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Content archived on 2024-06-10

New Machining Technique for Optical Components Using Photo-ablation


The overall goal of the MATOC project was to develop a production technology for manufacturing Diffractive/Refractive Optical Elements (DROE) using excimer lasers. At the moment, these elements are produced by grinding and polishing, molding, ion exchange or photolithography. IR-optics can even be manufactured by diamond turning. These processes either use expensive tools, so that a profit margin is only given for a large number of units, or they require long processing times. Especially for the micro-machining of glass for a small number of units, there is no suitable process available at the moment. Investigations on treating glass with excimer lasers could enable the fabrication of lenses of different geometric structures. A reduction of production time, which is considered to be one of the main parameters for the production requirements, is the aim of the optimized process.

The results of excimer laser machining strongly depend on the properties of the material. Investigations within the first year of the project have already shown that C2036 is the most suitable of all treated materials, so it has been chosen to be the focus of the further investigations.

At the beginning of the project excimer lasers at wavelengths of 308 nm and 248 nm were used. Experiments during the first year of the program have shown that production requirements could not be achieved at 308 nm. Therefore, it was decided to concentrate on the wavelength of the KrF laser at 248 nm.

All results from basic investigations were considered when developing the MATOC micro machine. The machine is based on a 4-axis stage, and an NC-controlled flexible mask with up to 4 additional axes. A special software enables the user to create various diffractive and refractive rotation symmetric optical elements. Optical parameters can be inserted in a Windows-based user interface that calculates the NC-code and shows the graph of the surface equation. Using the MATOC machine, a surface roughness of about 100 nanometers has already been achieved in glass samples.

The machined diffractive and refractive samples were measured and tested by the end-users. The optical properties of the samples were defined by the profile of the shape geometry and the surface roughness. The calculated curvature can be approximated by ablation steps of less than 100 nanometers depth for most kinds of glass. This makes it possible to achieve a surface profile where accuracy and reproducibility of the profile are sufficient. To achieve the required surface roughness of less than 10 nanometers, various polishing processes were tested to smoothen the laser machined structures. Polishing using CO2 lasers or using abrasive liquids while rotating the workpiece, for example, have shown a considerable reduction of surface roughness. The main disadvantage of all processes was a negative effect on the surface profile, especially at the sharp outlines of diffractive structures.

Another criterion to verify the quality of the machined optics was stipulated in fiber coupling tests. The light of a laser diode was focused by a lens and coupled into the end of a glass fiber. The transmission rate was determined by the portion of light transmitted through the lens. The coupling efficiency was stipulated by measuring the portion of transmitted through the fiber. These tests were also carried out for standard spherical and aspherical lenses. The results were compared with those of the laser-machined samples. In fact, the laser ablated optical elements showed transmission rates and coupling efficiencies of only about 30% of the standard optics, up to now. The premise for increasing optical quality is the further improvement of the surface roughness and the reduction of profile defects due to the polishing process.

At the end of the project, all partners agreed that the obtained results are essential for further development of laser machining in optical industry. In conclusion it can be mentioned that for excimer laser fabrication of glass, the emphasis must be placed on the improvement of surface quality.

BE95-1202 New Machining Technique for Optical Components Using Photo-ablation
For the MATOC-project, Europe's leading experts in complementary working areas, which are Sopelem/Sofretec (FR) for sophisticated optical elements, Lambda Physik (DE) for UV-laser sources, Laser Zentrum Hannover (DE) for UV-laser processing, Exitec Limited (UK) for laser based micromachining systems, will join together to create a new machining technique for hybrid optical components and micro-optics. This innovative technique will open new markets and areas to the European industry and will be of high economical benefit.
The industrial, scientific and medical applications demand for such optical components in the visible and infrared is widespread. Sopelem-Sofretec as a high technology optical system manufacturer needs low cost prototyping and batch production methods especially for the visible and infrared video camera market.

The optical industry has a strong need in new machining techniques for hybfid-optical (diffractive and refractive) macrocomponents (diameter = 50 mm to 100 mm) on the one hand and microoptics (diameter = 100 m up to 1 mm) on the other hand. An integration of these components into new and existing products will reduce costs, size, weight and at the same time will enhance the performance.

The very high competitive worldwide optical equipment market has pushed European optical manufacturers to increase their expertise and knowledge in the production of cost-effective high technological optical equipment. Thus, the project's objective is to build up a new machining technique, which will permit new diffractive/refractive macro optical elements and micro-lenses to be introduced into the industrial market.

This machining technique will offer, to the European optical industry, a cost-effective batch production tool. The current use of diamond turning tools is limited to ductile materials and offers insufficient economic benefits to fabrication of optical components. As a result, the production of the innovative optical components with the new technique will open up new horizons for the optical designer that can not be explored today with existing optical technologies.

The use of the UV-photo-ablation technology, which has shown radically new advances in recent years, will allow to set-up prototype machining system. Europeans leading laser based micro-machining systems manufacturer will create a new maching technique for optical components made of different materials, with the support of a research centre, which has developed a pioneering new processing technology for three-dimensional microstructures, and leading laser source manufacturer. The results will be spread to different new sectors, like micromechanics and microelectronics and therefore, mean extraordinary high exploitation.

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Laser Zentrum Hannover e.V.
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30419 Hannover

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