Laser Material Processing Applications Research Articles

Creating an Array of Parallel Vortical Optical Needles

We propose a method for creating parallel Bessel-like vortical optical needles with an arbitrary axial intensity distribution via the superposition of different cone-angle Bessel vortices. We analyzed the interplay between the separation of individual optical vortical needles and their respective lengths and introduce a super-Gaussian function as their axial profile. We also analyzed the physical limitations to observe well-separated optical needles, as they are influenced by the mutual interference of the individual beams. To verify our theoretical and numerical results, we generated controllable spatial arrays of individual Bessel beams with various numbers and spatial separations by altering the spectrum of the incoming laser beam via the spatial light modulator. We demonstrate experimentally how to implement such beams using a diffractive mask. The presented method facilitates the creation of diverse spatial intensity distributions in three dimensions, potentially finding applications in specific microfabrication tasks or other contexts. These beams may have benefits in laser material processing applications such as nanochannel machining, glass via production, modification of glass refractive indices, and glass dicing.

Actively Q switched radially polarized Ho:YAG laser with an intra-cavity laser-written S-waveplate.

Nanosecond Q switched pulses and radial polarization are established stand-alone techniques for enhanced laser materials processing applications, but are generally challenging to achieve simultaneously at high average power levels. Here, we demonstrate a 20.6 W radially polarized Ho:YAG rod laser which has been actively Q switched in order to generate 515 μJ, 210 ns pulses at 2097 nm. By utilizing an ultra-low-loss spatially variant birefringent wave plate (S-waveplate) inside the laser cavity, the linearly polarized fundamental mode has been converted to a radially polarized donut-shaped beam with very high conversion efficiency.

Generating double focal spots by focusing a radially polarized double-ring-shaped beam with an annular classical axicon

Focusing properties of a radially polarized double-ring-shaped beam with an annular classical axicon are numerically investigated based on vector Debye theory. Double focal spots and a flat-topped beam can be generated by choosing appropriate values of the pupil to the beam ratio β, numerical aperture, and annular obstruction. The distance between the twin spots, their depth of focus and the depth of the flat-topped beam are considerably affected by numerical aperture and annular obstruction. These focal shapes may find applications in laser printing, multifocal microscopy, and material processing.

Integration of blue–green electroluminescence structure in mid-infrared hollow optical fiber for targeting invisible CO2 laser beam

Mid-infrared (IR) CO2 lasers have important applications in laser medicine and material processing. However, the CO2 laser beam is invisible to human eyes, it is not easy to accurately indicate the beam position in practical applications. In this work, we designed a PEDOT (Poly (3, 4-ethylenedioxythiophene))/ZnS: Cu/BaTiO3/Ag electroluminescence (EL) structure and integrated it at the output end of the mid-IR hollow optical fiber. The blue–green light can be output along with the CO2 laser beam in this EL structure integrated fiber, thus achieving the optical path indication and illumination of invisible laser beam. This work provides a possible guide for aiming the CO2 laser beam in the min-IR optical fiber.

Modelling dynamic 3D heat transfer in laser material processing based on physics informed neural networks

Machine learning algorithms make predictions by fitting highly parameterized nonlinear functions to massive amounts of data. Yet those models are not necessarily consistent with physical laws and offer limited interpretability. Extending machine learning models by introducing scientific knowledge in the optimization problem is known as physics-based and data-driven modelling. A promising development are physics informed neural networks (PINN) which ensure consistency to both physical laws and measured data. The aim of this research is to model the time-dependent temperature profile in bulk materials following the passage of a moving laser focus by a PINN. The results from the PINN agree essentially with finite element simulations, proving the suitability of the approach. New perspectives for applications in laser material processing arise when PINNs are integrated in monitoring systems or used for model predictive control.

