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.
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.
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.
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.