In this contribution the design, realization, and characterization of a transmission grating spectrograph (TGS) designed for spectral characterization and monitoring tasks at extreme-ultraviolet (EUV) wavelengths are presented. The overall dimensions of the EUV-TGS have been minimized to allow for easy integration in existing beamlines and setups while maintaining sub-Ångström spectral resolution. The main module (length = 40 mm) of the realized EUV-TGS consists of a spectral and a spatial filter and a high-resolution phase-shifting transmission grating with a grating structure periodicity of 80 nm. For spectral characterization and monitoring of a plasma-based EUV radiation source, a two-dimensional detector is positioned in close proximity to the main module. The realized spectrograph prototype can be moved in and out of the beam path, and therefore does not affect spot profiling and general beam alignment tasks. The structural and optical design and performance measurements of the EUV-TGS are presented, demonstrating that this ultra-compact spectrograph can be easily integrated into EUV setups that will benefit from an inline diagnostic option.
This paper focuses on the design and fabrication of phase-shifting transmission masks tailored for high-resolution nanopatterning using a compact EUV exposure tool. The authors analyze various factors that influence the achievable resolution, aiming to push the boundaries towards the sub-10 nm range, approaching the theoretical resolution limit. The demand for high-resolution nanoscale patterns spans across diverse applications, driving the need for compact exposure tools and lithographic concepts. The developed EUV exposure tool can be operated at either 10.9 nm or 13.5 nm exposure wavelengths depending on the specific use case. This capability allows for large area nanopatterning with enhanced throughput as well as industrial resist qualification with focus on highest resolution. The utilized discharge-produced plasma (DPP) EUV source offers partially coherent radiation. For this radiation type, the (achromatic) Talbot lithography has proven to be the most effective with resolution in the sub-30 nm range and a theoretical resolution limit of less than 10 nm. To optimize the intensity distribution in the wafer plane, the authors use rigorous coupled-wave analysis (RCWA) simulations to fine-tune the material composition and geometry of the masks. Various factors influencing the achievable resolution are identified and presented. In addition to simulative optimization, the fabrication of dense periodic nanopatterns poses increasing challenges for smaller periods. In this work, the mask fabrication process is optimized to produce stable and high-resolution periodic mask patterns, leading to record resolutions for both line and contact hole periodic nanopatterns with the presented setup.
Using quantum annealers within tools for the automated optics design promises to yield advantages regarding computational costs and design quality. These advantages can be enhanced by using problem specific anneal schedules and initial states.
Diffractive neural networks enable robust three-dimensional beam shaping by treating systems of cascading diffractive optical elements as artificial neural networks, enabling the usage of neural network training methods for the design. Here, we demonstrate how this can be used to create intensity distributions with a large depth-of-field in an experimental realization through spatial light modulators.
In this paper we investigate the dependence between target-distribution contrast and decrease in optical performance of the free-form optics due to misalignment along the optical axis. It is shown that by reducing the target-contrast, the optical performance of the resulting illuminance distribution can be increased at different target distances and for different light source positions.
High power VCSEL systems are a versatile and powerful tool for thermal treatment in industrial production where they enable a very homogeneous and locally controllable irradiance distribution at small working distances. Due to the inherent divergence of VCSELs, both characteristics degrade with increasing working distance. Depending on the size of the used VCSEL system, already at distances of about 100 mm the irradiation is not homogenous anymore and the local controllability is strongly limited already at even smaller distances. To extend the application range of VCSEL systems for increased working distances while maintaining homogeneity and local controllability, two multi-aperture beam integrators have been designed. Simulation results as well as measurements of a prototype system are presented in this work.
