There is an increasing use of high-power fiber lasers in manufacturing and telecommunications industries operating in the infrared spectrum between 1000 and 2000 nm, which are advertised to provide as much as 10 kW continuous output power at 1070 nm. Safety standards have traditionally been based on experimental and modeling investigations with scant data available for these wavelengths. A series of studies using 1070-nm infrared lasers to determine the minimum visible lesion damage thresholds in skin using the Yucatan miniature pig (Sus scrofa domestica) for a range of beam diameters (0.6, 1.1, 1.9, 2.4, 4.7, and 9.5 cm) and a range of exposure durations (10 ms to 10 s) is presented. Experimental peak temperatures associated with each damage threshold were measured using thermal imaging. Peak temperatures at damage threshold for the 10-s exposures were ∼10°C lower than those at shorter exposures. The lowest and highest experimental minimum visible lesion damage thresholds were found to have peak radiant exposures of 19 and 432 J/cm 2 for the beam diameter-exposure duration pairs of 2.4 cm, 25 ms and 0.6 cm, 10 s, respectively. Thresholds for beam diameters >2.5 cm had a weak to no effect on threshold radiant exposure levels for exposure times ≤0.25 s , but may have a larger effect on thresholds for exposures ≥10 s .
A series of experiments were conducted in vivo using Yucatan miniature pigs (Sus scrofa domestica) to determine thermal damage thresholds to the skin from 1319-nm continuous-wave Nd:YAG laser irradiation. Experiments employed exposure durations of 0.25, 1.0, 2.5, and 10 s and beam diameters of ∼0.6 and 1 cm. Thermal imagery data provided a time-dependent surface temperature response from the laser. A damage endpoint of fifty percent probability of a minimally visible effect was used to determine threshold for damage at 1 and 24 h postexposure. Predicted thermal response and damage thresholds are compared with a numerical model of optical-thermal interaction. Resultant trends with respect to exposure duration and beam diameter are compared with current standardized exposure limits for laser safety. Mathematical modeling agreed well with experimental data, predicting that though laser safety standards are sufficient for exposures <10 s , they may become less safe for very long exposures.
A computer-based model has been built that simulates the response of the retinal pigmented epithelial (RPE) cell to laser
exposure in the photochemical (non-thermal) damage exposure range (≥ 100 s exposures). The modeling approach used
is knowledge-based, modular, and hierarchical, allowing the explicit modeling of the cascades of intracellular events in
response to laser application. Thus, the model can be used to both analyze existing in vitro data sets, as well as
efficiently direct sampling strategies for future in vitro and in vivo studies. This model has been validated using
laboratory data from several studies reported in the literature using blue light (413 nm and 458 nm) lasers with 100 s, 200
s, and 3600 s exposure durations. The model was able to predict the in vitro ED50 response curve from these studies, as
well as the results for which we have no in vitro data (extrapolated based on irradiance reciprocity), within 1-6% for the
shorter duration exposures. Based on exploration of this computer model using lethal vs. non-lethal laser exposure
scenarios, the RPE cell’s oxidative stress response differs quantitatively very little with respect to typical oxidative stress
sources such as superoxide and hydrogen peroxide. However, in the lethal exposure scenarios the model points to a
potential tipping point in the oxidative stress response of the mitochondrial-based cellular energetics. Further studies are
underway to explore issues related to the levels of ATP/ADP and GSH/GSSG that are predicted by the model in these
lethal vs. non-lethal exposure scenarios.
ABSTRACT
Damage thresholds (ED50) for skin using Yucatan mini-pig (Sus scrofa domestica)
have been determined at the operational wavelength of 1319 nm with beam diameters of
0.61 cm and 0.96 cm. Exposure durations of 0.25, 1.0, 2.5 and 10 seconds were used to
determine trends in damage threshold with respect to exposure time and beam diameter at
this moderately-high penetrating wavelength. A relatively narrow range of total radiant
exposure from 37.4 J/cm2 to 62.3 J/cm2 average was observed for threshold damage with
laser parameters encompassing a factor of two in beam area and a factor of forty in
exposure duration.
Balb/c wild type mice were used to perform in vivo experiments of laser-induced thermal damage to the retina. A
Heidelberg Spectralis HRA confocal scanning laser ophthalmoscope with a spectral domain optical coherence
tomographer was used to obtain fundus and cross-sectional images of laser induced injury in the retina. Sub-threshold,
threshold, and supra-threshold lesions were observed using optical coherence tomography (OCT), infrared reflectance,
red-free reflectance, fluorescence angiography, and autofluorescence imaging modalities at different time points post-exposure.
