Ultrafast lasers supplying highest average powers, pulse energies and pulse repetition rates require adequate system technology for high throughput and high quality laser processing. Depending on the application, multi-beam processing and/or high-speed scanning for fast beam deflection are required. Diffractive optical elements are used for beam splitting. Galvanometer scanners show high-performance at pulse repetition frequencies (PRF) up to a few MHz and polygon scanners provide high-speed scanning at even higher PRF for large surface processing. However, efficient line-oriented surface processing of small patterns using PRF in the MHz range cannot be achieved by existing scanning system technology. New resonant scanning systems have the potential to close this gap between galvanometer and polygon scanning with highest beam deflection speeds due to their oscillation frequencies in the kHz range and high duty cycles for all patterns. In addition to the high-speed scanning capabilities, resonant scanners can also enable new applications like ultrashort pulse laser (USPL) surface welding by moving the focused laser beam ultrafast along defined contours, inducing melting layers by heat accumulation. This publication describes the principles of high-speed processing with high power, high PRF ultrafast lasers in combination with high-speed resonant scanning systems. Potential applications and experimental results will be discussed like cutting, two-dimensional surface processing and functionalization, or welding applications. Moreover, a new approach is proposed to convert the nonlinear sinusoidal scanning of resonant systems into linear scanning by a spatially varying modulation of the unfocused laser beam.
Modern ultrashort pulsed high power (USP) lasers enable advanced and highly parallel micromachining processes. Due to constraints of the system technology, parallelization and beam shaping is mostly static during the process, which limits many interesting applications. The increasing power limits of liquid crystal on silicon (LCoS) spatial light modulators (SLMs) now open the door to dynamic beam shaping and splitting in the machining process. Although the frame rates of high-performance LCoS displays are typically in the range of 60 Hz and above, these devices are still mainly used as programmable diffractive optical elements (DOEs) and usually remain static during the fabrication process due to the high computational cost of conventional computer-generated hologram (CGH) algorithms. Herein, we report on the application of real-time CGH algorithms for dynamically generating and modifying 3D spot-distributions for advanced micromachining processes with high power USP lasers. We use a recently demonstrated algorithm based on compressive sensing combined with a highly parallelized computation on graphics processing units (GPUs). This allows for the calculation of phase holograms on the timescale of typical SLM response time and enables an online adjustment of the beam shape and spot distribution. In a detailed investigation of different CGH generation strategies, the impact of hardware response times, calculation speed and heat accumulations on the parallelized multi-spot laser process is experimentally evaluated on silicon. The presented results reveal the challenges and limits of on-the-fly CGH computation in USP laser material processing with SLMs and assesses its gain in processing flexibility for versatile USP micromachining applications.
In our work, we developed a LIDAR (LIght Detection And Ranging) source with high output energy of over 12 mJ and narrow spectral bandwidths < 30 MHz at a repetition rate of 1 kHz. Our approach contains the amplification of pulses with E ≤ 50 nJ out of a low energy seed source at the central wavelength 1064.475 nm and spectral width < 20 MHz. The seed pulses were amplified in four sequential Nd:YVO4 amplification stages. The amplification of parasitic ASE (Amplified Spontaneous Emission) from stage to stage due to the high emission cross section of the material had to be suppressed to guarantee maximal population inversion for the seed pulse amplification. By temporal separation of ASE and seed pulses with an AOM between the first and the second amplification stage, ASE could be suppressed sufficiently.
A density-dependent two-temperature model is applied to describe laser excitation and the following relaxation processes of silicon in an external electric field. Two approaches on how to describe the effects of the external electric field are presented. The first approach avoids the buildup of internal electric fields due to charge separation by assuming ambipolar diffusion and adds an additional carrier-pair current. In the second approach, electrons and holes are treated separately to account for charge separation and the resulting shielding of the external electric field inside the material. The two approaches are compared to experimental results. Both the first approach and the experimental results show similar tendencies for optimization of laser ablation in the external electric field.