The measurement of relevant process emissions is a challenging task, especially when access for measurement technology is limited. One example is the optical combustion chamber monitoring of internal combustion engines. The access is limited and spatial resolution for observation is limited by the possible use of optical elements in the combustion chamber. So far, data acquisition has been realized with the aid of spark plugs with integrated connections to an optical sensor. This optical spark plug has the function of a spark plug and simultaneously enables the detection of light in the engine. The optical spark plug is positioned in the center of the combustion chamber which allows for a symmetrical design for a 360° field of view. Our approach is to build an alternative fiber-based light sensor without the function of a spark plug, because if no ignition unit is installed, there is more space for additional optical elements for higher optical spatial resolution than conventional light sensors with ignition function. The main challenge is the miniaturization of the optical and mechanical set up. Due to the fixed position of the spark plug and the limited access to the combustion chamber, the light sensor must be inserted at an angle into the combustion chamber at a different location, so, the optical system must be asymmetric. This paper presents the results of the design and fabrication of a combustion chamber light sensor with respect to the optical and mechanical challenge of spatially resolved detection of light pulses in a combustion chamber of an engine under an oblique access to the combustion chamber.
The available (average) power of high-power lasers is steadily increasing. This poses the challenge of providing this power dynamically tailored to the respective laser processing application, be it surface structuring, cutting or 3D printing, in order to ensure efficient and high-quality processing. In dynamic high-power laser beam shaping, a compromise usually has to be made between the applicable amount of (average) laser power and the degrees of freedom for the beam shaping device. In general, the higher the damage threshold is, the fewer are the degrees of freedom for available beam shaping devices[1,2]. One way to overcome this deficit is to first shape the beam with a high resolution and low power output and then amplify the beam to the necessary laser power. The subsequent amplification introduces unwanted changes in the desired beam shape, which needs to be compensated. The current method to compensate the amplification induced changes is to exactly simulate the optical system at hand as well as the amplification process. For this purpose, an Iterative-Fourier- Transformation-Algorithm (IFTA) combined with an additional optimization is used. This method requires prior knowledge of all system and amplification defining parameters, which are non-trivial to determine. Another approach, pursued in this paper, is the use of an artificial neural network (ANN). The ANN is trained through the combinations of different phase masks and the resulting beam shape profiles. This training method should allow the ANN to indirectly map any optical system without any regard to its complexity. Through an appropriate choice of training samples and subsequent training the ANN is able to approximate the mapping function of the optical system including the amplification. The fully trained ANN generates phase masks for the beam shaping process in one step and thus allows highly dynamic beam shaping of arbitrary beam shape profiles.
The quality, efficiency and robustness of laser materials processing significantly benefits from an application-adapted beam shaping. For a successful realization of tailored beam shapes that utilizes state-of-the-art optic concepts, research in two different steps is necessary. First, information about the required beam shape (especially in terms of intensity distribution) is needed. During the last years, we have developed and validated a method to solve the so-called inverse heat conduction problem which enables the derivation of an intensity distribution that specifically tailors the induced temperature profile within the processed material. Here, we present the latest enhancement of the algorithm to complex time-dependent scenarios and new applications, e.g. from the field of surface treatment and tape placement. With the knowledge of the target beam shape, in a second step, optical systems must be designed that enable the realization of the achieved, highly complex intensity distributions. We further demonstrate the potential and limits of novel optics such as freeform mirrors, LCoS-SLMs or diffractive neural networks as well as VCSELs for application-adapted beam shaping. We especially present specific design methods that can contribute to a robust, flexible, and practical realization in various applications.
In this work, we compare four different design concepts for external-cavity laser diodes (ECDL) with respect to the maximum achievable output power before the onset of catastrophic optical damage (COD). A multiphysics model of the ECDL with a self-consistent description of the electrical, optical and thermal properties of the device is used to evaluate the COD level. The feedback-induced failure is provoked by shifting the fast axis collimation (FAC) lens along the fast axis (smile error) resulting in an absorption of the feedback radiation within the highly p-doped and contact metal layers. The investigated design concepts include three local modifications at the front facet of the laser diode chip itself which are supposed to suppress injected current, optical absorption and leakage current from the quantum well. Within the considered parameter space these approaches lead to an increase of the COD level by 8%, 27% and 27% respectively, however at the cost of drawbacks like slightly reduced efficiency or beam quality along the fast axis. By combining all three approaches the output power can be increased by 37%. The fourth approach uses an additional lens within the external resonator to make it bi-telecentric and allows for a feedback field without image reversal. This approach completely removes the sensitivity of the setup regarding a vertical misalignment of the FAC lens. The drawback in this case is the increase of the resonator size by approximately a factor of 20.