Lesions observed using all imaging modalities, except autofluorescence, were not visible immediately after
exposure but did resolve within an hour and grew in size over a 24 hour period. There was a decrease in fundus
autofluorescence at exposure sites immediately following exposure that developed into hyper-fluorescence 24-48 hours
later. OCT images revealed threshold damage that was localized to the RPE but extended into the neural retina over a 24
hour period. Volumetric representations of the mouse retina were created to visualize the extent of damage within the
retina over a 24 hour period. Multimodal imaging provides complementary information regarding damage mechanisms
that may be used to quantify the extent of the damage as well as the effectiveness of treatments without need for
histology.
A series of experiments are conducted in vivo using Yucatan mini-pigs (Sus scrofa domestica) to determine thermal damage thresholds to the skin from 1940-nm continuous-wave thulium fiber laser irradiation. Experiments employ exposure durations from 10 ms to 10 s and beam diameters of approximately 4.8 to 18 mm. Thermal imagery data provide a time-dependent surface temperature response from the laser. A damage endpoint of minimally visible effect is employed to determine threshold for damage at 1 and 24 h postexposure. Predicted thermal response and damage thresholds are compared with a numerical model of optical-thermal interaction. Results are compared with current exposure limits for laser safety. It is concluded that exposure limits should be based on data representative of large-beam exposures, where effects of radial diffusion are minimized for longer-duration damage thresholds.
An adaptive optics imaging system was used to qualitatively observe the types of aberrations induced by an infrared laser
in a rhesus eye. Thermal lensing was induced with an infrared laser radiation wavelength of 1150-nm. The adaptive
optics system tracked the temporal response of the aberrations at a frequency of 30 Hz for continuous-wave exposures.
Results are compared against thermal lensing aberrations induced in an artificial eye.
A study of retinal damage thresholds in non-human primates (NHP) in the near-infrared (NIR) wavelengths of 1110,
1130, 1150, and 1319 nm has recently been reported. The progression of damage in retinal areas that received exposures
below, greater than, and at threshold values for each respective wavelength are compared. Subjects were imaged using
an Adaptive Optics (AO) enhanced Spectral Domain Optical Coherence Tomographer (SD-OCT) a year post laser
exposure to examine damage characteristics and localization. The subject's retinas within the study exhibited a delayed
response to NIR exposures in that many of the lesions that were not visible at the 1-hour observation period continued to
grow in size over the 24-hour period and or became visible. Thermal lensing is believed to play a significant role in the
formation or retinal lesions in the NIR and may explain the delayed response.
A confocal imaging system mounted to a micrometer stage was used to image the thermal lens induced into a water filled
Cain-cell artificial eye. A dual-beam pump-probe geometry was used to quantify the 633-nm visible wavelength probe
beam's transient response when exposed to the near-infrared pump-beam source. The infrared laser radiation
wavelengths tested were 1110, 1130, 1150 and 1318 nm for 1-s exposures to 450-mW of power. Analysis of video data
revealed the amount of refractive shift, induced by the thermal lens, as a function of time. Data demonstrate how the
formation and dissipation of the thermal lens follow a logarithmic excitation and exponential decay in time respectively.
Confocal imaging showed that thermal lensing was strongest for the 1150-nm wavelength followed by 1130, 1318 and
1110-nm.
Near threshold retinal lesions were created in the eyes of non-human primate (NHP) subjects in the near infrared (NIR)
wavelength range of 1100 to 1319 nm, with 80 to 100 ms laser exposures. Two new in vivo imagining techniques,
Adaptive Optic enhanced-Spectral Domain Optical Coherence Tomography (AO-SDOCT) and Adaptive Optic enhanced
confocal Scanning Laser Ophthalmoscope imagery (AOcSLO) were utilized to pinpoint areas of chronic damage within
the retinal layers resulting from laser exposure. Advantages and limitations of each technology with regard to the study
of laser retinal tissue interaction are highlighted.
Near-infrared (NIR) laser exposures to the retina are affected by intraocular absorption, chromatic aberration and retinal
absorption. We present the latest results of retinal exposure to wavelengths between 1.0 to 1.319 micrometers and show
how the trends for long-pulse exposure are dramatically affected by intraocular absorption in the anterior portion of the
eye.
A series of experiments were conducted in vivo on porcine skin to determine the ED50 damage thresholds for 1214 nm continuous wave laser irradiation. These results provide new information for refinement of Maximum Permissible Exposure (MPE). The study employed exposure durations of 1 sec, 3 sec, and 10 seconds with nominal spot diameters of 6 mm, 8 mm and 10 mm and as a function of laser power. The effect of each irradiation was evaluated acutely, one hour after exposure, and 24 hours post exposure. Probit analysis was conducted to estimate the dose for 50% probability of laser-induced damage (ED50); Damage was defined as persistent redness at the site of irradiation for the pig skin after 24 hours. The results indicated that Maximum Permissible Exposure (MPE) limits should be lowered for the laser beam diameters larger than 6 mm.
