Svetlana Sheina

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

Abstract. We investigated optical damage (surface and bulk) in one of the most promising wide bandgap nonoxide nonlinear crystals, HgGa2S4, that can be used in ∼1-μm pumped optical parametric oscillators (OPOs) and synchronously pumped OPOs (SPOPOs) for generation of idler pulses above 4 μm 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/cm2, which corresponds to ∼100  MW/cm2 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/cm2 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 ∼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.

High-power, 100-Hz HgGa 2 S 4 OPO pumped at 1064 nm

HgGa₂S₄ is employed in a 1064-nm pumped optical parametric oscillator, to generate ~5-ns long idler pulses near 4 μm with energies as high as 6.1 mJ and average power of 610 mW at 100 Hz.

Optical damage limits in chalcogenide nonlinear crystals used in 1064nm pumped nanosecond optical parametric oscillators

We investigated optical damage (surface and bulk) in wide band-gap (absorption edge below 532 nm) sulphide and selenide nonlinear crystals that can be used in 1064-nm pumped optical parametric oscillators (OPOs) for generation of idler pulses above 4 μm without two-photon absorption losses at the pump wavelength. The optical damage has been characterized at the pump wavelength for different repetition rates. Surface damage has been studied for uncoated and antireflection-coated (mainly with a single layer for pump and signal wavelengths) samples. 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 and not to average intensities and in many of the studied crystals bulk damage in the form of scattering centers occurs before surface damage. Such bulk damage formation is faster at higher repetition rate. Lower repetition rates increase the lifetime of the crystal but do not solve the problem. In the most successful nonlinear crystal (both in terms of output energy and average power), orange-phase HgGa2S4, the safe pump intensity in extracavity measurements is below 100 MW/cm2 which corresponds to less than 1 J/cm2 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/cm2 limited by the formation of bulk scattering centers. The damage resistivity of yellow-phase HgGa2S4 or Cd-doped HgGa2S4 is higher and of the almost colorless CdGa2S4 it is roughly two times higher but the latter has no sufficient birefringence for phase-matching.

High-energy 1064-nm pumped optical parametric oscillator for the 6 µm spectral range based on HgGa2S4

The defect chalcopyrite crystal HgGa2S4 has been employed in a 1064-nm pumped optical parametric oscillator to generate <7 ns long idler pulses near 6.3 µm with energies as high as 3 mJ, tunable in a broad spectral range from 4.5 to 9 µm.