title: Improvement of the beam quality of a high-pulse-energy mid-infrared fractional-image-rotation-enhancement ZnGeP2optical parametric oscillator.
abstract: The beam quality of a high-energy mid-infrared ZnGeP2(ZGP) optical parametric oscillator has been improved by use of a negative lens inside the nonplanar fractional-image-rotation enhancement (FIRE) resonator compensating for the thermal lensing and gain guiding effects. With an f=-0.9  m CaF2lens, the beam quality is 1.5 and ∼2 for the resonant signal beam and the non-resonant idler beam, respectively. Up to 33 mJ of mid-infrared pulse energy in the 3-5 μm wavelength range is obtained with 94 mJ of pump energy on the crystal. This is an improvement of the beam quality by a factor of 3 for the signal beam and ∼2 for the idler beam compared without using a lens inside the FIRE resonator.
authors.forename: Martin
authors.initials: M
authors.lastname: Schellhorn
authors.forename: Gerhard
authors.initials: G
authors.lastname: Spindler
authors.forename: Marc
authors.initials: M
authors.lastname: Eichhorn