Fast and non-destructive non-imaging metrology of nanostructures is crucial for the development of integrated circuits and for the corresponding in-situ metrology within fabrication processes. Stochastic variations related to the gratings local period (line edge roughness, LER) and line width (line width roughness, LWR) are of special interest due to their key role for the minimal achievable structure size. Non-imaging metrology approaches taking these statistic variations into account are quite limited. For scatterometry, models predict a change of the grating’s diffraction efficiency according to a DebyeWaller factor but only in the non-zeroth diffraction orders. The authors perform simulations of nanoscale gratings that suggest an influence of LER and LWR on the reflectance (zeroth diffraction order efficiency) which motivate an extended study on LER and LWR measured by spectrally resolved EUV reflectometry here described as EUV spectrometry. The authors present reconstruction results of nanoscale gratings measured with a compact spectrometer utilizing extreme ultraviolet (EUV) radiation emitted by a discharged-produced plasma (DPP) EUV source. The use of two sequential spectrographs, one for the reference measurement of the source spectrum, the other one for the measurement of the spectrum after sample interaction, combined within the experimental setup allows to measure the broadband reflectance with 2% relative uncertainty of samples under various grazing incidence angles. The method offers a proven sub-nm reconstruction accuracy for critical grating parameters. Within the presented study, the measured samples are dedicated test samples, fabricated to exhibit well-defined LER and LWR at different grating periods and linewidths. In addition, the samples are also cross-characterized by the Physikalisch-Technische Bundesanstalt (PTB, Berlin). Experimental and simulative results are discussed to derive approaches to include LER and LWR as parameters in the physical model for reconstruction.
In this contribution, the authors present the design and fabrication of optimized phase-shifting transmission masks for high-resolution nanopatterning with a compact EUV exposure tool. Several influencing factors on the achievable resolution are determined and characterized, paving the way towards the theoretical resolution limit in the sub-10 nm range. Applications that require high resolution patterns are numerous, leading to an increasing demand for compact exposure tools and lithographic concepts. The realized exposure tool is a compact and versatile setup, that can be operated either at an exposure wavelength of 10.9 nm or 13.5 nm addressing both large area nanopatterning with maximized throughput and industrial resist qualification with highest resolution. For partially coherent radiation as provided by the utilized discharge-plasma produced (DPP) EUV radiation source of the setup, the (achromatic) Talbot lithography has proven to be the most suitable lithographic approach with a demonstrated resolution in the sub-30 nm regime and a theoretical resolution limit below 10 nm. To maximize the contrast of the resulting intensity distribution in wafer plane, the material composition and geometry of the mask are optimized by means of rigorous coupled-wave (RCWA) simulations. Different influencing factors on the achievable resolution are identified and presented. In addition to the simulative optimization of the phase-shifting masks, the fabrication of the dense periodic nanopatterns becomes more and more challenging for smaller periods. In this contribution the mask fabrication process is optimized to create stable and high-resolution periodic mask patterns, leading to record resolution for both line and pinhole periodic nanopatterns with the presented setup.
Nowadays, sophisticated ray-tracing software packages are used for the design of optical systems, including local and global optimization algorithms. Nevertheless, the design process is still time-consuming with many manual steps, and it can take days or even weeks until an optical design is finished. To address this shortcoming, artificial intelligence, especially reinforcement learning, is employed to support the optical designer. In this work, different use cases are presented, in which reinforcement learning agents are trained to optimize a lens system. Besides the possibility of bending lenses to reduce spherical aberration, the movement of lenses to optimize the lens positions for a varifocal lens system is shown. Finally, the optimization of lens surface curvatures and distances between lenses are analyzed. For a predefined Cooke Triplet, an agent can choose the curvature of the different surfaces as optimization parameters. The chosen surfaces and the distances between the lenses will then be optimized with a least-squares optimizer1 . It is shown, that for a Cooke Triplet, setting all surfaces as variables is a good suggestion for most systems if the runtime is not an issue. Taking the runtime into account, the selected number of variable surfaces decreases. For optical systems with a large number of degrees of freedom an intelligent selection of optimization variables can probably be a powerful tool for an efficient and time-saving optimization.
