Ng-cut Yb:KGd(WO4)2, thermal lens is positive for rays lying in all meridional planes and at least seven times weaker than that of Yb:Y3Al5O12. This is associated with lower quantum defect under 980-nm pumping (in comparison with 940-nm pumping for Yb:Y3Al5O12) and mutual compensation of negative impact of temperature dependence of the refractive index and positive impact of thermal expansion effect to the thermal lens optical power. Principal meridional planes of thermal lens in Ng-cut Yb:KGd(WO4)2 contain Np and Nm axes of optical indicatrix; corresponding sensitivity factors M = dD/dPabs equal 1.38 m−1 and 1.09 m−1 (with respect to absorbed pump power). This near-athermal crystal was utilized to produce air-cooled diode-pumped cw laser with output power of 14 W, impressive slope efficiency of 76% and excellent beam quality (M2 < 1.2).

► Ng-cut monoclinic ytterbium-doped potassium gadolinium tungstate KGd(WO4)2.
► Measurement of thermal lensing optical power under intense diode pumping.
► 14 W cw diode-pumped Yb:KGd(WO4)2-laser with impressive slope efficiency of 76%.