Laser-Induced Damage And The Role Of Self-Focusing

Abstract

We review the influence of self-focusing on the measurement of bulk laser-induced-damage (LID) thresholds in normally transparent optical materials. This role is experimentally determined by measuring the spot size and polarization dependence of LID and by observing beam distortion in the far field. Utilizing these techniques, we find that by using a tight focusing geometry in which the breakdown power is below P2, the effects of self-focusing can be practically eliminated in an LID experiment. P2 is the so-called second critical power for self-focusing, and P2 = 3.77P1, where P1 = cÀ2 /3272n2, where c is the speed of light in vacuum, X is the laser wavelength and n2 is the nonlinear index of refraction. This is in accordance with numerical calculations by J. H. Marburger [in Progress in Quantum Electronics, J. H. Sanders and S. Stenholm, eds., Vol. 4, Part 1, pp. 35-110, Pergamon, Oxford (1975)]. With this knowledge we determine that damage is only partially explained by avalanche ionization and that the initiation of damage is strongly influenced by extrinsic processes. We therefore conclude that we are measuring extrinsic LID.

Determination of scattering properties and damage thresholds in tissue using ultrafast laser ablation

“…The NA is chosen to reduce the effects of spherical aberration, which can decrease the Strehl ratio as ablation depth is increased, 21 and to avoid self-focusing, which can cause damage before the intended focus or create a tighter focus, and lead to an underestimation of the ablation fluence threshold. 22,23 We ablate a 40 × 40 μm 2 FoV by scanning 512 lines at 1/16 frames per second, causing ∼0.078 μm separation between lines and ∼1.28 μm separation between pulses on the same line, resulting in a ∼20 pulse overlap. An FoV is ablated rather than a single point to better sample the tissue and provide a more accurate representation of l s and F th .…”

Imaging of Phase Objects using Partially Coherent Illumination

“…The first metric is the surface damage initiation density <N> max , and the second is the peak-irradiance in the bulk material I pk , used to predict self-focusing induced bulk filamentation damage. [36,37] These two damageability metrics are assigned failure thresholds for each downstream optic.…”

Investigation of Electromagnetic Field Propagation in Reverse Saturable Absorbers

“…Optical limiting was first observed in Uquids [12]- [13] where these early experiments demonstrated that the limiting could be due to self-focusing or self-defocusing as well as absorption and scattering. Later experiments showed that semiconductors have large optical nonlmearities [24].…”

“…This device regulated the output power of a continuous wave (CW) Argon ion laser. It wasn't until 1983 that CSa [13] was used as an optical limiting material for nanosecond pulses. The experiment used a focused geometry and showed that the mechanisms involved in limiting transmission were catastrophic self-focusing together with absorption and scattering resulting from laser induced breakdown.…”