Volume 1, Issue 5

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Ultrafast lasers have excellent characteristics for materials processing in terms of precision and quality. This tutorial paper introduces the basic concepts of ultrafast laser materials processing and the structure and key components of typical ultrafast laser microprocessing systems. The emphasis is on the pulse-shaping devices and systems for controlling and manipulating the spatial, temporal, and polarization properties of focused femtosecond laser pulses.

Current laser cladding technologies have limited resolutions of about 50 μm. A new solution has been developed within the project ‘Nanomicro’ to improve the spatial resolution. The ‘micro hollow laser beam system’ and a deposition system for highly focused metal micropowders are innovative parts within a high-precision 3D micromanufacturing laser cladding platform. We demonstrate, here, a new hollow laser sintering concept that is aiming on more efficient laser cladding and manufacturing of highly precise and functional parts in the range of a few microns.

At high power densities in excess of, say, 10 9 W/cm 2 , such as may be created by focusing pulsed laser radiation of nanosecond pulse duration, optical glass is no longer transparent but absorbs the radiant power, resulting in stress and physical damage (cracks). While this is intended for certain techniques of machining glass and other normally transparent materials, secondary foci or ‘ghost images’ inside the focusing optics may also exceed the threshold for absorption and thereby cause damage to lenses if such ghost images fall inside an optical element. When designing optics for laser machining of glass, it is therefore important to control the formation of ghost images and to ensure that they are located outside all lens elements. This dominates the design process for such optical systems.

A grating waveguide mirror (GWM) results from the combination of a waveguide and a sub-wavelength grating which lead, for given incidence conditions, to polarization and or wavelength filtering. In the present paper, we report on the application of such elements for the selection of the polarization (linear, radial, or azimuthal) as well as the narrowing, stabilizing and tuning of the emission spectrum of high-power lasers. Using a leaky-mode circular GWM, beams with radial and azimuthal polarization with output powers of up to 275 W and 145 W, respectively, could be extracted from a Yb:YAG thin-disk with optical efficiencies of 52.5% and 43%, respectively. In both cases, the GWM was composed of a highly reflective (HR) mirror and a sub-wavelength grating as the end-mirror of the resonator. Using a leaky-mode linear GWM operating under Littrow condition, beams with linear polarization and a narrow spectral bandwidth were obtained from a Yb:YAG thin-disk laser. In multimode operation, an output power of 325 W was achieved at an optical efficiency of 53.5%. In near-fundamental mode operation, 110 W was extracted at an optical efficiency of 36.2%. In this latter case, the spectral bandwidth was measured to be around 25 pm. Moreover, continuous wavelength tuning from 1007 to 1053 nm was demonstrated with our device.

High power vertical-cavity surface-emitting laser (VCSEL) arrays can be used as a versatile illumination and heating source. They are widely scalable in power and offer a robust and economic solution for many new applications with moderate brightness requirements. The use of VCSEL arrays for high power laser diode applications enables multiple benefits: full wafer level production of VCSELs including combination with micro-optics; assembly technologies allowing large synergy with LED assembly thus profiting from the fast development in solid state lighting; outstanding reliability and a modular approach on all levels. With the use of μ-lenses, the described optical principle of near field imaging and superpositioning of many thousands of VCSELs gives perfect control over intensity distribution and is inherently robust. With a slightly modified approach, lines of any desired length can be built from modules of 1-cm length and are therefore scalable for a wide range of applications.

As laser brazing benefits from advantages such as smooth joints and small heat-affected zones, it has become established as a joining technology that is widely used in the automotive industry. With the processing of complex-shaped geometries, recent developed brazing heads suffer, however, from the need for continuous reorientation of the optical system and/or limited accessibility due to lateral wire feeding. This motivates the development of a laser brazing head with coaxial wire feeding and enhanced functionality. An optical system is designed that allows to generate an annular intensity distribution in the working zone. The utilization of complex optical components avoids obscuration of the optical path by the wire feeding. The new design overcomes the disadvantages of the state-of-the-art brazing heads with lateral wire feeding and benefits from the independence of direction while processing complex geometries. To increase the robustness of the brazing process, the beam path also includes a seam tracking system, leading to a more challenging design of the whole optical train. This paper mainly discusses the concept and the optical design of the coaxial brazing head, and also presents the results obtained with a prototype and selected application results.