Ultrashort pulse lasers are currently used in a variety of applications, including laser eye surgery. However, laser safety standards do not consider the potential hazard to the eye due to the nonlinear interaction of ultrashort lasers with the ocular tissues. We used a single NIR femtosecond pulse to determine the peak pulse energies that generated a supercontinuum within the eye of anesthetized porcine subjects and resulted in retinal alterations. The results of this study inform the laser safety standards about hazards to the eye due to the supercontinuum generated by nonlinear effects in the aqueous media of the eye.
Lasers with ultrashort pulse durations have become ubiquitous in a variety of applications, including medical procedures such as laser eye surgery. These sources generate high peak powers such that the role of nonlinear optical effects resulting from the interaction of femtosecond pulses with the surrounding media needs to be considered when evaluating their potential hazard to the eye. In the latest version of the ANSI laser safety standards, the safe exposure limits have been relaxed at wavelengths between 1.2 – 1.4 µm because of biological data collected for the nanosecond and millisecond pulse regimes. However, this increase did not consider nonlinear optical effects resulting from the interaction with femtosecond pulses. One manifestation of these nonlinear effects is the generation of broadband light known as supercontinuum. We sent a near infrared (NIR) femtosecond laser with peak energies at or below the energy corresponding to the maximum permissible exposure (MPE) limit listed in laser safety standards into the eye of anesthetized porcine subjects. Exposures were performed with both collimated and converging beams to simulate an eye focused at a far distance and one focused at a near distance, respectively, and have the potential to generate a supercontinuum within the eye. Nominally 1 h and 24 h after exposure, the retina was examined using a fundus camera. The presence or absence of any alteration of the retina was noted. The results of this study inform the laser safety standards committees about potential hazards to the eye due to the supercontinuum generated by nonlinear effects in the aqueous media of the eye.
Recently, pulsed lasers with ultrashort pulse durations have become ubiquitous in a variety of applications, including medical procedures such as laser eye surgery. These sources are capable of generating extremely high peak powers that can cause laser-induced tissue breakdown upon exposure. However, current laser safety standards do not provide exposure limits for wavelengths longer than 1400 nm and pulse durations shorter than 1 ns due to a lack of biological data. Instead, the recommendation is to limit the peak irradiance to the maximum permissible exposure (MPE) limits applicable to 1 ns pulse durations. We applied femtosecond laser pulses of varying energies at 1540 nm and 2000 nm to corneas of anesthetized rabbits. We used slit lamp biomicroscopy and optical coherence tomography to examine the exposure sites and determine the presence or absence of visible lesions 1 h and 24 h post-exposure. The dose-response data correlating the presence or absence of any alteration of the corneal surface to the pulse energy 1 h post-exposure was evaluated using probit analysis to extract the median effective dose (ED50) corresponding to the cornea damage threshold. We compared our results to the MPE limits applicable to 1 ns pulse durations and determined that the current safety standard procedures are not adequate to evaluate small diameter single pulse femtosecond exposures at 1540 nm and 2000 nm. The results of this study contribute to the knowledge base used for setting laser safety standards in the near infrared range for ocular exposure to ultrashort pulses.
Pulsed lasers with ultrashort pulse durations have become ubiquitous in a variety of applications, including laser eye surgery. Therefore, the role of nonlinear optical effects, such as supercontinuum generation, needs to be considered when evaluating their potential hazard. We used a NIR femtosecond laser to generate a supercontinuum within an artificial eye. We recorded the visible spectra of the supercontinuum generated and calculated the energy contained within the visible band. Our results indicate that for certain exposure conditions, the supercontinuum’s energy within the visible range surpasses the maximum permissible energy allowed for visible wavelengths by the laser safety standards.
Recent developments in high-energy regenerative amplifiers and broadly tunable optical parametric amplifiers (OPA) opened new spectral windows to study the impact of ultrashort laser pulses on biological tissues. These sources can generate extraordinarily high peak power capable of causing laser-induced breakdown. However, current laser safety standards (ANSI Z136.1-2014) do not provide guidance on maximum permissible exposure (MPE) values for the skin with pulse durations less than one nanosecond. This study measured damage thresholds in excised porcine skin in the mid-infrared (MIR) region of the electromagnetic spectrum. The laser system, comprised of a high-energy regenerative amplifier and OPA, was tuned to wavelengths between 4000-6000 nm to coincide with heightened absorption for both water and collagen. The laser operated at a fundamental repetition rate of 1 kHz and a nominal pulse width of 150 fs. The beam was focused at the sample surface with a 36X aluminum reflective objective and scanned over a 4 mm2 area for each exposure condition. Spectral domain optical coherence tomography (SD-OCT) imaging of the tissue provided a volumetric assessment of tissue morphology and identified changes in the backscattering profile within the laser-exposed regions. The determination of laser damage thresholds in the MIR for ultrafast lasers will guide safety standards and establish the appropriate MPE levels for exposure to sensitive biological tissue. These data will help guide the safe use of ultrafast MIR lasers in emerging applications across a multitude of industries and operational environments.
Understanding the optical properties of water is critical to both laser-tissue interactions as well as setting ocular laser safety standards. The nonlinear properties of water are responsible for supercontinuum generation; however, these effects are poorly understood for wavelengths longer than 1064 nm. A previous study suggested that the supercontinuum generation may convert retinal-safe femtosecond near-infrared pulses with wavelengths longer than 1064 nm into visible wavelength pulses that are above the maximum permissible exposure limit as defined by ANSI Z136.1-2014. To address this knowledge gap, we extend the Z-scan technique in distilled water to wavelengths between 1150 nm to 1400 nm, where linear absorption is strong. Utilizing wavelength tunable, nominally 100 fs laser pulses, we observe wavelength dependence of the nonlinear optical properties of water. The nonlinear refractive index at 1150 nm was consistent with measurements taken at 532 nm in previous studies, and was observed to increase at longer wavelengths. The nonlinear absorption was positive for wavelengths between 1150 nm and 1350 nm and reversed to saturable absorption at 1400 nm. Saturable absorption poses a previously unanticipated eye safety risk as current ocular laser safety standards assume strong absorption at 1400 nm. These results expand our current understanding of the nonlinear optical properties of water to wavelengths in the 1150 nm to 1400 nm region, and inform efforts to revise national and international exposure limits to account for retinal hazards due to nonlinear effects.
Ultrafast lasers have become a fixture in many biomedical, industrial, telecommunications, and defense applications in recent years. These sources are capable of generating extremely high peak power that can cause laser-induced tissue breakdown through the formation of a plasma upon exposure. Despite the increasing prevalence of such lasers, current safety standards (ANSI Z136.1-2014) do not include maximum permissible exposure (MPE) values for the cornea with pulse durations less than one nanosecond. This study was designed to measure damage thresholds in corneal tissue phantoms in the near-infrared and mid-infrared to identify the wavelength dependence of laser damage thresholds from 1200-2500 nm. A high-energy regenerative amplifier and optical parametric amplifier outputting ~100 femtosecond pulses with pulse energies up to 2 mJ were used to perform exposures and determine damage thresholds in transparent collagen gel tissue phantoms. Three-dimensional imaging, primarily optical coherence tomography, was used to evaluate tissue phantoms following exposure to determine ablation characteristics at the surface and within the bulk material. The determination of laser damage thresholds in the near-IR and mid-IR for ultrafast lasers will help to guide safety standards and establish the appropriate MPE levels for exposure sensitive ocular tissue such as the cornea. These data will help promote the safe use of ultrafast lasers for a wide range of applications.
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