Novel non-planar ring cavity for enhanced beam quality in high-pulse-energy optical parametric oscillators

A novel non-planar ring cavity is presented. It is shown that by using a fractional image rotation, i.e. a rotation whose angle cannot be expressed as 2π/n, where n is a small integer, the number of modes that can oscillate in the cavity is greatly reduced. This Fractional Image Rotation Enhancement (FIRE) cavity can thus be used to increase the beam quality in cases of low transversal mode discrimination, such as lasers with large pump and beam size and high-pulse-energy optical parametric oscillators (OPOs), especially when compactness and efficiency are key parameters. The experimental results obtained with a FIRE OPO for mid-IR (3–5 μm) generation based on a ZnGeP2 crystal pumped by a Ho3+:LiLuF4 MOPA system at 2.05 μm are compared to those realized with the same crystal and pump arrangement using a standard Rotated Image Singly-Resonant Twisted RectAngle (RISTRA) cavity. An increase of the beam quality from M2 of 2.08–2.29 to M2 of 1.88–1.99 is reached at 20 mJ pulse energy when the FIRE cavity is used.