The performance of collinear and noncollinear pulsed barium borate optical parametric oscillators with third-harmonic Nd:YAG pumping is analyzed. The effects of tangential phase matching and pump Poynting vector walk-off compensation are shown to enhance noncollinear operation. However, we show that these effects are mutually exclusive for a type I -barium borate optical parametric oscillator and furthermore that tangential phase matching is dominant. The selection of a collinear or a noncollinear configuration is determined by pump divergence and spot size as well as by crystal aperture. With a pump divergence of 4 mrad, noncollinear operation is optimal. The highest slope efficiency, 37%, and the lowest threshold, 5 mJ, are obtained with nearly perfect tangential phase matching. For a pump divergence of less than 2 mrad and a similar spot size, collinear operation gives the lowest threshold, 3.2 mJ, and the highest slope efficiency, 33%.
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