To support refinement of the Maximum Permissible Exposure (MPE) safety limits, a series of experiments were
conducted in vivo on Dutch Belted rabbit corneas to determine corneal minimum visible lesion thresholds for 2.0 &mgr;m
continuous-wave laser irradiation. Single pulse radiant exposures were made at specified pulse durations of 0.1 sec, 0.25
sec, 0.5 sec, 1.0 sec, 2.0 sec and 4.0 seconds for spot 1/e2 diameters of 1.17 mm and 4.02 mm. Lesions were placed in rows without overlap on rabbit cornea. The effect of each irradiation was evaluated within one minute post exposure
and the final determination of lesion formation was made using a slit lamp one hour post exposure. Threshold lesions
were defined as the presence of a superficial surface whitening one hour after irradiation. Probit analysis was conducted
to estimate the dose for 50% probability (ED50) of laser-induced damage. Approximately 20 different radiant exposures
were made for each exposure duration-spot size combination. At the threshold level, the diameters of barely visible
opaque white lesions were smaller than the Gaussian 1/e2 beam diameter. In selected survival animals, most of the
threshold lesions were still visible 24 hours after exposure. The average lesion radius was approximately 0.4 ± 0.12 mm
diameter for the 1.17 mm spot size and 1.0 ± 0.20 mm diameter for the 4.02 mm spot size. The exposure duration
dependence of threshold average radiant exposure was described by an empirical power law equation: Threshold radiant
exposure[J/cm2] = a x exposure duration[s] b, experimentally derived coefficient a was 9.79 and b was 0.669 for the
1.17 mm spot diameter; values of a and b were 4.57 and 0.456 respectively for the 4.02 spot diameter. Based on the
experimental data and the empirical power law, the safety factors which were defined as threshold radiant exposure
divided by MPE values were predicted for the 2.0 &mgr;m wavelength at various exposure durations and spot diameters.
The minimum limit of the safety factor was approximately a factor of four for both 4.02 mm and 1.17 mm spot
diameters. Due to the very sharp boundary and small uncertainties of damage threshold determination, it is suggesting
that a factor of 4 "padding" is adequate and safety standard may not need to be changed.
A series of experiments in a new animal model for retinal damage, cynomolgus monkeys (Macaca fascicularis), have been conducted to determine the damage threshold for 12.5-nanosecond laser exposures at 1064 nm. These results provide a direct comparison to threshold values obtained in rhesus monkey (Macaca mulatta), which is the model historically used in establishing retinal maximum permissible exposure (MPE) limits. In this study, the irradiance level of a collimated Gaussian laser beam of 2.5 mm diameter at the cornea was randomly varied to produce a rectangular grid of exposures on the retina. Exposures sites were fundoscopically evaluated at post-irradiance intervals of 1 hour and 24 hours. Probit analysis was performed on dose-response data to obtain probability of response curves. The 50% probability of damage (ED50) values for 1 and 24 hours post-exposure are 28.5(22.7-38.4) &mgr;J and 17.0(12.9-21.8) &mgr;J, respectively. These values compare favorably to data obtained with the rhesus model, 28.7(22.3-39.3) &mgr;J and 19.1(13.6-24.4) &mgr;J, suggesting that the cynomolgus monkey may be a suitable replacement for rhesus monkey in photoacoustic minimum visible lesion threshold studies.
A diagnostic system was designed, fabricated and tested for measuring a Nd:YAG laser's performance using a five-axis laser machining center. Far field beam profile, beam quality, beam tilt and beam power were measured at high frequencies while holes were simultaneously percussion drilled in aerospace materials used in gas turbine engines. The system allowed determination of effects of varying laser and machine operating parameters on materials processing performance. Variations were made in laser pulse length, pulse energy, pulse repetition rate, drilling standoff distances from part, incident angles and other parameters. Measured laser beam performance was compared directly with drilled holes. A number of identically drilled holes provided hole-to-hole variance, allowing the machine center operators to optimize materials processing by tuning the laser operating parameters. The results also gave the operator additional understanding of variations of processing performance.
A comprehensive diagnostic instrumentation suite was designed and successfully utilized to characterize the performance parameters from two coupled lasers. The characterization included such standard diagnostic measurements as: absolute and RMS power, polarization, jitter, near-field and far-field irradiance profiles, near-field phase, spectral content, and beam quality. Coupling strength between the two lasers was also measured as well as their mutual coherence, locking range, and the heterodyne frequencies between axial modes in the resonator.
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