Correction of temperature influences on plastic lenses for applications in laser material processing

Today, the use of plastic lenses in processing optics for laser material processing is fundamentally limited to laser powers lower than 1 W. This is due to thermo-optical effects in the plastic material which primarily lead to a shift in the focus position. To overcome this laser power limitation, a design routine was developed for injection molded lenses, which simulates the optical properties of the lens in thermal equilibrium state during laser irradiation and which iteratively adapts the lens design for the manufacturing process in this way that the lens is compensated for thermal effects and manufacturing errors like shrinkage. The approach is demonstrated for an injection molded focusing lens used at an average laser power of 15 W. The focusing properties of the compensated manufactured lens are compared with the underlying simulations as well as with an uncompensated lens geometry used at the same laser power.

Squared Focal Intensity Distributions for Applications in Laser Material Processing.

Tailored intensity profiles within the focal spot of the laser beam offer great potential for a well-defined control of the interaction process between laser radiation and material, and thus for improving the processing results. The present paper discusses a novel refractive beam-shaping element that provides different squared intensity distributions converted from the Gaussian output beam of the utilized femtosecond (fs) laser. Using the examples of surface structuring of stainless-steel on the micro- and nano-scale, the suitability of the beam-shaping element for fs-laser material processing with a conventional f-Theta lens is demonstrated. In this context, it was shown that the experimental structuring results are in good agreement with beam profile measurements and numerical simulations of the beam-shaping unit. In addition, the experimental results reveal the improvement of laser processing in terms of a significantly reduced processing time during surface nano-structuring and the possibility to control the ablation geometry during the fabrication of micro-channels.

Few-mode random fiber laser with a switchable oscillating spatial mode.

Random fiber lasers are of tremendous interest to diverse applications for optical fiber sensing, speckle-free imaging. To date, random fiber lasers with fundamental mode oscillation have been well developed. However, controllable oscillating spatial mode in random fiber lasers have not been reported yet. Here, we propose and demonstrate a few-mode random fiber laser with a switchable oscillating spatial mode based on mode injection locking. An external signal light is injected to realize the locking of transverse mode in this random fiber laser and the direct oscillations of the fundamental mode, hybrid mode, and high order mode can be realized, respectively. This random fiber laser operates in the high-order LP11 mode stably with a threshold of as low as 88 mW. High efficiency and high purity cylindrical vector beams can be obtained by removing the degeneracy of the LP11 mode. This work may pave a path towards random fiber lasers with controllable spatial modes for specific applications in mode division multiplexing, imaging, and laser material processing.

Study on processing characteristics and mechanisms of thermally assisted laser materials processing

Nanosecond pulsed lasers have been widely applied to interact with and characterize many different materials, and industrial users are exploring more avenues for various application processes. For the purpose of achieving broader application, the current challenge is to achieve a high processing efficiency for laser materials processing. In this study, thermally assisted laser materials processing (TALMP), which introduces heat from an external source to increase the workpiece material's temperature and make it easier to carry out ablation or modification, was developed and investigated. Moreover, to investigate the materials processing characteristics and mechanisms of TALMP, practical applications using TALMP, including surface ablation on WC-Co, and surface polishing on additive manufactured Ti-6Al-4V, were carried out. The results showed that TALMP can be successfully applied to various materials by different laser materials processing applications. Due to the workpiece material's high temperature and heat content, the coupling between laser and target materials can be enhanced; thus, physical ablation and modification for laser surface ablation can be achieved effectively. However, with a high offset temperature, a little decrease of ablation accuracy and quality could occur, and a two-step approach is proposed to eliminate this unwanted effect. Moreover, this strategy is proven to be able to achieve the desired surface ablation with high efficiency and high-quality surfaces. For thermally assisted surface polishing, due to higher temperature field with long duration, a smoother and flatter polished surface can be obtained more easily. The experimental results proved the superiority and versatility of TALMP.

Design and Manufacturing Method of Fundamental Beam Mode Shaper for Adapted Laser Beam Profile in Laser Material Processing.

Many laser material processing applications require an optimized beam profile, e.g., ring shape or Top-Hat profiles with homogeneous intensity distribution. In this study, we show a beam shaping concept leading to a phase shifting element with binary height profile as well as a very low periodicity with near diffraction limited spot size. Further advantages of so-called Fundamental Beam Mode Shaping (FBS) elements are the simplified handling, and a high efficiency and homogeneity. The calculated height profile of FBS elements are transferred in fused silica substrates using a combination of microlithography technologies, reactive ion etching (RIE) and ion beam etching (IBE). The experiments demonstrated a linear relation between the etching depth after RIE and IBE. The optical evaluation of the manufactured FBS beam mode shaper confirmed the presented concept design.

