A system being able to in situ measure and control not simply the distance between the workpiece and the focusing
optics, but the true focal position on the workpiece including the thermally induced focal shift in a laser processing head
is presented. In order to achieve this, a bundle of astigmatic measurement beams is used following the same optical path
as the welding beam. A camera and a software algorithm allow to keep the focal position constant within a range of 4
mm and with a resolution between 150 μm and 500 μm.
A new approach is described to couple light from high-power blue LEDs (1x1mm2) into a thin large area light guide
using a dielectric interference multilayer as an angular filter. The goal is to achieve large area luminaires such as
backlight systems. The method overcomes the drawback of structuring holes or recessions in light guides when using a
system with side-emitting LEDs. Several new LED-masking filters have been designed to improve the polarization
dependency and coupling efficiency from a previous design that used a stack of inorganic bi-layers. Firstly, a polymeric
multilayer manages to increase the coupling efficiency from 52% to 69%. Secondly, a birefringent multilayer filter
enables to fully suppress the Brewster's angle effect and to realize identical behaviour for both s- and p- polarized light
at large angles. Thirdly, by adding layers of a third inorganic material to the original bi-layer stack with refractive index
in between the other two, the transmittance equality is improved and the coupling efficiency can reach up to 63%.
A novel angular filter is proposed for a large area planar collimating luminaire. A planar collimator of 2x18o cone angle
with scalable and simplified structure is demonstrated. It has a thickness of only 5mm. Compact collimators and planar
collimators have been analyzed with the ray tracing software LightTools. Measurements on this collimator filter and
optical designs match well with the predicted performance.
A new approach to thin film filter design, the so called saddle-point method of global optimization was explored and
showed promising initial results. A 40-layer collimator filter with a 30% lower merit function was obtained and is
discussed.
A passive few-mode multicore fiber consisting of 7 coupled cores is investigated. The fiber is compared to a largemode-
area step-index fiber, with the same number of modes and a similar mode field area. Based on the 7-core
fiber results a single transverse mode multicore fiber, with a mode field area of 465 μm2 at 1050 nm, delivering
virtually diffraction limited output beam quality is demonstrated. Stimulated Raman threshold measurements
are presented and a fundamental mode high-power beam transport with more than 350 W is shown.
This paper reports on a new arrangement for CO2 laser welding, allowing the fast variation of the polarization of radiation (DP). The experiments with steel St37 and Ck45 show a significant influence of a DP on the melt pool dynamic, especially a remarkable smoothing of the top bead at certain frequencies of DP.
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