Optical damage studies of mercury thiogallate nonlinear crystals for use in 1-μm pumped optical parametric oscillators

We investigated optical damage (surface and bulk) in one of the most promising wide bandgap nonoxide nonlinear crystals, HgGa2S4, that can be used in similar to 1-mu m pumped optical parametric oscillators (OPOs) and synchronously pumped OPOs (SPOPOs) for generation of idler pulses above 4 mu m without two-photon absorption losses at the pump wavelength. The optical damage has been characterized at the pump wavelength for different repetition rates using uncoated and antireflection-coated (mainly with a single layer for pump and signal wavelengths) samples. HgGa2S4 is the most successful nonlinear crystal (both in terms of output energy and average power) for such OPOs, but optical damage inside the OPO has a lower threshold and represents at present the principal limitation for the achievable output. It is related to peak pulse and not to average intensity, and bulk damage in the form of scattering centers occurs before surface damage. Such bulk damage formation is faster at higher repetition rates. Lower repetition rates increase the lifetime of the crystal but do not solve the problem. The safe pump fluence in extracavity measurements is <1 J/cm(2), which corresponds to similar to 100 MW/cm(2) for the 8-ns pulse duration (both values peak on-axis). In the OPO, however, peak on-axis fluence should not exceed 0.3 J/cm(2) limited by the formation of bulk scattering centers in orange-phase HgGa2S4. In the nanosecond OPO regime, the damage resistivity of Cd-doped HgGa2S4 is higher and that of the almost colorless CdGa2S4 is roughly two times higher, but the latter has no sufficient birefringence for phase-matching. In SPOPOs operating in the similar to 100 MHz regime, the damage limitations are related both to the peak pulse and the average intensities, but here HgGa2S4 seems the best nonoxide candidate to obtain first steady-state operation with Yb-based mode-locked laser pump sources. (C) 2013 Society of Photo-Optical Instrumentation Engineers (SPIE)