Hard-tissue ablation was already investigated for a broad variety of pulsed laser systems, which cover almost the entire range of available wavelengths and pulse parameters. Most effective in hard-tissue ablation are Er:YAG and CO2 lasers, both utilizing the effect of absorption of infrared wavelengths by water and so-called explosive vaporization, when a thin water film or water–air spray is supplied. The typical flow rates and the water layer thicknesses are too low for surgical applications where bleeding occurs and wound flushing is necessary. We studied a 20 W ps-laser with 532 nm wavelength and a pulse energy of 1 mJ to effectively ablate bones that are submerged 14 mm under water. For these laser parameters, the plasma-mediated ablation mechanism is dominant. Simulations based on the blow-off model predict the cut depth and cross-sectional shape of the incision. The model is modified considering the cross section of the Gaussian beam, the incident angle, and reflections. The ablation rate amounts to 0.2 mm3/s, corresponding to an increase by at least 50% of the highest values published so far for ultrashort laser ablation of hard tissue.
Next-generation infrared (IR) optical components based on chalcogenide glasses (ChGs) may include structures which benefit from the enhanced optical function offered by spatially modifying regions with a nanocrystalline phase. Such modification may be envisioned if the means by which such spatial control of crystallization can be determined using the advantages offered through three-dimensional direct laser write (DLW) processes. While ChGs are well known to have good transparency in the IR, they typically possess lower thresholds for photo- and thermally- induced property changes as compared to other glasses such as silicates. Such low thresholds can result in material responses that include photoexpansion, large thermo-optic increases, mechanical property changes, photo-induced crystallization, and ablation. The present study examines changes in ChG material response realized by exposing the material to different laser irradiation conditions in order to understand the effects of these conditions on such material property changes. Thresholds for photoexpansion and ablation were studied by varying the exposure time and power with sub-bandgap illumination and evidence of laser induced phase change were examined. Simulations were carried out to estimate the temperature increase from the irradiation and the tolerances and stability of the calculations were examined. The models suggest that the processes may have components that are non-thermal in nature.
Within the past 10 years, thulium (Tm)-doped fiber lasers have emerged as a flexible platform offering high average power as well as high peak power. Many of the benefits and limitations of Tm:fiber lasers are similar to those for ytterbium (Yb)-doped fiber lasers, however the ~2 µm emission wavelength posses unique challenges in terms of laser development as well as several benefits for applications. In this presentation, we will review the progress of laser development in CW, nanosecond, picosecond, and femtosecond regimes. As a review of our efforts in the development of power amplifiers, we will compare large mode area (LMA) stepindex and photonic crystal fiber (PCF) architectures. In our research, we have found Tm-doped step index LMA fibers to offer relatively high efficiency and average powers at the expense of fundamental mode quality. By comparison, Tm-doped PCFs provide the largest mode area and quasi diffraction-limited beam quality however they are approximately half as efficient as step-index fibers. In terms of defense related applications, the most prominent use of Tm:fiber lasers is to pump nonlinear conversion to the mid-IR such as supercontinuum generation and optical parametric oscillators/amplifiers (OPO/A). We have recently demonstrated Tm:fiber pumped OPOs which generate ~28 kW peak power in the mid-IR. In addition, we will show that Tm:fiber lasers also offer interesting capabilities in the processing of semiconductors.
Semiconductors such as Si and GaAs are transparent to infrared laser radiation with wavelengths >1.2 μm. Focusing
laser light at the back surface of a semiconductor wafer enables a novel processing regime that utilizes this transparency.
However, in previous experiments with ultrashort laser pulses we have found that nonlinear absorption makes it
impossible to achieve sufficient optical intensity to induce material modification far below the front surface. Using a
recently developed Tm:fiber laser system producing pulses as short as 7 ns with peak powers exceeding 100 kW, we
have demonstrated it is possible to ablate the “backside” surface of 500-600 μm thick Si and GaAs wafers. We studied
laser-induced morphology changes at front and back surfaces of wafers and obtained modification thresholds for multipulse
irradiation and surface processing in trenches. A significantly higher back surface modification threshold in Si
compared to front surface is possibly attributed to nonlinear absorption and light propagation effects. This unique
processing regime has the potential to enable novel applications such as semiconductor welding for microelectronics,
photovoltaic, and consumer electronics.
Additive manufacturing, also known as 3D-printing, is a near-net shape manufacturing approach, delivering part
geometry that can be considerably affected by various process conditions, heat-induced distortions, solidified melt
droplets, partially fused powders, and surface modifications induced by the manufacturing tool motion and processing
strategy. High-repetition rate femtosecond and picosecond laser radiation was utilized to improve surface quality of
metal parts manufactured by laser additive techniques. Different laser scanning approaches were utilized to increase the
ablation efficiency and to reduce the surface roughness while preserving the initial part geometry. We studied post-processing
of 3D-shaped parts made of Nickel- and Titanium-base alloys by utilizing Selective Laser Melting (SLM) and
Laser Metal Deposition (LMD) as additive manufacturing techniques. Process parameters such as the pulse energy, the
number of layers and their spatial separation were varied. Surface processing in several layers was necessary to remove
the excessive material, such as individual powder particles, and to reduce the average surface roughness from asdeposited
22-45 μm to a few microns. Due to the ultrafast laser-processing regime and the small heat-affected zone
induced in materials, this novel integrated manufacturing approach can be used to post-process parts made of thermally
and mechanically sensitive materials, and to attain complex designed shapes with micrometer precision.
Several laser systems in the infrared wavelength range, such as Nd:YAG, Er:YAG or CO<sub>2</sub> lasers are used for efficient ablation of bone tissue. Here the application of short pulses in coaction with a thin water film results in reduced thermal side effects. Nonetheless up to now there is no laser-process for bone cutting in a clinical environment due to lack of ablation efficiency. Investigations of laser ablation rates of bone tissue using a rinsing system and concerning bleedings have not been reported yet. In our study we investigated the ablation rates of bovine cortical bone tissue, placed 1.5 cm deep in water under laminar flow conditions, using a short pulsed (25 ps), frequency doubled (532 nm) Nd:YVO<sub>4</sub> laser with pulse energies of 1 mJ at 20 kHz repetition rate. The enhancement of the ablation rate due to debris removal by an additional water flow from a well-directed blast pipe as well as the negative effect of the admixture of bovine serum albumin to the water were examined. Optical Coherence Tomography (OCT) was used to measure the ablated volume. An experimental study of the depth dependence of the ablation rate confirms a simplified theoretical prediction regarding Beer-Lambert law, Fresnel reflection and a Gaussian beam profile. Conducting precise incisions with widths less than 1.5 mm the maximum ablation rate was found to be 0.2 mm<sup>3</sup>/s. At depths lower than 100 μm, while the maximum depth was 3.5 mm.