Adjustable‐Function Beam Shaping Methods

AbstractLaser welding is used in a wide range of industrial applications including automotive, aerospace, semiconductors, electronics, medical, power, defense, and others. The raw laser beam is not the optimal shape for many industrial applications in general and especially in the field of welding, brazing, soldering, and other similar processes. Compared to other laser material processing applications, these processes stand out in terms of the required laser power (multi kW) and the highly multimode beams often used. A process‐specific tailored laser intensity distribution can improve throughput, seam height, strength, and the edge smoothness of the joints.

Vector Beams with Parabolic and Elliptic Cross-Sections for Laser Mate-rial Processing Applications

Vector Beams with Parabolic and Elliptic Cross-Sections for Laser Mate-rial Processing Applications

High-order mode Yb-doped fiber lasers based on mode-selective couplers.

We experimentally investigate high-order modes (HOMs) generating at the wavelength of 1.0 μm in all-fiber Yb-doped lasers based on mode-selective couplers (MSCs). Broadband MSCs for HOMs conversion are achieved by optimizing the phase-matching condition at 1.0 μm. Efficient generation of HOMs is demonstrated with the MSCs inserted both into continuous-wave and mode-locked fiber lasers. The slope efficiencies of HOMs (LP11, LP02 and LP21) are presented and discussed. Additionally, cylindrical vector and orbital angular momentum beams are produced out of the MSCs by controlling the polarization states. The results show proof-of-concept implementation of the MSCs, which enables high efficiency of HOMs generation especially in pulsed fiber lasers and holds promising applications in laser material processing.

Vector Mathieu beam profile engineering for laser material processing applications

Beam profile engineering where a desired optical intensity distribution can be generated by an array of phase shifting elements is a promising approach in laser material processing. Scalar Mathieu beams have elliptical intensity distribution perceivable as “optical knives” in the transverse plane. They exhibit non-diffracting properties similar to Bessel beams, where a relatively long focal depth retains unchanging intensity distribution, which makes Mathieu beams a promising approach in laser processing. Here, we introduce additionally vector Mathieu beams with controllable polarization and investigate numerically their spatial spectra. Further, we attempt to generate individual components of vector Mathieu beams experimentally using SLMs. We report on our achievements in the control over the beam shape and dimensions along the propagation axis.

Wavelet analysis of light emission signals in laser beam welding

This paper introduces a compact, robust, and versatile generation of a laser processing head coupled with a single-mode Ytterbium source StarFiber 150 P which not only is used in a wide range of laser material processing applications but also enables in situ and real-time quality control in production lines. In this study, the effect of power density on the weld quality of bead on plate in a SHADOW (Stepless high speed accurate and discrete one pulse welding) welding process was considered. Furthermore, the co-axial back reflected/emitted light from the process zone was measured using three optical sensors; each one measures the light emission in a certain waveband (450–850, 1000–1200, and 1250–1700 nm). From these sensors, the recorded signals were first used as benchmarks to correlate with the weld quality features and defects. Discrete wavelet transform has been used to analyze the signal within the time–frequency domain. The detail signals at different decomposition levels disclosed information with regard to the type (mode) and dynamics of welding process. Accordingly, the deep penetration welding (keyhole) was confidentially distinguished from the shallow (conduction) welding mode. Several defects such as process instabilities (high fluctuations in the keyhole dynamics), formation, and collapse of the keyhole, as well as the lack of penetration, were detected from the corresponding signal signatures. In addition, this information permits us to locate the defect/transition site through the in situ and real-time quality control of the process in production lines. In contrast, porosity and cracks could not be sensed.

Light self-focusing in the atmosphere: thin window model.

