Mid-infrared femtosecond laser damage thresholds in skin

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.