Paul K. Kennedy

Laser-induced breakdown in aqueous media

Introduction (p.156). Laser-induced breakdown (p.157). Breakdown thresholds in aqueous media (p.161). Experimental measureemnts (p.177). Plasma expansion and emission in aqueous media (p.193). Mechanical effects of breakdown in aqueous media (p.205). Applications of laser-induced breakdown in liquids (p.231).

Theory and simulation on the threshold of water breakdown induced by focused ultrashort laser pulses

A comprehensive model is developed for focused pulse propagation in water. The model incorporates self-focusing, group velocity dispersion, and laser-induced breakdown in which an electron plasma is generated via cascade and multiphoton ionization processes. The laser-induced breakdown is studied first without considering self-focusing to give a breakdown threshold of the light intensity, which compares favorably with existing experimental results. The simple study also yields the threshold dependence on pulse duration and input spot size, thus providing a framework to view the results of numerical simulations of the full model. The simulations establish the breakdown threshold in input power and reveal qualitatively different behavior for picoand femto-second pulses. For longer pulses, the cascade process provides the breakdown mechanism, while for shorter pulses the cooperation between the self-focusing and the multiphoton plasma generation dominates the breakdown threshold.

A first-order model for computation of laser-induced breakdown thresholds in ocular and aqueous media. II. Comparison to experiment

For pt. I see ibid., vol.31, no.12, p.2241-9 (1995). 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.

Retinal damage and laser-induced breakdown produced by ultrashort-pulse lasers

Abstract• Background: In vivo retinal injury studies using ultrashort-pulse lasers at visible wavelengths for both rabbit and primate eyes have shown that the degree of injury to the retina is not proportional to the pulse energy, especially at suprathreshold levels. In this paper we present results of calculations and measurements for laser-induced breakdown (LIB), bubble generation, and self-focusing within the eye. • Methods: We recorded on video and measured the first in vivo LIB and bubble generation thresholds within the vitreous in rabbit and primate eyes, using external optics and femtosecond pulses. These thresholds were then compared with calculations from our LIB model, and calculations were made for self-focusing effects within the vitreous for the high peak power pulses. • Results: Results of our nonlinear modeling and calculations for self-focusing and LIB within the eye were compared with experimental results. The LIB ED50 bubble threshold for the monkey eye was measured and found to be 0.56 μJ at 120 fs, compared with the minimum visible lesion (MVL) threshold of 0.43 μJ at 90 fs. Self-focusing effects were found to be possible for pulsewidths below 1 ps and are probably a contributing factor in femtosecond-pulse LIB in the eye. • Conclusions: Based on our measurements for the MVL thresholds and LIB bubble generation thresholds in the monkey eye, we conclude that in the femtosecond pulsewidth regime for visible laser pulses, LIB and self-focusing are contributing factors in the lesion thresholds measured. Our results may also explain why it is so difficult to produce hemorrhagic lesions in either the rabbit or primate eye with visible 100-fs laser pulses even at 100 μJ of energy.

Variation of laser induced retinal-damage threshold with retinal image size

The dependence of retinal damage threshold on laser spot size was examined for two pulse width regimes; nanosecond-duration Q-switched pulses from a doubled Nd:Yttrium–aluminum–garnet laser and microsecond-duration pulses from a flashlamp-pumped dye laser. Threshold determinations were conducted for nominal retinal image sizes ranging from 1.5 to 100 mrad of visual field, corresponding to image diameters of ∼22 μm to 1.4 mm on the primate retina. In addition, base line collimated-beam damage thresholds were determined for comparison to the extended source data. Together, this set of retinal damage thresholds reveals the functional dependence of threshold on spot size. The threshold dose was found to vary with the area of the image for larger image sizes. This experimentally determined trend was shown to agree with the predictions of thermal model calculations of laser-induced retinal damage for spot sizes ≳150 μm. The results are compared to previously published extended source damage thresholds and to th...

Ultrashort laser pulse bioeffects and safety

Recent studies of retinal damage due to ultrashort laser pulses have shown that less energy is required for retinal damage for pulses shorter than 1 ns than that for longer pulses. It has also been shown that more energy is required for near-infrared (NIR) wavelengths than in the visible because the light focuses behind the retina, requiring more energy to produce a damaging fluence on the retina. We review the progress made in determining the trends in retinal damage from laser pulses of 1 ns to 100 fs in the visible and NIR wavelength regimes. We have determined the most likely damage mechanism(s) operative in this pulse width regime.

