Damage resistance of AR-coated germanium surfaces for nanosecond CO2 laser pulses

An evaluation of the state-of-the-art of AR coatings on galium doped germanium, used as a saturable absorber at 10.6 micrometers, was conducted. Both 1-on-1 and N-on-1 laser damage thresholds were measured with 1.2 ns pulses on bare and coated surfaces. Only front surface damage was observed. With few exceptions, the thresholds for coated surfaces were centered at 0.49 +-0/3J/sq. cm. Bare Ge had a threshold ranging from 0.65 to 0.70 J/sq. cm. No significant differences due to substrate polish, crystallinity or doping level were evident and multiple-shot conditioning resulted in the same threshold as for single shot tests. From an analysis of standing wave electric fields, damage for AR coated Ge appeared to be limited by the surface properties of Ge. Measurements at both 1.2 and 70 ns indicated that the threshold (J/sq cm) of both coated and uncoated Ge increases as the square root of the pulse width.