Microstructures as high efficient diffusers
- Responsible : Roland BITTERLI, Toralf SCHARF
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Micro-optics includes a family of optical components and systems that are fabricated with the aid of modern micromachining, such as optical lithography, direct laser and e-beam writing, reactive ion-etching etc. The functionality of such elements has been proven in various applications, including laser-beam shaping, illumination systems for DUV-lithography, optical interconnects, lab-on-chip and miniaturized imaging systems. Of strong interest is the combination of micro-optics with micro mechanics.Laser lines with a uniform flat-top profile are interesting for many applications in the domain of surface treatment and fabrication (e.g. laser annealing for TFTs ). 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 reduce the contrast of the interference pattern. Losses due to undesired diffusion in large angles have to be minimized.
Figure 1. SEM picture (left) optical picture (right) showing the topograpy of the 1D diffuser.
Our concept is based on a statistical array of cylinder lenses. The lenses have a constant radius of curvature with a statistical distribution in width. For narrow diffusion angles it is necessary to have lenses with a small sag. To avoid specular transmission which it is necessary to avoid flat zones at the bottom of the lens as well as at the top. As a material we use fused silica for its large transmission range and good damage resistivity in high power laser applications.The fabrication is based on isotropic etching of fused silica in hydrofluoric acid and a typical diffuser is shown in Figure 1. The surface profile can be measured with interferometric techniques and allows investigations of the optical functionality.
Figure 2. Surface profile measured with a Mach Zehnder interferometer. One recognizes the statistical variations of the optical element.
Characterization is done by measuring the light distribution as a function of angle and wavelengths at high angular resolution. A typical result is shown in Figure 3 and compared with a simulation.
Figure 3. Measurement and simulation of optical properties in the far field of one dimensional diffusers. A typical speckle image is shown that is caused by random interferences of laser light.
Actual Projects in this field:
CTI PFNM-NM ALBS No. 9143_1 PFNM-NM
-  Miyasaka, M., Stoemenos, J., J.Appl. Phys. 86, 5556–5565
-  Schröder, G., Technische Optik, Grundlagen und Anwendungen, Vogel Verlag (2002).