Burst mode (BM) processing with femtosecond laser pulses is emerging as a versatile tool for manufacturing micro-components on different materials, thanks to its ability to reduce the thermal load, which ensures highly precise and accurate miniaturization. However, a systematic investigation of the influence of the experimental parameters introduced by such irradiation mode, i.e., the number of pulses within the burst, their polarization and the intra-burst frequency, on the ablation process has not been reported, yet. In this work, we exploited a statistical approach based on the Design of Experiment (DoE) to study the micro-milling process of steel with bursts. Two prediction models were defined, describing the relationship between the working parameters, i.e., average power, number of overscans, laser repetition rate, scan speed and number of pulses within the bursts, and the response variables, i.e., ablated depth and surface roughness, revealing burst mode as a very promising solution to improve the surface finishing of ultrashort laser pulses micromilled components.

We report on an experimental investigation of ultrafast laser ablation of silicon with bursts of pulses. The pristine 1030nm-wavelength 200-fs pulses were split into bursts of up to 16 sub-pulses with time separation ranging from 0.5ps to 4080ps. The total ablation threshold fluence was measured depending on the burst features, finding that it strongly increases with the number of sub-pulses for longer sub-pulse delays, while a slowly increasing trend is observed for shorter separation time. The ablation depth per burst follows two different trends according to the time separation between the sub-pulses, as well as the total threshold fluence. For delays shorter than 4ps it decreases with the number of pulses, while for time separations longer than 510ps, deeper craters were achieved by increasing the number of subpulses in the burst, probably due to a change of the effective penetration depth.

We study the incubation effect during laser ablation of stainless steel with ultrashort pulses to boost the material removal efficiency at high repetition rates. The multi-shot ablation threshold fluence has been estimated for two pulse durations, 650-fs and 10-ps, in a range of repetition rates from 50kHz to 1 MHz. Our results show that the threshold fluence decreases with the number of laser pulses N due to damage accumulation mechanisms, as expected. Moreover, approaching the MHz regime, the onset of heat accumulation enhances the incubation effect, which is in turn lower for shorter pulses at repetition rates below 600 kHz. A saturation of the threshold fluence value is shown to occur for a significantly high number of pulses, and well fitted by a modified incubation model. (C) 2014 Optical Society of America

The recent development of ultrafast laser ablation technology in precision micromachining has dramatically increased the demand for reliable and real-time detection systems to characterize the material removal process. In particular, the laser percussion drilling of metals is lacking of non-invasive techniques able to monitor into the depth the spatial- and time-dependent evolution all through the ablation process. To understand the physical interaction between bulk material and high-energy light beam, accurate in-situ measurements of process parameters such as the penetration depth and the removal rate are crucial. We report on direct real time measurements of the ablation front displacement and the removal rate during ultrafast laser percussion drilling of metals by implementing a contactless sensing technique based on optical feedback interferometry. High aspect ratio micro-holes were drilled onto steel plates with different thermal properties (AISI 1095 and AISI 301) and Aluminum samples using 120-ps/110-kHz pulses delivered by a microchip laser fiber amplifier. Percussion drilling experiments have been performed by coaxially aligning the diode laser probe beam with the ablating laser. The displacement of the penetration front was instantaneously measured during the process with a resolution of 0.41 ?m by analyzing the sawtooth-like induced modulation of the interferometric signal out of the detector system.

We demonstrate that a non-invasive sensing technique based on optical feedback interferometry is capable to instantaneously measure the ablation front displacement and the removal rate during ultrafast laser percussion drilling of metallic plates. The sawtooth-like modulation of the interferometric signal out of the detecting sensor has been analyzed to reveal the time dependence of the removal depth with sub-micrometric resolution. Various dynamic factors related to the influence of laser pulse duration and peak energy have been assessed by in-situ spatial- and time-dependent characterization all through the ablation process. The importance of realtime measurement of the ablation rate is crucial to improve the basic understanding of ultrafast lasermaterial interactions. Moreover, the detection system results high-sensitive, compact, and easily integrable in most industrial workstations, enabling the development of on-line control to improve the ablation efficiency and the quality of laser micromachining processes.

High-energy ultra-short pulse laser ablation is a fast-growing technology in precision laser micromachining of transparent as well as opaque materials. Accurate in-situ measurements of physical parameters such as the penetration depth and the removal rate are crucial to fully characterize the ultrafast laser-material interactions [1-5]. Nonetheless, the laser drilling is still lacking of a real-time technique able to monitor and control the spatial- and time-dependent evolution of the hole-depth in metallic plates.

We present an experimental study of the drilling of metal targets with ultrashort laser pulses with pulse durations from 800 fs to 19 ps at repetition rates up to 1 MHz, average powers up to 70 Watts, using an Ytterbium-doped fiber CPA system. Particle shielding and heat accumulation have been found to influence the drilling efficiency at high repetition rates. Particle shielding causes an increase in the number of pulses for breakthrough. It occurs at a few hundred kHz, depending on the pulse energy and duration. The heat accumulation effect is noticed at higher repetition rates. Although it overbalances the particle shielding thus making the drilling process faster, heat accumulation is responsible for the formation of a large amount of molten material that limits the hole quality. The variations of the pulse duration reveal that heat accumulation starts at higher repetition rates for shorter pulse lengths. This is in agreement with the observed higher ablation efficiency with shorter pulse duration. Thus, the shorter pulses might be advantageous if highest precision and processing speed is required.

We use novel high-finesse polymeric resonant grating waveguide structures (GWS) for strong enhancement of two photon fluorescence (TPF). At a specific wavelength and angular orientation of the incident beam, the grating waveguide structure resonates. This resonance results in a field enhancement at the surface that can be exploited for TPF spectroscopy, without the need for highly focused laser excitation light. We compare the TPF obtained from a thin layer of tetramethylrhodamine (TMR) deposited on top of a GWS at resonance and off-resonance. Our procedure and results indicate that the detection of TPF can indeed be improved with the resonant GWS by at least fifteen times. These results have been also demonstrated using a GWS and a 2 ?m thick layer of TMR aqueous solution. © 2004 Elsevier B.V. All rights reserved.