Laser structuring is a powerful tool for functionalizing surfaces, e.g., improving the tribological properties. To achieve small structures in the < 2 μm range, microscope objectives are typically used in laser material processing. There are two main challenges to achieve small structures: On the one hand, the limited working distance between the focusing optics and the workpiece results in a comparatively small processing area of a few square millimeters. On the other hand, the depth of field is limited when structuring with microscope lenses due to their large numerical aperture. As a result, the intensity of the laser beam is strongly dependent on the position in the propagation direction, so that the process window for material removal is only a few μm and small deviations disrupt the process. For highly productive large-area laser structuring in a roll-to-roll (R2R) process, the processing area must be enlarged, and the depth of field must be increased at the same time to enhance process robustness. With a given R2R process speed of the moving material of 2 m/min, and a material width of 0.5 m, we want to structure an area of 1 m²/min. The structuring pattern is a hexagonal arrangement of spots with a spot diameter of 1-2 μm and a spot distance of 2 μm. Additionally, we want to achieve a depth of field of 45-50 μm to enhance the process robustness. Given this background, this paper presents an approach in which a laser beam is split into numerous sub-beams and these sub-beams are subsequently shaped in such a way that the depth of field is increased for each individual beam. For beam shaping, a combination of static optical elements is used to transform a uniform into a Bessel-like intensity distribution to achieve a greater depth of field. By a skillful arrangement of the focusing elements, structure sizes of 1-2 μm as well as structure distances of 2 μm are achieved with the given R2R process speed.
Background: In the extreme ultaviolet (EUV) lithography process the performance of the photoresist is a crucial factor regarding the quality and critical dimensions of the fabricated structures.
Aim: The characterization of the latent image structures in photoresists during the process steps before the development of the resist is key to understand the relation between the material of the resists, the selection of process parameters, and the resulting quality of fabricated structures.
Approach: Spectroscopic EUV reflectometry is a nondestructive metrology technique that measures the broadband reflectance of samples in the EUV spectral range and under grazing incidence angles. The technique offers a combination of high sensitivity to nanoscale structural parameters of periodic structures as well as a high sensitivity to the material composition samples, enabling the characterization of latent images of periodic structures.
Results: Measurements of the reflectance of an EUV-exposed and unexposed photoresist reveal the contrast in optical constants after the resists are treated with a post-exposure bake as well as shrinkage of the resist layer thickness. Based on this data, simulative studies on latent images of periodic grating structures are conducted showing the possibility to extract information on the structure parameters including the latent image profile and surface topography.
Conclusion: Spectroscopic EUV reflectometry shows to be sensitive to the contrast of exposed and unexposed photoresist which commends the technique to be adequate for the characterization of latent images in photoresists.
KEYWORDS: Semiconductor lasers, High power lasers, Temperature metrology, Continuous wave operation, Near field, Optical testing, Near field optics, Thermography
Results of an extended series of experimental studies into the beam parameter product (BPP) of high-power diode lasers are summarized, covering efforts to clearly diagnose the limiting factors and develop novel device technology to address these limits. We review diagnostic studies, separating BPP empirically into bias-dependent (thermal) and bias-independent (non-thermal) terms for convenience of analysis. First, we use monolithically grating-stabilized lasers to confirm the presence of a well-defined series of guided modes, rather than filaments. Second, we present results from a series of custom devices and tests (guided by targeted simulations). Third, we show that effects driven by thermal lensing and current spreading dominate the variation in BPP with bias. The residual bias-independent BPP background remains around 30- 50% of the total, and is most likely partly limited by gain-guiding effects. Fourth, the presence of longitudinal temperature variation due to non-uniform optical intensity along the resonator further degrades the bias-independent background level. Lateral current blocking technology is shown to reduce current spreading, and improve the bias-dependent BPP. Thermal engineering also improves bias-dependent BPP, achieved by varying epitaxial layer structure and by targeted changes in bar layout, clarified using measurements in thermography cross-referenced to simulation. In summary, we contend that experimental studies have allowed the effects that limit lateral BPP to be largely clarified, so that research efforts can now focus on developing device technology suitable for reducing BPP without other penalties. The background level to BPP remains an open topic, and further study is needed to better understand and address this.
