Micro-optical components for fiber and integrated optics realized by the LIGA technique

Abstract

For a cost effective mass production of high precision micro-optical and micro-mechanical elements and devices for the rapidly growing market of telecommunication micro-systems and sensors, the LIGA technique (German acronym for the process steps lithography, electroforming and moulding) is a very promising fabrication tool [1,2]. The excellent precision and high aspect ratios that are possible make LIGA structures well adapted for applications concerning data transfer via glass fibers in combination with optical packaging and interconnection [3]. Fiber grooves with additional fixing elements have been realized, allowing exact alignment of the fiber and positioning of the fiber-end face. Furthermore, various micro-mechanical elements have been fabricated which can adjust fibers in ribbon connectors to achieve high coupling efficiency [4]. Transparent polymeric materials have been used for injection moulding or embossing to realize microoptical structures that form a complete "micro-optical bench" with lenses, mirrors, beam splitters and prisms. Here, a light beam may be guided, its intensity profile may be altered or it may pass additional elements placed in the beam path [SI. The "side-wall surfaces" obtained by LIGA show a roughness below 50nm (rms) and, therefore, are suitable for optical purpose in the visible and IR. Recently, it has been shown that replication technologies like embossing, casting or injection moulding are a very powerful tool to realize polymer waveguides [6-lo]. By replication it is possible to integrate waveguides, passive fiber alignment structures and micro-optical components onto a substrate by a single fabrication step. Another remarkable advantage compared with conventional planar techniques is that the realization of new types of three-dimensional structures (e.g. in the field of fiberchip-coupling) can be accomplished. X-ray lithography, conventional photoresist lithography or laser-micromachining followed by electroplating have been used to fabricate metal mould inserts which subsequently served for the replication of waveguide patterns by hot embossing into polymer substrates. These waveguide-'grooves' have been filled with a second material with higher index of refraction. Ysplitters, straight and curved waveguides for single and multimode operation have been realized. In a similar way, micro-cuvettes can be filled with substances showing nonlinear-optical properties or may also be used as flow channels in optical sensor devices. Also a modularly designed micro-chemical analysis system with micro-cuvettes and a micro-optical bench, allowing the measurement o f continuous flow liquid probes, is under development. All of these products are subject to the same requirements. These are: structure dimensions from some microns (waveguide) to some hundred microns (fiber alignment), accuracy of the micro structures in the sub-micron range, mass production of such micro-optical products,