Industrial grade ultrashort pulsed USP laser systems offer pulse durations of about 300 fs and higher whereas shorter pulse durations could only be achieved with research systems, as e.g. Ti:Sapphire, lacking from stability and high repetition rates as demanded for industrial laser micro processing. Former experiments showed that on the one hand and improve of the energy specific volume (process efficiency) can be achieved with shorter pulses especially for steels. On the other hand, an ablation regime with very smooth surfaces on soda-lime glass was observed with pulses shorter than about 500 fs. Thus, it is worth to investigate the sub 100 fs regime for industrial laser micromachining processes. We will show first results from basic investigations concerning the energy specific volume, the surface quality and the minimum achievable structure size for metals, semiconductors and glasses with an industrial grade NKT Aeropulse system at a waveleng6th of 1030 nm in combination with MIKS1 S pulse compressor from N2 Photonics (offering sub 100 fs pulses) and a high-end galvo scanner from Scanlab for repetition rates up to 1.2 MHz.
We report the experimental studies of UV laser ablation with a microspot scanning system comprising of a 10 ps, 355nm ultrashort pulse laser in combination with a standard galvoscanner employing a Microscan MSE-G2-UV extension from Pulsar Photonics. The UV wavelength provides the first step in downscaling the feature size and the microscan extension allows the laser beam to be tightly focused down to a very small diameter of < 1.5 μm. The small focal spot size allows highly precise ablation of microstructures. Laser ablation characteristics with the microscan objective is investigated in steel and copper for different laser repetition rates. The threshold fluence and the energy penetration depth for steel and copper were found to be comparable for repetition rates from 200 kHz-2 MHz. The advantages and also the limitations of laser ablation using the microscan objective is discussed, especially with respect to its small Rayleigh range of ~ 5 μm. The sample positioning tolerances and maximum achievable ablation depths count among the latter. Initial experiments on laser drilling in 10 μm steel foils is also reported, with the exit hole diameter of the order of the focused laser beam diameter.
The development of ultrashort pulsed laser systems actually goes far beyond the kW level. But e.g. for metals and single pulses todays standard methods like galvo scanners are not suited for higher average powers and alternative approaches have to be developed. We will get an insight into actual developments using multi-pulse strategies in temporal representation as pulse bursts and in spatial representation as multi-beams or with direct beam forming. A combination of these methods with synchronized scanning or real pulse on demand option could pave the way for using high average powers.
The commercially available Synova Laser MicroJet® technology combines conventional laser capabilities with compressed water jet that precisely guide laser beam in a similar manner to optical fibers. Due to physical water breakdown, technology is typically focused on the nanosecond pulse duration range. A stable beam shaping setup with a diffusor and commercial fiber to couple into water jet, was developed allowing to test Laser MicroJet® at 100-300 ps pulse duration. The change to energy intensity profile with diffuser allowed to triple coupled energy without inducing the physical breakdown in water and could be further increased by implementing 2 and 3 pulse bursts into the setup. High quality scribing was achieved at Si wafer at high scanning speed. Preliminary results on multilayer Si-wafer have demonstrated that scribing quality is in this case more feedrate dependent, limited chipping occurring at speed of commercial interest. Cutting tests were performed on semiconductors as well as on metals. On both, it was possible to achieve high quality cuts with high feedrate up to 12 mm/s with Ra < 0,3 μm.
We investigated GHz pulse bursts ablation on metals, silicon, zirconium dioxide, soda-lime glass and sapphire for surface structuring applications with a commercial laser system providing a burst pulse frequency of 5.4 GHz and a maximum of 25 pulses per burst pulse train. The results on metal show dramatic decrease of the ablation efficiency and a reduction of the machining quality. For silicon we also observed a reduction of the ablation efficiency for GHz pulse bursts but found a strong increase for MHz pulse bursts using a 10 ps laser system. On glass an increase of the ablation rate for GHz pulse bursts was observed, however with pure machining quality indicated by crakes in the surface and boarders. Zirconium oxide was the only investigated material, where a GHz pulse bursts induces a moderate higher ablation efficiency with comparable surface qualities, however a 10% higher ablation rate was obtained with a 10 ps laser system.
