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 (ZR); Soileau’s approximation [ Opt. Eng. 28, 113 ( 1989), SFF ≃ 1/(1 − P/P2)] holds only if the condition z0 ≫ ZR 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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