1. Introduction
Low cost, compact cyan lasers at 500–510 nm wavebands are useful in medical diagnostic, laser spectroscopy, color display and high-density data storage, which have attracted a lot of interests. Due to the lack of corresponding fundamental spectral lines, the cyan lasers are hard to be directly obtained from the traditional frequency doubling method. The SFM with nonlinear crystal is a possible approach to obtain new wavelength laser. The cyan lasers based on the Nd3+-doped crystals have been reported previously [1] and [2]. Comparing with the Nd3+-doping, the Yb3+-doping has many advantages, such as higher doping level, broader emission band and smaller quantum defect. What is more important, the Yb3+-doped lasers are always tunable [3] and [4], which can greatly expand the available fundamental wavelengths around 1 µm.
KTP is an excellent nonlinear optical (NLO) crystal which can be used for second harmonic generation (SHG), third harmonic generation (THG) and SFM [5]. Compared with other NLO crystals, KTP possesses wide phase matching scope and large effective nonlinear coefficient. What’s more, the phase matching direction of KTP exhibits less variation with the temperature, so the temperature control is not strict, which is favorable for stable laser output.