[100], [110] and [111] oriented silicon shows different behavior when it is machined with 10 ps pulses in the NIR. For the [100] orientation the roughness increases to 2.8 µm when the peak fluence is raised to 1.6 J/cm2 then drops down to a value below 200 nm for a fluence of 2 J/cm2 and stays below 300nm for fluences up to 7.5 J/cm2. For the other orientations a completely different behavior is observed. The roughness constantly increases to 900 nm at 1.6 J/cm2 and then further to about 8 µm for a peak fluence of 7.5 J/cm2.
The upscaling of laser micromachining processes with ultrashort pulses is limited due to heat accumulation and shielding effects. Multibeam scanning represents one of the strategies to overcome this drawback. It is in general realized by combining a diffractive beam splitter with a galvanometer scanner. A full synchronization with the laser repetition rate offers new possibilities with minimum thermal impact. We will demonstrate this by means of a multipulse-drilling on the fly process with a regular 5x5 spot pattern having a spot to spot spacing of 160µm. With a constant speed (synchronized to the laser) this pattern can be moved by exactly this spacing between two laser pulses. At a repetition rate of 100 kHz and an average power of 16 W we were able to drill more than 1'500 holes/s in a 10µm thick steel foil without any thermal impact. In a next step we will extend this technology with an SLM to different periodic patterns.
Self-organized structures like cavities and LIPSS are often used to generate specials surface functionalities with ultrashort pulsed laser machining. For a better understanding of the formation of cavities and LIPSS we performed a layer by layer surface visualizations on single crystal iron with different specific lattice orientations. The experiments indicate that formation and grow rate of cavities depends on the orientation of the crystal lattice. Based on this information we have been able to predict the cavity formation for given fluence on an electrical steel sheet with non-oriented grains with an accuracy better than 90% by analyzing the lattice orientations of the grains by a previous EBSD measurement.
Bursts of 230 fs pulses with up to 25 pulses having a time spacing of 180 ps were applied to steel AISI304, copper DHP, brass and silicon in real surface texturing (milling) application by machining squares. The previously reported very high removal rates for GHz bursts could not be confirmed, on the contrary, the specific removal rate tremendously drops down to less than 10% for the metals and 25% for silicon when the number of pulses per burst is increased. This drop is fully in line with shielding effects already observed in case of MHz pulses and double pulse experiments. The increase of the number of pulses per burst directly goes with strongly increased melting effects which are assumed to additionally re-fill the already machined areas in this milling application. Calorimetric experiments show an increasing residual heat with higher number of pulses per burst. Further the removal rates of the GHz bursts directly follow the tendency of single pulses of identical duration. This fosters the hypothesis that in case of metals and silicon only melting and melt ejection lead to the high reported removal rates for GHz bursts in punching applications and that no additional "ablation cooling" effect is taking place.
In ultra-short pulsed laser micromachining of metals shorter pulses generally lead to higher throughput e.g. for copper the specific removal rate drops to about 25 % if the pulse duration is raised from 10 ps to 50 ps and it is increases by about 35 % for a decreased pulse duration of 350 fs. In contrast, a significantly higher increase of 100 % was observed for stainless steel AISI 304 when the pulse duration is decreased from 10 ps to 350 fs, whereas the drop for the longer pulse duration of 50 ps was of the same order of magnitude as for copper. Recent results of a two-pulse burst experiment on copper with temporal varying intra-burst pulse distance have shown, that pulse distances from 10 ps to 100 fs cause only a slight increase in efficiency of approximately 10 % whereas for pulse distances from 10 ps to 500 ps the efficiency is reduced to nearly 50 %. Although, these factors differ from the well-studied pulse duration experiments, they show a similar tendency. Therefore, we assume a dominant common cause.
In literature the diminished efficiency is often referred to particle- or plasma shielding for tens of picoseconds pulse durations. In this sense, the ablation of the material during the pulse is faded by itself for longer pulses as well as the ablation of the material from the first pulse is restricted by the second pulse in the burst experiments. Additional investigations concerning reflection and calorimetric considerations will help to underline this hypothesis.