In this work, we use a multiphysics model of an external-cavity laser diode to study the influence of misaligned external optical feedback on the COD level of the device. The model solves the drift-diffusion equations for the electrical transport in the vertical-longitudinal plane self-consistently with a wave-optical model (including semiconductor chip and external resonator) and a 3D thermal model of chip and submount. A vertical misalignment of the FAC lens in an external resonator configuration consisting only of the FAC lens and feedback mirror leads to strongly reduced COD levels within the simulation if the feedback radiation hits the metal layers on the p-side of the device. The absorbed feedback radiation is the initial driver for the COD, whereas vertical leakage currents lead to ever increasing temperatures during thermal runaway. Experimental data of pulsed COD tests confirm the simulation results qualitatively. The minimum absorbed optical feedback power leading to COD depends on the operating point of the device. It increases with increasing external reflectivity due to the onset of COD at lower currents and corresponding lower internal optical power densities. For a low external reflectivity the output power is limited by thermal rollover instead of COD. The surface recombination velocity as the parameter quantifying the facet passivation quality has only a minor influence on the COD level in the simulation as for a low surface recombination velocity (high facet quality) the carriers can still recombine nonradiatively in the bulk layers due to the vertical leakage currents.
Applications that require high resolution patterns are numerous, leading to an increasing demand for compact patterning tools and alternative lithographic concepts. For many scientific applications like biosensing or fabrication of metamaterials, or artificial crystals, the achievable resolution and the patterned area of the fabrication process are of main importance. In the field of high-volume manufacturing, there is a need for high-resolution patterning at the industrial exposure wavelength of 13.5 nm. The main industrial application for compact exposure tools is EUV photoresist development and its related process optimization. The overall patterned area is of minor interest. Instead, the focus is placed on the achievable resolution and quality of the intensity distribution used for the patterning tests. The realized EUV dual beamline allows to address both application fields in a single in-lab setup. By operating the source with an argon/xenon (Ar/Xe) gas mixture, a narrowband spectrum with a main wavelength of 10.9 nm is created without the need of spectral filtering. The resulting intensity of up to 2 mW/cm2 in wafer plane allows large area patterning with highest throughput of several mm2/min. Single exposure fields of 2 x 2 mm2 can be stitched together to achieve an overall patterned area of up to several cm² with minimal stitching borders of ~ 1 μm. By inserting a customized multilayer mirror into the beamline, the emission spectrum of the DPP source (operated with pure Xe gas) is in-band filtered to 13.5 nm, thus allowing qualification of industrial photoresists regarding sensitivity, contrast and resolution. The mask-wafer positioning system for the 13.5 nm beamline is designed for maximum rigidity to minimize relative movements between the mask and wafer that would lower the achievable resolution. Multi-field resolution test masks are created in-house and are exposed in a parallel manner to determine the achievable resist resolution in an efficient manner. Transmission mask designs are optimized by a rigorous simulation model. By tuning the pattern geometry on mask, different patterns like contact holes or nanopillars can be created on the wafer, tailored to the required application.
Freeform optics are used to create complex application-adapted illuminance distributions due to their high number of adjustable degrees of freedom. As with conventional optics, this can result for some applications in large optics that exceed the available installation space or lead to high production costs. While the use of Fresnel lenses is common in such cases for conventional optics, the Fresnelization step in the design process of freeform optics is more complex due to the lack of rotational symmetry and only done for specific cases. Therefore, this paper presents a method to examine different segmentation strategies on freeform lenses and to optimize Fresnelization parameters. In this work, the method will be demonstrated using a head-up display (HUD) as example, in which the combination of a Fresnel lens and a display creates a moiré pattern in the illuminance distribution. In order to accelerate the simulation of complex Fresnel freeform optics considerably, a special ray tracing algorithm is developed, which takes advantage of the segmentation characteristics. The simulation approach of the illuminance distribution and the optimization of the Fresnelization are shown and discussed on a lens with about 300 segments visibly reducing the moiré pattern.
