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12 March 2013 Ho3+:LLF MOPA pumped RISTRA ZGP OPO at 3-5 μm
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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 ZnGeP2 (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/cm2 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 Tm3+ fiber laser pumped Ho3+:LuLiF4 (Ho3+:LLF) MOPA system was developed delivering > 100 mJ at 2053 nm in TEM00 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 M2 ~ 2.5.
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Georg Stoeppler, Martin Schellhorn, and Marc Eichhorn "Ho3+:LLF MOPA pumped RISTRA ZGP OPO at 3-5 μm", Proc. SPIE 8604, Nonlinear Frequency Generation and Conversion: Materials, Devices, and Applications XII, 86040I (12 March 2013);


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