For silicon machined with 10 ps pulses at 1064nm it was found that the specific removal rate increases by a factor of about 2.5 when an 8-pulse burst is used instead of single pulses [1]. This increase in the specific removal rate directly scales with a higher surface roughness.
For copper and a 3-pulse burst the absorptance of a machined surface increases to about 200% of the one obtained with single pulses [2]. This can serve as an explanation for the higher specific removal rate observed in this case [3]. Actual calorimetric measurements on silicon show that the fraction of the incoming energy which is converted to heat is almost independent on the number of pulses per burst (a behavior which was observed for copper too) and the absorptance depends on the number of pulses. However, as the observed variations in the absorptance are only in the order of a few % and do not monotonically increase with the number of pulses per burst the change in the absorptance cannot explain the increase in the specific removal rate for pulse bursts on silicon. Additional experiments including calorimetry with varying intra-burst time differences and pulse energies as well as experiments concerning reflectivity and transmission will help to understand this behavior of silicon.
[1] B. Jaeggi, D.J. Förster, B. Neuenschwander, OSA Technical Digest, CLEO (2018), AM1M.3
[2] B. Jaeggi, D. J. Foerster, R. Weber, B. Neuenschwander, Adv. Opt. Techn. 7, 175 (2018)
[3] B. Neuenschwander, B. Jaeggi, D.J. Foerster, Th. Kramer, S. Remund, to be published in Proc. of ICALEO (2018)
In the burst mode the reported removal rates were often higher than the ones achieved with single pulses at identical repetition rate and average power. But this effect is mainly caused by the reduced energy per single pulse in the burst and the corresponding fluence which is then nearer its optimum value showing highest specific removal rate. But there exist special situations where the burst mode shows a higher efficiency and therefore an increased specific removal rate. For copper e.g. it was found that a 3-pulse burst with a time spacing of 12 ns at a wavelength of 1064 nm leads to an about 15% higher specific removal rate.
We extended the burst investigations to semiconductors and isolators and measured the specific removal rate as a function of the applied peak fluence for different materials, number of pulses in the burst and time spacing. For 1064 nm silicon e.g. shows a maximum specific removal rate which amounts about 1.7 µm3/µJ for single pulses and a 2 pulse burst as well. Then it almost linearly increases up to about 5 µm3/µJ when the number of pulses in the burst is raised to 8. A similar effect is found for machining grooves into diamond-like nanocomposite films with single pulses and a 2- and 3-pulse burst, respectively. In contrast, for silicon and 532 nm wavelength where the photon energy exceeds the bandgap, only a small difference in the maximum specific removal was observed.
Heat accumulation is assumed to cause the higher specific removal rates but further experiments are needed gain a clearer picture.
The resulting surface roughness and waviness after processing with ultra-short pulsed laser radiation depend on the laser parameters as well as on the machining strategy and the scanning system. However the results depend on the material and its initial surface quality and finishing as well. The improvement of surface finishing represents effort and produces additional costs. For industrial applications it is important to reduce the preparation of a workpiece for laser micro-machining to optimize quality and reduce costs.
The effects of the ablation process and the influence of the machining strategy and scanning system onto the surface roughness and waviness can be differenced due to their separate manner. By using the optimal laser parameters on an initially perfect surface, the ablation process mainly increases the roughness to a certain value for most metallic materials. However, imperfections in the scanning system causing a slight variation in the scanning speed lead to a raise of the waviness on the sample surface.
For a basic understanding of the influence of grinding marks, the sample surfaces were initially furnished with regular grooves of different depths and spatial frequencies to gain a homogenous and well-defined original surface. On these surfaces the effect of different beam waists and machining strategy are investigated and the results are compared with a simulation of the process. Furthermore the behaviors of common surface finishes used in industrial applications for laser micro-machining are studied and the relation onto the resulting surface roughness and waviness is presented.
