Harmonic Generation from 1.079 μm to 0.54 μm with 85% Efficiency

For many years, potassium titanyl phosphate (KTP) has been a well known crystal for frequency doubling of Nd:YAG lasers (1.064 μm). The large nonlinear coefficient of KTP crystal together with the wide temperature width, large acceptance angle, and very low absorption loss make KTP a promising candidate for high conversion efficiency. One problem with the use of KTP is that phase matching at 1.064 μm is possible only by Type II angle tuning, so that walk-off of the ordinary and extraordinary beams inside the crystal limits its utility in situations involving high finesse resonators. Recently however, Garmash et al. [1] have reported that Type II 90° noncritical phase-matching is possible with an a-cut KTP at 1.079 μm, thus enhancing the possibilities for intracavity cw frequency doubling. Indeed by following this lead, we have achieved 85% nonlinear conversion efficiency for doubling of 1.079 μm radiation to 0.54 μm.