KEYWORDS: 3D modeling, Thermal modeling, Optical damage, High power lasers, Diodes, Quantum wells, Waveguides, Thermography, Temperature metrology, Semiconductor lasers
The process of catastrophic optical damage (COD) in 9xx-nm laser diodes is typically divided into three phases. In this work we model the first phase of COD by placing a localized additional heat source near the front facet corresponding to accumulated defects or misaligned optical feedback. We then compare two different multiphysical models to investigate thermal runaway, the second phase of COD. The first model considers only the carrier density within the quantum well coupled to a lateral-longitudinal optical model and a 3D thermal model. For this model, the temperature distribution converges within a few iteration steps without indication of thermal runaway and irrespective of the power of the additional heat source. The second model self-consistently computes the electrical and optical properties in the vertical-longitudinal plane and the 3D temperature distribution of the device. A critical power of the additional heat source is found above which the temperature distribution does not converge anymore and the maximum temperature increases to values above 1000 K. This strong temperature increase is accompanied by a thermally induced current crowding near the front facet and excessive carrier leakage from the quantum well. An analysis of the contributions of different heat sources shows that the nonradiative recombination in the waveguide and cladding layers exhibits the strongest changes during thermal runaway. The results of the two models indicate that the frequently proposed explanation of the feedback loop for thermal runaway consisting of a thermally induced bandgap shrinkage and increasing nonradiative recombination needs to be supplemented by thermally induced current crowding.
We present a direct diode laser with an optical output power of more than 800 W ex 100 μm with an NA of 0.17. The system is based on 6 commercial pump modules that are wavelength stabilized by use of VBGs. Dielectric filters are used for coarse and dense wavelength multiplexing. Metal sheet cutting tests were performed in order to prove system performance and reliability. Based on a detailed analysis of loss mechanisms, we show that the design can be easily scaled to output powers in the range of 2 kW and to an optical efficiency of 80%.
The potential of diamond as an optical material for high-power laser applications in the wavelength regime from the visible spectrum (VIS) to the near infrared (NIR) is investigated. Single-crystal diamonds with lateral dimensions up to 7×7mm2 are grown with microwave plasma assisted chemical vapor deposition (MPACVD) in parallel with up to 60 substrates and are further processed to spherical optics for beam guidance and shaping. The synthetic diamonds offer superior thermal, mechanical and optical properties, including low birefringence, scattering and absorption, also around 1 μm wavelength. We present dielectric (AR and HR) coated single-crystal diamond optics which are tested under high laser power in the multi-kW regime. The thermally induced focal shift of the diamond substrates is compared to the focal shift of a standard collimating and focusing unit for laser cutting made of fused silica optics. Due to the high thermal conductivity and low absorption of the diamond substrates compared to the fused silica optics no additional focal shift caused by a thermally induced refractive index change in the diamond is observed in our experiments. We present experimental results regarding the performance of the diamond substrates with and without dielectric coatings under high power and the influences of growth induced birefringence on the optical quality. Finally, we discuss the potential of the presented diamond lenses for high-power applications in the field of laser materials processing.
Multiplexing technologies enable the development of high-brightness diode lasers for direct industrial applications. We present a High-Power Dense Wavelength Division Multiplexer (HP-DWDM) with an average channel spacing of 1.7 (1.5) nm and a subsequent external cavity mirror to provide feedback for frequency stabilization and multiplexing in one step. The "self-optimizing" multiplexing unit consists of four reflective Volume Bragg Gratings (VBGs) with 99% diffraction efficiency and seven dielectric mirrors to overlay the radiation of five input channels with an adjustable channel spacing of 1-2 nm. In detail, we focus on the analysis of the overall optical efficiency, the change of the beam parameter product and the spectral width. The performance is demonstrated using five 90 μm multimode 9xx single emitters with M2≤17. Because of the feedback the lateral (multimodal) spatial and angular intensity distribution changes strongly and the beam parameter product decreases by a factor of 1.2 to 1.9. Thereby the angular intensity distribution is more affected than the width of the beam waist. The spectral width per emitter decreases to 3-200 pm (FWHM) depending on the injection current and the reflectance of the feedback mirror (0.75%, 1.5%, 4%, 6% or 8%). The overall optical multiplexing efficiency ranges between 77% and 86%. With some modifications (e.g. enhanced AR-coatings) we expect 90-95%.
