ZnGeP2 and CdGeAs2 have long been recognized as promising crystals for infrared frequency generation. They exhibit the highest nonlinear optical coefficients (d36 equals 75 pm/V and 236 pm/V for ZnGeP2 and CdGeAs2, respectively) among all known compounds that possess adequate birefringence for phase matching. ZnGeP2's transparency range (0.62−13 μm) makes it the optimum material for shifting the wavelength of 2-μm pump lasers into the 3–5-μm range via optical parametric oscillation (OPO), whereas that of CdGeAs2 (2.3–18 μm) is better suited for doubling the frequency of CO2 lasers (9–11 μm) into the same range via second-harmonic generation. In both cases however, the application of these materials has been hindered by great difficulty in achieving crack-free single crystals, and by large defect-related absorption losses.The horizontal-gradient-freeze (HGF) growth technique has been instrumental in overcoming these difficulties. “Ultralow” axial gradients (1–3°C/cm) have been used to control stoichiometry by minimizing vapor transport as well as to eliminate cracking due to anisotropic thermal expansion. (The a-axis and c-axis thermal-expansion coefficients of ZnGeP2 differ by a factor of two, whereas those of CdGeAs2 differ by a factor of 15.) In addition, oriented seeds were used to ensure monocrystalline nucleation (because even a small degree of polycrystallinity can lead to cracking even in low gradients) and growth along preferred directions to facilitate fabrication of device crystals. Finally growth was performed in a two-zone, transparent furnace in order to monitor and control the seeding-and-growth process.
We have demonstrated what we believe to be the first mid-infrared optical parametric oscillator (OPO) pumped directly by a pulsed Tm-doped fiber laser. The Tm-fiber pump laser produces 30 ns pulses with a repetition rate of 30 kHz at a wavelength of 2 microm. The ZnGeP2 (ZGP) OPO produces 20 ns mid-IR pulses in the 3.4-3.9 microm and 4.1-4.7 microm spectral regions simultaneously. More than 658 mW of mid-IR output power has been generated with a total OPO slope efficiency greater than 35%.
We report a resonantly fiber-laser-pumped Er:YAG laser operating at the eye-safe wavelength of 1645 nm, exhibiting 43% optical efficiency and 54% incident slope efficiency and emitting 7-W average power when repetitively Q switched at 10 kHz. To our knowledge, this is the best performance (conversion efficiency and average power) obtained from a bulk solid-state Q-switched erbium laser. At a 1.1-kHz pulse repetition frequency the laser produces 3.4-mJ pulses with a corresponding peak power of 162 kW. Frequency doubling to produce 822.5-nm, 4.7-kW pulses at 10 kHz was performed to demonstrate the laser's utility.
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