During the last decades laser micromachining became a valuable tool for many applications in automotive, medicine, tool construction, or mobile technology. Beside the quality, processing time and production costs are crucial questions. A promising approach to achieve high throughput combined with good processing quality represent laser ablation by an assisting magnetic-field. Therefore, we studied the influence of an applied magnetic field to the ablation behaviour of silicon by using short and ultrashort laser pulses. Based on the experimental results we report on a first theoretical model that addresses the energy distribution of the heated electrons in the irradiated area.
We present a new fabrication method to realize smooth structures in lithium niobate. Therefore a detailed study on laser micro-polishing using ultrashort laser pulses is carried out by separating the effects of spatial pulse overlap and temporal pulse overlap. The adventage of this approach is the smoothing of the processed area by a simultaneous ablation. That will allow ablation depths between 1 µm and 4 µm with rms-roughness values of ≈20 nm.
We compare conventional bursts with GHz bursts in respect of their applicability for surface treatments such as polishing, hardening or surface alloying. It turns out that GHz bursts are more suitable than conventional bursts, especially with respect of energy efficiency and thus for gentle surface treatments with less thermal load for the bulk material. The temporal delay between the pulses is the key factor for surface melting. It is shown that the application of GHz bursts corrects the surface degradation caused by heat accumulation during ablation. This allows a dual process strategy to increase the ablation rates while maintaining the surface quality.
In many laser micro machining applications, ultra-short pulse (USP) lasers with a dynamical adaptive pulse repetition frequency (PRF) would allow for a significant increase of processing speed. Machining with ultra-fast resonant scanners or new techniques for fast processing narrow beam paths are two examples. These applications require USP lasers which provide a real-time PRF-synchronization to the deflection speed of the laser beam. Moreover, despite the dynamical change of the PRF between single pulses up to 10 MHz the pulse energy shall remain constant for high quality processing. Even today, this seems to be not easily realized. We present a laser system with constant pulse energies for all PRFs from single pulse to 10 MHz with an extremely fine PRF step size. The laser system consists of a mode-locked diode laser, running at a basic PRF of 4:3 GHz and a succeeding ultrafast pulse picker. This leads to a temporal accuracy of 233 ps, 40 times higher than with a typical solidstate mode-locked laser. To guarantee constant pulse energy over the entire PRF-range, the pulsed signal is superimposed with a cw-signal of the same polarization, serving as an inversion control in subsequent fiber and slab-amplifiers. In the end a SHG-stage serves as the desired cw-filter. The output power at 515nm is up to 109W. The pulse duration is 5 ps. With this setup, first experiments were performed showing the advantage of real USP pulse on demand laser systems.
A new generation of resonant scanners in the kHz-range shows ultra-high deflection speeds of more than 1000m/s but suffer from an inherent nonlinear mirror oscillation. If this oscillation is not compensated, a typical bitmap, written point by point, would be strongly distorted because of the decreasing spot distance at the turning point of the scanning mirror. However, this can be avoided by a dynamic adaption of the repetition rate (RR) of the ultrafast laser. Since resonant scanners are operated in the 10 kHz-range, this means that the RR has to be continuously swept up to several 10 000 times per second between e.g. 5MHz and 10 MHz. High-speed continuous adaption of the RR could also optimize laser micromachining of narrow curved geometries, where nowadays a time consuming approximation with numerous vectors is required. We present a laser system, which is capable of sweeping the RR more than 32 000 times per second between 5MHz and 10MHz at an average output power of more than 120W at 515nm with a pulse duration of about 40 ps. The laser consists of a semiconductor oscillator, a 3-stage fiber pre-amplifier, a solid state InnoSlab power amplifier and a SHG stage. We systematically analyzed the dynamic of the laser system as well as the spectral and temporal behavior of the optical pulses. Switching the repetition rate typically causes a varying pulse energy, which could affect the machining quality over one scanning line. This effect will be analyzed and discussed. Possible techniques to compensate or avoid this effect will be considered.
Characteristics by laser micromachining of congruent, stoichiometric and doped lithium niobate by using ultrashort laser pulses with different wavelengths from ultraviolet up to infrared were investigated. The ablation thresholds were determined in dependence of c+-side and accordingly c−-side. The strong impact of crystal orientation by micromachining lithium niobate will be additionally shown by the use of a high pulse repetition rate of 1000 kHz. Furthermore, we demonstrate the advantage of processing smooth ridges with high-repetition UV picosecond laser-pulses in combination of post-processing thermal annealing and a low-loss ridge waveguide in congruent LiNbO3 will be demonstrated.