Ultra-high power (exceeding the self-focusing threshold by more than three orders of magnitude) light beams from ground-based laser systems may find applications in space-debris cleaning. The propagation of such powerful laser beams through the atmosphere reveals many novel interesting features compared to traditional light self-focusing. It is demonstrated here that for the relevant laser parameters, when the thickness of the atmosphere is much shorter than the focusing length (that is, of the orbit scale), the beam transit through the atmosphere in lowest order produces phase distortion only. This means that by using adaptive optics it may be possible to eliminate the impact of self-focusing in the atmosphere on the laser beam. The area of applicability of the proposed “thin window” model is broader than the specific physical problem considered here. For instance, it might find applications in femtosecond laser material processing.

Focusing light into desired patterns through turbid media by feedback-based wavefront shaping

We demonstrate that the focusing of light into desired patterns through turbid media can be realized using feedback-based wavefront shaping. Three desired focused patterns—a triangle, a circle, and a rectangle—are used as examples to study this ability. During the process of modulating scattered light, the Pearson’s correlation coefficient is introduced as a feedback signal. It is found that the speckle field formed by the turbid media gradually transforms into the desired pattern through a process of modulation of the input beam wave front. The proposed approach has potential applications in biomedical treatment and laser material processing.

Optimization of the Liquid Crystal on Silicon Technology for Laser Microprocessing Applications

Phase-only spatial light modulators (SLMs) allow to bring adaptive optics and diffractive optics methods into laser material processing. We focus our research on the liquid crystal on Silicon (LCOS) technology, which is known from display and photonic applications for its high resolution, flexibility, technological and economical feasibility.The broad range of applications in laser material processing imposes various requirements on such dynamic components.Most important is the ability of the SLM to withstand the higher intensities and energies over the long time. This requires better understanding of the laser induced damage threshold (LIDT) levels of these devices on system and component level as well as long term performance degradation, especially if ultra-short pulse (USP) light sources are used. The measures to improve LIDT and other important parameters while keeping the economically feasible wafer-scale fabrication of LCOS SLMs will be discussed.

Tubular filamentation for laser material processing.

An open challenge in the important field of femtosecond laser material processing is the controlled internal structuring of dielectric materials. Although the availability of high energy high repetition rate femtosecond lasers has led to many advances in this field, writing structures within transparent dielectrics at intensities exceeding 1013 W/cm2 has remained difficult as it is associated with significant nonlinear spatial distortion. This letter reports the existence of a new propagation regime for femtosecond pulses at high power that overcomes this challenge, associated with the generation of a hollow uniform and intense light tube that remains propagation invariant even at intensities associated with dense plasma formation. This regime is seeded from higher order nondiffracting Bessel beams, which carry an optical vortex charge. Numerical simulations are quantitatively confirmed by experiments where a novel experimental approach allows direct imaging of the 3D fluence distribution within transparent solids. We also analyze the transitions to other propagation regimes in near and far fields. We demonstrate how the generation of plasma in this tubular geometry can lead to applications in ultrafast laser material processing in terms of single shot index writing, and discuss how it opens important perspectives for material compression and filamentation guiding in atmosphere.

Impact of the thermal lens effect in atmospheric pressure DBD-plasma columns on coaxially guided laser beams

The combination of laser and low-temperature plasmas is of growing interest for micro-structuring purposes for a wide range of materials. This combination can be used for a reduction of the ablation threshold as well as an improvement of the machining quality in various laser material processing applications. The plasma involved in such combination, however, leads to a thermally generated influence on the laser beam quality. In this paper, a DBD-based argon plasma as typically used in low-temperature laser plasma hybrid arrangements was investigated by interferometric and beam profile measurements. The radial temperature profile as well as the beam propagation characteristics in terms of focal shift and effective pointing stability was determined. Due to the argon plasma, a temperature increase ΔT by up to 25 K, and thus a thermal lens was observed within the light path of the laser beam featuring an increase in refractive index Δn by maximum 2.86 × 10−5. In the given setup, the plasma-induced thermal lens caused a focal shift by up to 4 mm. Further, the lateral focus position was deviated by a maximum of about 30 µm, which is in the order of magnitude of the beam waist radius.