DNA-Damaging Effect of Different Wavelength (206 and 257 nm) Femtosecond Laser Pulses.

Objective: The purpose of this study was to investigate the possible cytotoxic and genotoxic impact of new-generation 206 nm femtosecond solid-state laser irradiation on murine skin cells in vitro, and to compare the cell and DNA damage caused by different wavelength (206 vs. 257 nm) femtosecond laser pulses. Background data: The first attempts to evaluate the possible genotoxic impact of ultrashort laser pulses on the murine bone marrow cells in vitro revealed the unlooked-for DNA-damaging effect. However, the impact of far-ultraviolet (UV) radiation on genetic material of internal and external organs' cells may differ due to differences in size, structure, and biochemical composition of the cells. Methods: Mouse skin cells were exposed to different doses of 206 and 257 nm wavelength femtosecond laser, and 254 nm UV lamp irradiation. Comet assay in two versions-the standard alkaline and the enzyme-linked-was used for the evaluation of DNA damage. Results: The irradiation determined by different parameters demonstrated intensity-dependent genotoxic impact. The pyrimidine dimers made up the greater part of DNA photodamage, but with rising exposure dose the increase of relative amount of more energy-consuming primary damage-DNA strand breaks-was detected. Conclusions: The 206 nm femtosecond laser irradiation was much more cytotoxic but caused less primary DNA damage than the same pulse duration longer wavelength (257 nm) laser irradiation. DNA-damaging effect of 206 nm femtosecond laser pulses with extremely low penetration force may highly depend on the size, structure, and biochemical composition of the cells of organ or tissue targets.

DNA Damage in Bone Marrow Cells Induced by Femtosecond and Nanosecond Ultraviolet Laser Pulses.

OBJECTIVE: The purpose of this study was to investigate the possible genotoxic impact of new generation 205 nm femtosecond solid-state laser irradiation on the DNA of murine bone marrow cells in vitro, and to compare the DNA damage caused by both femtosecond and nanosecond UV laser pulses. BACKGROUND DATA: Recent experiments of corneal stromal ablation in vitro and in vivo applying femtosecond UV pulses showed results comparable with or superior to those obtained using nanosecond UV lasers. However, the possible genotoxic effect of ultrashort laser pulses was not investigated. METHODS: Mouse bone marrow cells were exposed to different doses of 205 nm femtosecond, 213 and 266 nm nanosecond lasers, and 254 nm UV lamp irradiation. The comet assay was used for the evaluation of DNA damage. RESULTS: All types of irradiation demonstrated intensity-dependent genotoxic impact. The DNA damage induced depended mainly upon wavelength rather than on other parameters such as pulse duration, repetition rate, or beam delivery to a target. CONCLUSIONS: Both 205 nm femtosecond and clinically applied 213 nm nanosecond lasers' pulses induced a comparable amount of DNA breakage in cells exposed to the same irradiation dose. To further evaluate the suitability of femtosecond UV laser sources for microsurgery, a separate investigation of the genotoxic and mutagenic effects on corneal cells in vitro and, particularly, in vivo is needed.

High-speed photorefractive keratectomy with femtosecond ultraviolet pulses.

Femtosecond near-infrared lasers are widely used for a number of ophthalmic procedures, with flap cutting in the laser-assisted in situ keratomileusis (LASIK) surgery being the most frequent one. At the same time, lasers of this type, equipped with harmonic generators, have been shown to deliver enough ultraviolet (UV) power for the second stage of the LASIK procedure, the stromal ablation. However, the speed of the ablation reported so far was well below the currently accepted standards. Our purpose was to perform high-speed photorefractive keratectomy (PRK) with femtosecond UV pulses in rabbits and to evaluate its predictability, reproducibility and healing response. The laser source delivered femtosecond 206 nm pulses with a repetition rate of 50 kHz and an average power of 400 mW. Transepithelial PRK was performed using two different ablation protocols, to a total depth of 110 and 150 µm. The surface temperature was monitored during ablation; haze dynamics and histological samples were evaluated to assess outcomes of the PRK procedure. For comparison, analogous excimer ablation was performed. Increase of the ablation speed up to 1.6 s/diopter for a 6 mm optical zone using femtosecond UV pulses did not significantly impact the healing process.

Table top TW-class OPCPA system driven by tandem femtosecond Yb:KGW and picosecond Nd:YAG lasers.