Ridge waveguides in ferroelectric materials like LiNbO3 attended great interest for highly efficient integrated optical devices, for instance, electro-optic modulators, frequency converters and ring resonators. The main challenges are the realization of high index barrier towards the substrate and the processing of smooth ridges for minimized scattering losses. For fabricating ridges a variety of techniques, like chemical and wet etching as well as optical grade dicing, have been investigated in detail. Among them, laser micromachining offers a versatile and flexible processing technology, but up to now only a limited side wall roughness has been achieved by this technique. Here we report on laser micromachining of smooth ridges for low-loss optical waveguides in LiNbO3. The ridges with a top width of 7 µm were fabricated in z-cut LiNbO3 by a combination of UV picosecond micromachining and thermal annealing. The laser processing parameters show a strong influence on the achievable sidewall roughness of the ridges and were systematically investigated and optimized. Finally, the surface quality is further improved by an optimized thermal post-processing. The roughness of the ridges were analysed with confocal microscopy and the scattering losses were measured at an optical characterization wavelength of 632.8 nm by using the end-fire coupling method. In these investigations the index barrier was formed by multi-energy low dose oxygen ion implantation technology in a depth of 2.7 μm. With optimized laser processing parameters and thermal post-processing a scattering loss as low as 0.1 dB/cm has been demonstrated.
Irradiation of focused laser pulses to transparent materials leads to structural changes and can be used for the fabrication of e.g. LED light guiding components. In these applications both small spot sizes and a high lateral resolution in the μm range are absolutely essential. In order to achieve the industrially required throughput of nearly one million laser markings per second, ultrafast lasers with 100 W of average power and pulse repetition frequencies of several MHz are required. Laser machining of polymers additionally necessitates a wide spatial separation of the markings to avoid heat accumulation effects. Therefore, neither commercially available galvanometer based nor Polygon based scanners with their limited scan speed can be used for beam deflection. In our work, we developed an experimental setup based
We report on the creation of Fiber Bragg Gratings (FBGs) based sensors in large mode area (LMA) fibers. By using a quintupled Nd:YVO4 laser at 213 nm wavelength, FBGs are produced in conventional double clad fibers with core diameters of up to 50 µm. Bragg wavelength shifts depending on applied tensile strain as well as temperature changes are recorded and operating ranges of strain sensors identified. The ease of coupling with light sources into LMA fibers as well as their comparatively elevated robustness makes them promising sensor solutions for harsh environments.
In this study, ps-laser micromachining of different types of metals like copper, aluminum, titanium, tungsten and zinc have been investigated. Their single-pulse damage thresholds for the laser wavelengths of 355 nm, 532 nm and 1064 nm were experimentally determined. The laser-induced surface morphology both in the low and high fluence regime, as well as in the transition area, were examined for all investigated metals by scanning electron microscopy. Single pulse experiments and multi-pulse ablation experiments for up to 6 pulses using time-delays between successive pulses of 1 s and 20 ns were carried out. Our observations show that the surface morphologies significantly change from single-pulse ablation to the application of a second pulse. By comparing different separation times in the multi-pulse experiments we show that the burst-mode in ps-laser processing accumulates heat. This results in strong arising melting films, smoothing of the ablation craters and melt splashes outside of the crater. We found out, that copper and aluminum as well as titanium and zinc show similar ablation behavior by using the burst mode.
A new transversal pumping scheme of fiber lasers based on the optimized manufacturing of an array of large scale ridge waveguides in fused silica is presented. Moreover their application as directional couplers interacting with a double clad optical active fiber for laser application is discussed. Conventional broad area emitters without slow axis collimation (SAC) can be used to couple light (wavelength λp = 976 nm) into the waveguide array.
