Z-scan measurements of nonlinear refraction in Yb3+: KY(WO4)2

Recently, laser performance of new Yb3+: KY(WO4)2 crystal (Yb3+:KYW) has been obtained at 1025 nm [1]. The Yb3+:KYW crystal is promising for the microchip and passive mode-locked lasers tuning in the range of 1020-1060 nm. It is known that Kerr-lens mode-locking technique based on the nonlinear refractive index n2 of the laser medium itself is a manner for femtosecond laser pulse generation. Such a regime on Yb3+:KYW crystal is in progress. The efficiency of Kerr-lens mode-locking depends on the n2 magnitude, and it is very important to know the n2 of laser medium for the development of Kerr-lens mode-locking. In this paper, the nonlinear refractive index of the Yb3+:KYW crystal has been measured using the Z-scan technique with a picosecond laser pulses Using a single laser beam in a tight focus geometry one measures the transmittance of a nonlinear medium through a finite aperture in the far field as a function of the sample position z measured with respect to the focal plane. The 15-ps laser pulses having an energy of 0.45 mJ are focused into the sample to the beam waist of 80 µm Figure shows a Z-scan of a 1.5 mm thick Yb3+:KY(WO4)2 crystal which has no absorption at 1.08 µm. The curve displays positive induced changes in the refractive index, and, as consequence, positive sign of n2. The index change is calculated from the phase distortion ϕ(z,r) in the sample, which is found from fitting experimental and model curves using “Gaussian decomposition” method.