New data on the variation of laser induced retinal-damage threshold with retinal image size

In earlier studies, we examined the dependence of the laser induced retinal damage threshold on retinal image size for extended-source ocular exposures. We reported the spot-size dependence of the retinal threshold (based on ophthalmic observations at 24 h postexposure) for two pulsewidth regimes: nanosecond-duration (Q-switched) pulses from a doubled Nd:yttrium–aluminum–garnet laser and microsecond-duration pulses from a flashlamp-pumped dye laser at 590 nm. In either case, the retinal threshold was shown to vary with the area (i.e., diameter squared) for image diameters >5 mrad. More recently, we have collected additional data for the intermediate spot-size range (1.5–10 mrad) and have compared both the absolute values and the spot-size trend of retinal thresholds determined via ophthalmoscopic observation at 1 h postexposure to the analogous threshold data collected with observations at 24 h postexposure. These additional data and analyses reinforce our earlier conclusions regarding the threshold vs. s...

Trends in melanosome microcavitation thresholds for nanosecond pulse exposures in the near infrared

Abstract. Thresholds for microcavitation of bovine and porcine melanosomes were determined using nanosecond laser pulses in the near-infrared (1000 to 1319 nm) wavelength regime. Isolated melanosomes were irradiated by single pulses (10 or 50 ns) using a Q-switched Spectra Physics Nd:YAG laser coupled with an optical parametric oscillator (1000 to 1200 nm) or a continuum laser at 1319 nm. Time-resolved nanosecond strobe photography after the arrival of the irradiation beam allowed imaging of microcavitation events. Average fluence thresholds for microcavitation increased nonlinearly with increasing wavelength from ∼0.5  J/cm2 at 1000 nm to 2.6  J/cm2 at 1319 nm. Fluence thresholds were also measured for 10-ns pulses at 532 nm and found to be comparable to visible nanosecond pulse values published in previous reports. Calculated melanosome absorption coefficients decreased from 925  cm−1 at 1000 nm to 176  cm−1 at 1319 nm. This trend was found to be comparable to the decrease in retinal pigmented epithelial layer absorption coefficients reported over the same wavelength region. Estimated corneal total intraocular energy retinal damage threshold values were determined in order to compare to current and proposed maximum permissible exposure (MPE) safe levels. Results from this study support recently proposed changes to the MPE levels.

In vivo laser-induced breakdown in the rabbit eye

Threshold measurements for femtosecond laser pulsewidths have been made for retinal minimum visible lesions (MVLs) in Dutch Belted rabbit and rhesus monkey eyes. Laser-induced breakdown (LIB) thresholds in biological materials including vitreous, normal saline, tap water, and ultrapure water have been measured and reported using an artificial eye. We have recorded on video the first LIB causing bubble formation in any eye in vivo using albino rabbit eyes (New Zealand white) with 120- femtosecond (fs) pulses and pulse energies as low as 5 microjoules ((mu) J). These bubbles were clearly formed anterior to the retina within the vitreous humor and, with 60 (mu) J of energy, they lasted for several seconds before disappearing and leaving no apparent damage to the retina. We believe this to be true LIB because of the lack of pigmentation or melanin granules within the albino rabbit eye (thus no absorptive elements) and because of the extremely high peak powers within the 5-(mu) J, 120-fs laser pulse. These high peak powers produce self-focusing of the pulse within the vitreous. The bubble formation at the breakdown site acts as a limiting mechanism for energy transmission and may explain why high-energy femotsecond pulses at energies up to 100 (mu) J sometimes do not cause severe damage in the pigmented rabbit eye. This fact may also explain why it is so difficult to produce hemorrhagic lesions in either the rabbit or primate eye with 100-fs laser pulses.

Dynamic bidirectional reflectance distribution functions: Measurement and representation

With high-energy lasers, not only the direct laser beam can pose significant eye and skin hazards, but also light reflecting off material illuminated by the beam. Proper hazard analysis for a material irradiated by a laser relies upon the reflecting properties of the material surface, as these properties determine the magnitude and direction of the reflected laser energy commonly characterized by the bidirectional reflectance distribution function (BRDF). However, a high-energy laser heating and possibly melting a material can change the reflecting properties of that material, so these changes must be included in the hazard analysis. Traditional methods for measuring the BRDFs of materials are not practical for measurement of materials with rapidly-changing surface properties. However, BRDF measurement by imagery of a witness screen allows for practical measurements of the dynamically-changing BRDFs of materials under high-energy laser irradiation. Using this technique, the dynamic BRDFs of stainless stee...