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Effect of laser pulse repetition frequency on the optical breakdown threshold of quartz glass

The thresholds of optical breakdown in the volume of quartz glass were measured in relation to the number of pulses under irradiation by ultrashort laser pulses with different pulse repetition frequencies (). Increasing this frequency from 10 to was found to substantially lower the breakdown threshold for long pulses (at a wavelength of ) and to lower to a smaller degree for long pulses (). A strong frequency dependence of the breakdown threshold is observed under the same conditions as a manifold decrease of the breakdown threshold with increase in the number of pulses in a pulse train. The dependence of the optical breakdown on the number of pulses is attributable to the accumulation of point defects under multiple subthreshold irradiation, which affects the mechanism of collisional ionisation. In this case, the frequency dependence of the breakdown threshold of quartz glass is determined by the engagement of short-lived defects in the ionisation mechanism.