Measurement of the nonlinear refractive index of Yb:KGd(WO/sub 4/)/sub 2/ laser crystal

Abstract

In the past few years it has been shown that Yb-doped solid-state laser materials are very attractive for the development of diode-pumped high power femtosecond mode-locked oscillators. Among the variety of Yb-doped solid-state laser materials, Yb-ion doped potassium gadolinium tungstate (Yb:KGW) crystals [l] are among the most promising. They have broad amplification bandwidths, high emission cross sections, good thermal conductivity and are suitable for efficient diode pumping using commercially available laser diodes in the 930-980 nm region. Using a semiconductor saturable absorber mirror for passive mode locking, a diodepumped femtosecond Yb:KGW oscillator was demonstrated, resulting in more than 1W of average output power in I76 fs pulses [2]. Another attractive passive mode locking technique for ultrashort pulse generation is Ken-lens mode locking (KLM) which relies on intensity dependent nonlinear refractive index n2 of the gain material. So far for Yb-based lasers, KLM under direct diode-pumping has been realized only for Yb:KYW material. Pulses as short as 71 fs with 120 mW average output power were generated [3].It is well known that the larger the value of n2, the more efficient the Kerr-lensing effect.In our experiments we tried to assess the potential of KLM in Yb:KGW by measuring its nonlinear refractive index. We believe that because Yb:KGW has a broader amplification bandwidth than Yb:KYW [l], it should be possible to generate even shorter pulses with this material using a KLM technique. Moreover, as recent research has shown, the magnitude of n2 plays an important role in determining the overall KLM stability [4]. Therefore, a knowledge of the nonlinear refractive index of the laser crystal is crucial.Measurements were done using the z-scan technique [5] with picosecond pulses. A 1 mm thick sample of 5% at. wt. Yb-doped KGW crystal was cut such that the measurements with pump polarization parallel to np and n, axes of the crystal were possible. For the experiments we used -1 ps pulses with -1 pJ energy from a Tisapphire laser pumped optical parametric amplifier. The measurements were done in the 800 nm to 1600 nm wavelength region. Using as a reference material SF59 glass, our data indicate that the nonlinear refractive indices of Yb:KGW in the 800-1600 nm range are 15 x cm2/W for Elln,. These values can be compared with 3.1 x cm2/W for cm2/W for Elln, and 20 x c m * N for Tisapphire [6] and 8.7 x