From Ferroelectrics to Polymers - Perspectives for Thin Film Optical Waveguides and Devices
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Abstract Presently commercial optical fiber links experience an explosive growth. On the other hand, the integration of optical functions on a single substrate ("Integrated Optical Device") is still mostly limited to the R & D-laboratories. Only Mach-Zehnder modulators, made from ferroelectric crystals, have found commercial applications so far. The development phase of the integrated optical devices, based on bulk crystalline LiNbO 3 , is now covering a time span of nearly 30 years. In comparison, the development of the thin film growth of optical oxide ferroelectrics or of tailored optical polymers is just a few years old and we are still at the beginning of the learning curve. 1. Introduction
Optocommunication is in a phase of tremendous growth, because computers are at work everywhere and these computers now communicate directly with each other via long distance lines. This constitutes the major part of the constantly growing volume of data traffic. The most important components for the communication systems are laser diodes, glass fibers, fast photodetectors, passive multiplexers, splitters and filters. Also in use are alloptical Er-based amplifiers, fast LiNbO 3-based modulators and magneto-optical isolators. This arsenal satisfies todays needs and is probably sufficient for the demands of the near future. As far as the electrooptic or magnetooptic materials for modulators or optical isolators are concerned, typically bulk single crystalline materials are of the best quality. The materials science and art of growing these crystals, the understanding of their properties and the fabrication of optical components from these bulk crystals have been developed over many decades.
133 Mat. Res. Soc. Symp. Proc. Vol. 597 ©2000 Materials Research Society
2. Four reasons for developing the thin film technology of thin ferroelectrical optical
films First, optical fibers need optical waveguides at their terminations in order to match the mode profile of the propagating light signal. Most waveguides are fabricated in the near-surface regime, especially in the case of the optical ferroelectrics. If one "reverses" this observation, it is fair to say that most of the crystalline substrate is simply for mechanical stability, not for optical functions. Therefore the combination of a cheap substrate and an optimized thin film for performing the functions makes sense - if it can be achieved. Second, many ferroelectric materials with the desired electro-optic properties are very
difficult to grow in the form of large crystals, which then have to be cut and polished for wafers. Two examples are BaTiO 3 and KNbO 3. Therefore the development of the growth of thin optical films of these materials on large substrates is extremely important. Third, looking a little ahead and assuming, that we can achieve some of these objectives, we may compare the future oxide technology with the present progress in semiconductor devices. Semiconducting heterostructures with modulated electronic and optical properties are the basis for all modern
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