We used laser ablation to fabricate sub-wavelength structure anti-reflection coating (SWS-ARC) on a 5 cm diameter alumina lens. With an aspect ratio of 2.5, the SWS-ARC are designed to give a broad-band low reflectance response between 110 and 290 GHz. SWS shape measurements give 303 μm pitch and total height between 750 and 790 μm height, matching or exceeding the aspect ratio design values. Millimeter-wave transmittance measurements in a band between 140 and 260 GHz show the increase in transmittance expected with the ARC when compared to finite element analysis electromagnetic simulations. To our knowledge, this is the first demonstration of SWS-ARC on an alumina lens, opening the path for implementing the technique for larger diameter lenses.
To develop a safe and sustainable infrastructure for future missions, In-Space Manufacturing must become state of the art. This paper will propose a novel handling mechanism for powder-based material suitable for the microgravitational environment. Ultrasonic levitation is a promising technology for gravity-independent material handling. The fundamental challenge lies in the trapping of powder-based materials. To assist the material deposition process and stabilize the material handling water is used as a carrier material. A multi-emitter single-axis ultrasonic levitator is employed to levitate PA 12 SLS-powder in a fixed state and initiate a laser melting process to bind the powder material. The spherical levitator uses 72 piezoelectric transducers by Murata, with a levitation radius of 37 mm, which can generate a levitation force of up to 0.15 mN. A 20 W 1064 nm fiber laser is employed to evaporate the water and bind 0.4 μg PA 12 particles together. The experiments will be performed under earth and atmospheric conditions. This paper investigates the evaporation time of water inside a levitation field depending on the laser power. The properties and parameters of distinguished materials will be evaluated to determine the boundary conditions of the acoustic levitator. The data will be compared to a simplified analytical model and used to initiate a melting process with the SLS material.
Current strategies for closed loop control of the temperature sensitive selective laser sintering (SLS) process are often based on slow thermal cameras. Another method is single scanner pyrometry. In that case the pyrometer uses the same scanner as the laser. The pyrometer measuring spot and the laser spot are always in the same position. It is not possible to measure temperature outside of the laser spot. In a novel approach, a highly dynamic double scanner system is used to position the measuring beam of a high speed pyrometer. With this approach it is possible to position the laser and measurement spot independently on the powder bed surface (e.g. leading or trailing measurement relative to the laser spot). A very fast pyrometer which outputs measurement data up to 50 kilohertz and FPGA (Field Programmable Gate Array) technology will be used for real time processing of the measured temperature. With this it is possible to process temperature fluctuations on the surface and respond to them very quickly. This technique should be applied to make the SLS process more stable and to get the best results out of the entire build volume. For this purpose, the measured temperature is used to dynamically control the power of the laser.
In micro-material processing with ultrashort laser pulses (USP), the surface quality during drilling and thin film ablation varies with the scanning speed and the pulse repetition rate. However, while high pulse repetition rates tend to be desirable, local heat accumulation caused by increasing pulse overlap is counterproductive. Thus, the scanning speed must be scaled with the pulse repetition rate, preferably by supplementing the already existing setup.
In this work, we present a dynamic extension through the combination of an acousto-optical deflector (AOD) with a galvanometer scanner. This combines the best of two worlds: the dynamic beam deflection of the AOD and the large scanning field of the galvanometer scanner. The integrated AOD is able to deflect the laser beam pulse by pulse within its scanning field and to modulate the beam intensity simultaneously. The mechanical limitations and problems of the galvanometer scanner, such as vibrations and overshoots due to fast mirror rotations, can be specifically compensated by the high precision of the AOD. As a result, in addition to process time reduction, the surface and image quality improves significantly. In any case, the laser source needs synchronization with the AOD because the propagation of sound waves within the AOD crystal is slower than the laser pulse propagation through the medium. In the first step, a comparatively slow AOD based on tellurium dioxide with a transversal crystal alignment is used. The process time of a thin film ablation with 4 μJ at 1 MHz was reduced considerably by applying a USP laser system (Coherent Monaco).
Radial and azimuthal polarizations have attracted new interest in the process development community due to improved
beam propagation and absorption conditions in the ablation cavity. This paper presents our recent activities and results
on polarization converted ultrashort laser pulses by use of segmented half-wave-plates for the generation of ripple
structures with predetermined sub-patterns. The formation of ripples fabricated in metals, ceramics, and semiconductors
is analyzed by the morphological investigation of the structures (spacing and orientation) as a function of the polarization
state of the laser beam.
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