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Advances in fiber laser technology have resulted in the increased use and availability of several high-power laser systems operating in the mid-infrared band with short switching times and high modulation rates. The current American National Standard for the Safe Use of Lasers (ANSI Z136.1-2014) defines the calculation of the maximum permissible exposure (MPE) on the skin in terms of the exposure duration a single pulse or the total exposure time limit, and is based on continuous-wave laser skin exposure minimum visible lesion (MVL) data. This study determined the MVL data thresholds in Yucatan miniature pig skin for multiple-pulse 1940-nm laser exposures with pulse repetition frequencies (PRFs) of 100, 200, and 1000 Hz, and trains of 300, 1000, or 3000 pulses. The individual pulse duration in each exposure train was 500 µs. We report the MVL thresholds as the median effective dose (ED50) based on varying individual pulse energy. The results highlight the effect of PRF on the thresholds for multiple-pulse cases. Comparison with the existing ANSI Z136.1 MPE limits provides a calculation of the safety margin for each parameter case.

Neodymium-doped yttrium aluminum garnet (Nd:YAG) lasers are among the most commonly used lasers with a wide variety of applications from biomedicine to manufacturing. The ubiquity of these lasers increases the likelihood of accidental ocular injury resulting in permanent visual impairment. We performed dosimetry studies to determine retinal damage thresholds and hemorrhagic lesions in the porcine eye with Qswitched Nd:YAG lasers. The Yucatan miniature pig model exhibited similarities in ocular anatomy to human eyes. The Nd:YAG laser, tuned to 1064 nm with a pulse width of seven nanoseconds, delivered laser energy to the retina. Retinal imaging modalities including fundus photography, real-time video, confocal scanning laser ophthalmoscopy (cSLO), and spectral domain optical coherence tomography (SD-OCT) provided visualization of retinal morphology at multiple time points. Retinal damage thresholds were grouped into three categories: minimum visible lesion (MVL), contained hemorrhagic lesion (CHL), and vitreal hemorrhagic lesion (VHL). Probit analysis determined the effective dose for 50% probability of damage (ED50) for each lesion category. The threshold to produce a MVL was 0.193 mJ based on 24-hour assessments of the retina. The one-hour hemorrhagic lesion thresholds were 0.408 mJ and 1.52 mJ for CHL and VHL, respectively

Pulsed high-energy lasers operating in the near-infrared (NIR) band are increasingly being used in medical, industrial, and military applications, but there are little available experimental data to characterize their hazardous effects on skin tissue. The current American National Standard for the Safe Use of Lasers (ANSI Z136.1-2014) defines the maximum permissible exposure (MPE) on the skin as either a single-pulse or total exposure time limit. This study determined the minimum visible lesion (MVL) damage thresholds in Yucatan miniature pig skin for the single-pulse case and several multiple-pulse cases over a wide range of pulse repetition frequencies (PRFs) (10, 125, 2,000, and 10,000 Hz) utilizing nanosecond-scale pulses (10 or 60 ns). The thresholds are expressed in terms of the median effective dose (ED50) based on varying individual pulse energy with other laser parameters held constant. The results confirm a decrease in MVL threshold as PRF increases for exposures with a constant number of pulses, while also noting a PRF-dependent change in the threshold as a function of the number of pulses. Furthermore, this study highlights a change in damage mechanism to the skin from melanin-mediated photomechanical events at high irradiance levels and few numbers of pulses to bulk tissue photothermal additivity at lower irradiance levels and greater numbers of pulses. The observed trends exceeded the existing exposure limits by an average factor of 9.1 in the photothermally-damaged cases and 3.6 in the photomechanicallydamaged cases.

Laser-induced microcavitation refers to the rapid formation and expansion of a vapor bubble inside the bio-tissue when it is exposed to intense, pulsed laser energy. With the associated microscale dissection occurring within the tissue, laserinduced microcavitation is a common approach for high precision bio-surgeries. For example, laser-induced microcavitation is used for laser in-situ keratomileusis (LASIK) to precisely reshape the midstromal corneal tissue through excimer laser beam. Multiple efforts over the last several years have observed unique characteristics of microcavitions in biotissues. For example, it was found that the threshold energy for microcavitation can be significantly reduced when the size of the biostructure is increased. Also, it was found that the dynamics of microcavitation are significantly affected by the elastic modules of the bio-tissue. However, these efforts have not focused on the early events during microcavitation development. In this study, a direct numerical simulation of the microcavitation process based on equation of state of the biotissue was established. With the direct numerical simulation, we were able to reproduce the dynamics of microcavitation in water-rich bio tissues. Additionally, an experimental setup in deionized water and 10% PAA gel was made to verify the results of the simulation for early micro-cavitation formation for 10% Polyacrylamide (PAA) gel in deionized water.

Thresholds for microcavitation of isolated bovine retinal melanosomes were determined as a function of temperature using single nanosecond laser pulses at 532 nm and 1064 nm. Melanosomes were irradiated using a 1064-nm Qswitched Nd:YAG (doubled for 532-nm irradiation). Time-resolved microscopy was accomplished by varying the delay between the irradiation beam and an illumination beam allowing stroboscopic imaging of microcavitation events. Results indicated a decrease in microcavitation fluence threshold with increasing sample temperature for both 532-nm and 1064-nm single pulse exposures. The nucleation temperature at both wavelengths was extrapolated through the linear relationship between the temperature increases and the decrease in fluence threshold. In addition, absorption coefficients of melanosomes for visible and near-infrared wavelengths were estimated using the calculated nucleation temperatures.

Thresholds for microcavitation of isolated bovine and porcine melanosomes were determined using single nanosecond (ns) laser pulses in the NIR (1000 – 1319 nm) wavelength regime. Average fluence thresholds for microcavitation increased non-linearly with increasing wavelength. Average fluence thresholds were also measured for 10-ns pulses at 532 nm, and found to be comparable to visible ns pulse values published in previous reports. Fluence thresholds were used to calculate melanosome absorption coefficients, which decreased with increasing wavelength. This trend was found to be comparable to the decrease in retinal pigmented epithelial (RPE) layer absorption coefficients reported over the same wavelength region. Estimated corneal total intraocular energy (TIE) values were determined and compared to the current and proposed maximum permissible exposure (MPE) safe exposure levels. Results from this study support the proposed changes to the MPE levels.