A First - Order Model for Computation of Laser - Induced Breakdown Thresholds in Ocular and Aqueous Media : Part II - Comparison to Experiment

An analytic, first-order model has been developed to calculate irradiance thresholds for laser-induced breakdown (LIB) in condensed media, including ocular and aqueous media. A complete derivation and description of the model was given in a previous paper (Part I). The model has been incorporated into a computer code and code results have been compared to experimentally measured irradiance thresholds for breakdown of ocular media, saline, and water by nanosecond, picosecond, and femtosecond laser pulses in the visible and near-infrared. The comparison included both breakdown data from the literature and from our own measurements. Theoretical values match experiment to within a factor of 2 or better, over a range of pulsewidths spanning five orders of magnitude. The model includes two different definitions of breakdown, corresponding to two different experimental endpoints. The "flash" endpoint (hot, dense, visibly emitting plasma) is calculated using critical densities of 1020/cm3. The "bubble" endpoint (cooler , more diffuse plasma with little or no emission in the visible) is calculated using critical densities of 1018/cm3. The model has been incorporated into a computer code for comparison to experiment. This paper documents the code and gives a comparison of code results to experimentally measured irradiance thresholds for breakdown of ocular media, saline, and water by nanosecond (ns), picosecond (ps), and femtosecond (fs) laser pulses in the visible and near-infrared. The comparison includes data from the literature [4]-[8], for long pulse (rp ~ 30 ps) infrared breakdown, and from our own measurements, for visible and infrared pulses ranging from 7 ns to 100 fs. Threshold values for visible wavelengths and for pulsewidths of less than 30 ps, have not, to our knowledge, previously been measured in these media. Theoretical and experimental values agree fairly well for data corresponding to both experimental endpoints: 1. ns and long ps pulse breakdown with a flash endpoint, and 2. short ps and fs pulse breakdown with a bubble endpoint. Theoretical values match experiment to within a factor of 2 or better, over a range of pulsewidths spanning five orders of magnitude, a reasonably good match for a first order model. A description of the code is given in Section II. Experimental equipment and techniques for the Armstrong Lab LIB measurements are described in Section III. In Sections IV and V experimental data is compared with code results for long pulse breakdown with a flash endpoint and for short pulse breakdown with a bubble endpoint, respectively. Conclusions are given in Section VI. II. CODE DESCRIPTION The Laser Induced Breakdown Irradiance Threshold code, or LIBIRT, incorporates the equations, parameters, a.ld definitions of the model derived previously [1]. The code computes the cascade breakdown threshold, lth, multiphoton breakdown threshold, IMP, and multiphoton initiation threshold, Im, for a specified medium, laser pulse, and breakdown end point. For pulsewidths too short to allow cascade breakdown from the minimum initial density, po(min), the code computes the alternate values I:n and I:h, corresponding to a higher initial