Ho3+:LLF MOPA pumped RISTRA ZGP OPO at 3-5 μm

High pulse energies in the mid-IR with a comfortable tuning possibility are required in a number of areas, including remote sensing, medicine and counter measure applications. Frequency converters based on the crystal ZnGeP 2 (ZGP) are widely used for generating tunable IR radiation in the 2.5 μm - 12 μm region. ZGP possesses high nonlinearity, however, due to its limited transparency range it requires pump wavelengths longer than 2 μm. A significant drawback of ZGP and of most other IR non-linear crystals is a low damage threshold of ~ 1 J/cm 2 for nanosecond pulses. Therefore nanoseconds OPOs at pulse energies in the 10 mJ – 100 mJ range require large diameter pump beams to reduce the risk of optical damage to the crystal. We used the “Rotated-Image, Singly resonant, Twisted-RectAngle” (RISTRA) ring cavity concept to improve the beam quality of the OPO stage. A Tm 3+ fiber laser pumped Ho 3+ :LuLiF 4 (Ho 3+ :LLF) MOPA system was developed delivering > 100 mJ at 2053 nm in TEM 00 operation at a repetition rate of 100 Hz. Using a RISTRA as a single OPO stage we demonstrated an overall OPO pulse energy of 23.8 mJ in the wavelength region 3 μm - 5 μm at a pump energy of about 45.6 mJ and a repetition rate of 100 Hz. The beam quality was measured to be M 2 ~ 2.5.