For most micromachining applications, the laser focus has to be moved across the workpiece, either by steering the beam or by moving the workpiece. To maximize throughput, this movement should be as fast as possible. However, the required positioning accuracy often limits the obtainable speed. Especially the machining of small and complex features with high precision is constrained by the motion-system’s maximum acceleration, limiting the obtainable moving spot velocity to very low values. In general, processing speed can vary widely within the same processing job. To obtain optimum quality at maximum throughput, ideally the pulse energy and the pulse-to-pulse pitch on the workpiece are kept constant. This is only possible if laser-pulses can be randomly triggered, synchronized to the current spot velocity. For ultrafast lasers this is not easily possible, as by design they are usually operated at a fixed pulse repetition rate. The pulse frequency can only be changed by dividing down with integer numbers which leads to a rather coarse frequency grid, especially when applied close to the maximum used operating frequency.
This work reports on a new technique allowing random triggering of an ultrafast laser. The resulting timing uncertainty is less than ±25ns, which is negligible for real-world applications, energy stability is <2% rms.
The technique allows using acceleration-ramps of the implemented motion system instead of applying additional override moves or skywriting techniques. This can reduce the processing time by up to 40%.
Results of applying this technique to different processing geometries and strategies will be presented.
To be competitive in industrial applications the throughput is a key factor in laser micro machining using ultra-short pulsed laser systems. Both, ps and fs laser systems are suitable for industrial applications. Therefore one has to choose the right pulse duration for highest ablation efficiency. As shown in earlier publications the efficiency of the ablation process can be described by the specific removal rate, which has a maximum value at an optimum fluence. But its value often bases on a calculation using the threshold fluence and energy penetration depth deduced by measuring the depth of ablated cavities machined with different fluences and number of pulses. But this calculated specific removal rate often differs from the one deduced from ablated squares as recently shown in literature. Further an unexpected drop of the specific removal rate was reported for stainless steel when the pulse duration was reduced from 900 fs to 400 fs. Thus the influence of the pulse duration in the fs and low ps regime onto the specific removal rate is investigated with different methods for industrial relevant materials
A tilted fiber Bragg grating (TFBG) was integrated as the dispersive element in a high performance biomedical imaging system. The spectrum emitted by the 23 mm long active region of the fiber is projected through custom designed optics consisting of a cylindrical lens for vertical beam collimation and successively by an achromatic doublet onto a linear detector array. High resolution tomograms of biomedical samples were successfully acquired by the frequency domain OCT-system. Tomograms of ophthalmic and dermal samples obtained by the frequency domain OCT-system were obtained achieving 2.84 μm axial and 10.2 μm lateral resolution. The miniaturization reduces costs and has the potential to further extend the field of application for OCT-systems in biology, medicine and technology.
A compact, fiber-based spectrometer for biomedical application utilizing a tilted fiber Bragg grating (TFBG) as
integrated dispersive element is demonstrated. Based on a 45° UV-written PS750 TFBG a refractive spectrometer with
2.06 radiant/μm dispersion and a numerical aperture of 0.1 was set up and tested as integrated detector for an optical
coherence tomography (OCT) system. Featuring a 23 mm long active region at the fiber the spectrum is projected via a
cylindrical lens for vertical beam collimation and focused by an achromatic doublet onto the detector array. Covering
740 nm to 860 nm the spectrometer was optically connected to a broadband white light interferometer and a wide field
scan head and electronically to an acquisition and control computer. Tomograms of ophthalmic and dermal samples
obtained by the frequency domain OCT-system were obtained achieving 2.84 μm axial and 7.6 μm lateral resolution.
KEYWORDS: Optical coherence tomography, Field programmable gate arrays, Signal processing, Fourier transforms, Computer simulations, Data acquisition, Digital signal processing, Convolution, Algorithm development, Imaging systems
We developed an FPGA-based engine for Fourier-domain OCT that performs real-time signal processing based on Non- Uniform Fast Fourier Transform (NUFFT). The basic NUFFT algorithm is discussed and compared with cubic-spline interpolation regarding efficient re-sampling in k-space with different phase nonlinearities of sinusoidal swept sources. The NUFFT algorithm was adapted for an implementation in an FPGA and its accuracy is analyzed and assessed using simulated numerical data. When implemented, the NUFFT algorithm allows a processing performance at a sampling rate of 100 MS/s. The real-time processing capability was tested with sinusoidal bi-directional swept sources with A-scan rates of 50 kHz.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.