With the advent of high power and narrow bandwidth 969 nm pump diodes, direct pumping into the upper laser
level of Yb:YAG and hence quasi-2-level lasers became possible. Pumping directly into the emitting level leads
to higher quantum efficiency and reduction of non-radiative decay. Consequently, thermal load, thermal lensing
and risk of fracture are reduced significantly. Moreover pump saturation and thermal population of uninvolved
energy-levels in ground and excited states are benefical for a homogenous distribution of the pump beam as
well as the reduction of reabsorption loss compared to 3-level systems, which allows for high-power DPSS lasers.
Beside continuous-wave (cw) operation, nanosecond pulses with a repetition rate between 1 and 5 kHz are an
attractive alternative to flashlamp-pumped systems (10-100 Hz) in various measurement applications that require
higher data acquisition rates because of new faster detectors. Based on measurements of the absorption and
a detailed numerical model for pump beam distribution, including beam propagation and saturation factors,
power-scaling of a ceramic rod Yb:YAG oscillator was possible. Finally a cw output power of 50 W with 33
% pump efficiency at 1030 nm has been demonstrated (M2 < 1.2). Nanosecond pulses have been produced by
cavity-dumping of this system. The cavity-dumped setup allowed for 3-10 ns pulses with a pulse energy of 12.5
mJ at 1 kHz (M2 < 1.1). In order to achieve these results a systematic experimental and numerical investigation
on gain dynamics and the identification of different stable operating regimes has been carried out.
We present a widely tunable mid infrared coherent light source based on a tandem optical parametric oscillator (OPO)
and subsequent optical parametric amplification (OPA). The output wavelengths can be seamlessly tuned in the Mid-IR
from 2.6 μm to 12 μm or 4000 cm-1 to 833 cm-1 respectively. Maximal output energy of 26 mJ was obtained at a wavelength of 4.25 μm.
Carbon fiber reinforced plastic (CFRP) as a lightweight material with superior properties is increasingly being used in industrial manufacturing. Using ultrashort laser pulses can improve the quality in cutting or drilling applications, but at high power levels it is more complicated to maintain the accuracy and precision in CFRP drilling. According to the application requirements for the extent of the heat affected zone, the geometric precision and the productivity different drilling tools can be used. Therefore we report on the application of three different beam delivery systems to drilling processes of CFRP: Galvanometer scanner, trepanning head and diffractive optical elements.
We demonstrate a two-stage three-pass non-collinear optical parametric chirped-pulse amplifier delivering 125 μJ
pulse energy at 20 kHz repetition rate, corresponding to an average power of 2.5W. The system is pumped by
a 20 kHz Nd:YVO4 regenerative amplifier system. A grism-pair stretcher stretches the 6 fs seed pulses to more
than 100 ps from 650nm to 1000 nm. The amplified signal pulses are compressed with SF57 and fused silica
glass blocks. Using an acousto-optical programmable dispersive filter to compensate the residual higher-order
dispersion pulses of 9.6 fs duration are obtained which corresponds to a peak power of 13GW. We estimate the
level of parametric superfluorescence with the spectral hole technique.
We compared the performance of DQW and TQW edge-emitters in a passively mode-locked 1GHz MOPA
system at 1075 nm wavelength. Passive mode-locking is induced by applying a reverse DC voltage to the absorber
section. The average power is increased up to 0.9Wby a single-stripe pre-amplifier and a tapered amplifier. After
compensation of the quadratic chirp in a grating compressor we achieved a pulse duration of 342 fs. We found
that the oscillator gain current and the absorber bias voltage have significant impact on the pulse duration. Both
parameters were used to optimize the MOPA system with respect to the shortest pulse length after compression.
We report on a Nd:YVO4 regenerative amplifier (RA), end pumped by 888 nm-diode lasers. The output power
was about 46W at repetition rates from 150 to 833kHz with an M2-factor of 1.2. The amplifier was seeded by
a gain switched diode laser, generating pulses with a duration of 65 ps and a pulse energy of ≈ 5 pJ. The high
gain of the RA of more than 70 dB provides amplified pulse energies as high as 180μJ. Bifurcations of the pulse
energy could be avoided. Pulse amplitude fluctuations of only 1.2% for 10,000 consecutive pulses were measured.
The long term output power stability of the laboratory setup was 0.3%.
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