We present a compact TW-class OPCPA system operating at 800 nm. Broadband seed pulses are generated and pre-amplified to 25 µJ in a white light continuum seeded femtosecond NOPA. Amplification of the seed pulses to 35 mJ at a repetition rate of 10 Hz and compression to 9 fs is demonstrated.

Corneal stromal ablation with femtosecond ultraviolet pulses in rabbits.

PURPOSE: To determine the effectiveness of femtosecond ultraviolet (UV) pulses in ablating corneal stroma in a rabbit model and to compare the healing response between eyes treated with femtosecond UV pulses and eyes treated with standard excimer photorefractive keratectomy. SETTING: Laser Research Center, Vilnius University, Vilnius, Lithuania. DESIGN: Experimental study. METHODS: Myopic photoablation using a femtosecond UV solid-state laser system was applied to corneas of pigmented rabbits. Experiments in 16 eyes were performed for optimization of the laser system parameters (fluence, spot size, pulse repetition rate) and calibration of ablation rate. In 7 rabbits, deep femtosecond UV ablation (∼130 µm) in 1 eye and shallow ablation (∼30 µm) in the contralateral eye were performed. Nine rabbits received an approximately 30 µm ablation with femtosecond UV pulses in 1 eye and with a conventional excimer system in the contralateral eye. Two eyes were used as controls. The ablation process and surface-temperature dynamics were monitored and recorded. Surface quality and haze development were evaluated. Rabbits were humanely killed 0 to 6 months after surgery, and eyes were enucleated for histological examination. RESULTS: Rabbit corneas ablated with femtosecond UV pulses or excimer laser radiation were similar in terms of the corneal wound-healing process, surface quality, and histology. CONCLUSIONS: The experiments indicate the feasibility of clinical application of femtosecond UV lasers for stromal ablation. The ability to switch between laser harmonics allows fast changeover from infrared to the UV mode, implying that a wide range of ophthalmic procedures can be performed using a single solid-state laser device. FINANCIAL DISCLOSURE: Ms. Gabryte and Mr. Danielius are paid employees of Light Conversion Ltd. Mr. Danielius is a shareholder of Light Conversion Ltd. No other author has a financial or proprietary interest in any material or method mentioned.

Corneal shaping and ablation of transparent media by femtosecond pulses in deep ultraviolet range.

PURPOSE: To assess the performance of a newly developed solid-state femtosecond ultraviolet (UV) laser system in corneal ablation. SETTING: Vilnius University, Laser Research Centre, Vilnius, Lithuania. METHODS: Femtosecond pulses in the deep UV range (205 nm) were obtained by the generation of the fifth-harmonic of an amplified Yb:KGW laser system (fundamental output at 1027 nm). Coupled with galvanometric beam-scanning mirrors, this system allowed ablation shaping of transparent media, including poly(methyl methacrylate) (PMMA), collagen, and ex vivo porcine corneas. The surfaces of ablated structures were characterized using a noncontact confocal optical profiler. RESULTS: Spherical structures were successfully formed in all 3 materials tested. A 10.0 diopter refraction change in the cornea was produced in 180 seconds. The resulting surface quality was significantly higher (roughness length >100 microm versus approximately 6 microm) in gelatin and ex vivo corneas than in PMMA. CONCLUSION: The solid-state femtosecond UV laser system seems an attractive alternative to the currently used ophthalmic argon-fluoride excimer laser system because of its small footprint, silent operation, and ability to generate femtosecond light pulses at both 1027 nm (suitable for flap creation) and 205 nm (corneal ablation).

Self-compression of millijoule 1.5 microm pulses.

We demonstrate a four-stage optical parametric chirped-pulse amplification system that delivers carrier-envelope phase-stable approximately 1.5 microm pulses with energies up to 12.5 mJ before recompression. The system is based on a fusion of femtosecond diode-pumped solid-state Yb technology and a picosecond 100 mJ Nd:YAG pump laser. Pulses with 62 nm bandwidth are recompressed to a 74.4 fs duration close to the transform limit. To show the way toward a terawatt-peak-power single-cycle IR source, we demonstrate self-compression of 2.2 mJ pulses down to 19.8 fs duration in a single filament in argon with a 1.5 mJ output energy and 66% energy throughput.

Energy extraction improvement in picosecond amplifiers by pulse tilting.

We report a novel tilted pulse amplification technique that, in contrast with the chirped-pulse amplification method, allows stretching of ultrashort pulses independently upon the initial pulse bandwidth. A gain factor of 4.5 and good reconstruction of a 15 ps pulse after stretching to 200 ps in a double-pass end-pumped Nd:YAG amplifier have been achieved.