No abstract

Certain high power laser applications require thin homogeneous laser lines. A possible concept to generate the necessary flat-top profile uses multi-aperture elements followed by a lens to recombine separated beamlets. Advantages of this concept are the independence from entrance intensity profile and achromaticity. However, the periodic structure and the overlapping of beamlets produce interference effects especially when highly coherent light is used. Random optical elements that diffuse only in one direction can reduce the contrast of the interference pattern. Losses due to undesired diffusion in large angles have to be minimized to maintain a good quality and high efficiency of beam shaping. We have fabricated diffusers made of fused silica for a wide range of wavelengths that diffuse only in one direction. Structures are based on an array of concave cylindrical microlenses with locally varying size and position following a well defined statistical distribution. The scattering angle can be influenced by process parameters and is typically between 1 • and 60 • . To predict the influence of process parameters on the optical properties, a simplified model for the fabrication process and geometrical optics have been used. Characterization of the fabricated devices was done by stylus measurements for the surface shapes, microinterferometry to measure phase profiles and high resolution goniometry to obtain far field distribution of light. The simulated data compare very well to measured optical properties. Based on our simulation tool we discuss limits of our fabrication method and optimal fabrication parameters.

A wide range of lasers from the UV to the IR are selected based on their optical power and spectral characteristics to match the particular absorption behavior for the material to be processed. Periodic microlens arrays are often used as multi-aperture integrators to transform the Gaussian or non-uniform beam profile into a homogenized intensity profile either in 1-D or 2-D distribution. Each microlens element samples the input inhomogeneous beam and spreads it over a given angular distribution. Incoherent beams that are either temporally or spatially incoherent can produce very uniform intensity profiles. However, coherent beams will experience interference effects in the recombination of the beams generated by each individual microlens element. For many applications, for example pulsed laser sources, it is not possible to use a rotating or moving element, such as a rotating diffuser, to circumvent the interferences resulting from the beam coherence. Micro-optical elements comprised of a randomly varying component can be used to help smooth out the interference effects within the far-field intensity profile.