Penetration and Precision of Subsurface Photodisruption in Porcine Skin Tissue With Infrared Femtosecond Laser Pulses

The surgical precision of photodisruption with ultrafast optical pulses depends on the accurate delivery of optical energy to sites of interest. As light penetration is limited in turbid tissues, localization and precision of subsurface breakdown highly depend on the interacting effects of increased power requirements and external focusing conditions. Infrared femtosecond breakdown extent in excised porcine skin tissue was investigated using a high-frequency ultrasonic technique which sensitively detected laser-induced bubbles. Using a focused laser source, optical parameters including laser fluence and focusing numerical aperture (NA) were controlled. Decreasing NA improved penetration, while increasing NA improved precision. At lower NA, penetrations of up to 1 mm could be achieved with a single laser excitation at the cost of an expanded breakdown region which reduced precision. Even at higher NA, however, maximum penetration reduced to 400 mum and precision was still limited with extended breakdown regions for focusing depths greater than 100 mum. Glycerol was used as an index matching material, which helped reduce scattering and improved penetration by 150 to 400 mum for all NAs. Nonetheless, multiple breakdown sites and the corresponding reduction in precision were observed even with glycerol treatment.