Abstract

For sufficiently intense pulses propagating in a dielectric medium of positive nonlinear index, the axial peak intensity of an incident Gaussian beam will be enhanced, but a comprehensive expression for the self-focusing factor (SFF) of interest in assessing laser-induced breakdowns is not yet available. A simple formula derived from Marburger’s theory of self-focusing [ Prog. Quantum Electron. 4, 35 ( 1975)] and suitable for numerical evaluation shows that any irradiance enhancement depends not only on the peak pulse power but also on the position of the diffraction prefocus (z₀) and the length of the Rayleigh range (Z_R); Soileau’s approximation [ Opt. Eng. 28, 113 ( 1989), SFF ≃ 1/(1 − P/P₂)] holds only if the condition z₀ ≫ Z_R applies. On reevaluating an earlier laser-damage experiment performed on diamond, I find that the threshold for dielectric breakdown is at 1.22 TW/cm² for 30-ps pulses of 1.06-μm light and spot sizes of 3.92 μm (HW 1/e M) in intensity.

© 1993 Optical Society of America

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