Abstract

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.

© 1998 IEEE