Spatial and spectral emission characteristics and efficiency of high-power diode laser (HPDL) based pump sources
enable and define the performance of the fundamental solid state laser concepts like disk, fiber and slab lasers.
HPDL are also established as a versatile tool for direct materials processing substituting other laser types like CO2 lasers
and lamp pumped solid state lasers and are starting to substitute even some of the diode pumped solid state lasers. Both,
pumping and direct applications will benefit from the further improvement of the brightness and control of the output
spectrum of HPDL.
While edge emitting diodes are already established, a new generation of vertical emitting diode lasers (VCSELs) made
significant progress and provides easy scalable output power in the kW range. Beneficial properties are simplified beam
shaping, flexible control of the temporal and spatial emission, compact design and low current operation. Other
characteristics like efficiency and brightness of VCSELs are still lagging behind the edge emitter performance.
Examples of direct applications like surface treatment, soldering, welding, additive manufacturing, cutting and their
requirements on the HPDL performance are presented. Furthermore, an overview on process requirements and available
as well as perspective performance of laser sources is derived.
Four different external resonator concepts including VBGs for spectral stabilization of HPDLs are modelled and numerically evaluated to be compared to each other with respect to stabilization efficiency and sensitivity to the “smile-error". The coupled resonators including the external system and the diode laser are solved with a Fox-Li approach. The paper gives a brief summary about the applied simulation model and proceeds with the results for the different feedback concepts. The effective reflectivity, losses in the optical system, losses due to the back-coupling into the waveguide and the averaged optical confinement factor are calculated.
The active alignment of fast axis collimator lenses (FAC) is the most challenging part in the manufacturing process of optical systems based on high power diode laser bars. This is due to the high positioning accuracy in up to 5 degrees of freedom and the complex relations between FAC misalignment and properties of the resulting power density distribution. In this paper an experimental approach for FAC alignment automation is presented. The alignment algorithm is derived from a beam propagation model based on wave optics. The model delivers explicit relations between FAC misalignment and properties of the distorted power density distribution in the near and far field. The model allows to calculate the FAC misalignments and to correct them in one or multiple steps. The alignment algorithm is tested with a demonstrator system. The demonstrator contains an optical system which allows a real time analysis of the near field and far field power distribution of individual emitters. For the tests two different types of FAC lenses and high power diode laser bars are used. The FAC lenses are prealigned within a range of ±50 μm and 0.5 degree around the suitable position. During the automated alignment process the translational and rotational remaining misalignment and the properties of the far field power density distribution are recorded. The experimental results are evaluated regarding reliability and flexibility of the presented FAC alignment algorithm.
We present a compact High-Power DenseWavelength Division Multiplexer (HP-DWDM) based on Volume Bragg Gratings (VBGs) for spectrally stabilized diode lasers with a low average beam quality M2 ≤50. The center wavelengths of the five input channels with a spectral spacing of 1.5 nm are 973 nm, 974.5 nm, 976 nm, 977.5 nm and 979 nm. Multiplexing efficiencies of 97%±2% have been demonstrated with single mode, frequency stabilized laser radiation. Since the diffraction efficiency strongly depends on the beam quality, the multiplexing efficiency decreases to 94% (M2 = 25) and 85%±3% (M2 = 45) if multimode radiation is overlaid. Besides, the calculated multiplexing efficiency of the radiation with M2 = 45 amounts to 87:5 %. Thus, calculations and measurements are in good agreement. In addition, we developed a dynamic temperature control for the multiplexing VBGs which adapts the Bragg wavelengths to the diode laser center wavelengths. In short, the prototype with a radiance of 70GWm-2 sr-1 consists of five spectrally stabilized and passively cooled diode laser bars with 40Woutput after beam transformation. To achieve a good stabilization performance ELOD (Extreme LOw Divergence) diode laser bars have been chosen in combination with an external resonator based on VBGs. As a result, the spectral width defined by 95% power inclusion is < 120pm for each beam source across the entire operating range from 30 A to 120 A. Due to the spectral stabilization, the output power of each bar decreases in the range of < 5 %.
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