**1. Introduction**

594 Acoustic Waves – From Microdevices to Helioseismology

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Nowadays, in-fiber acousto-optic devices are increasingly used as frequency shifters, multiplexers, modulators, and tunable filters. They can be easily spliced into optical fiber systems, and the consequent low insertion loss, make them an attractive alternative to bulk optics devices. Our group, established at the Institute of Materials Science, Department of Applied Physics, of the Valencia University (ICMUV, Valencia, Spain), has been involved in this field for the last ten years, and our fabrication facilities allow us the development of new in-fiber acousto-optic devices for novel applications in different fields such as sensors, microwave photonics, lasers, and optical communications. Our aim here is to present the great potential shown by in-fiber acousto-optic devices for different photonic applications. Although this chapter is focused in our latest developments, the reader will also find discussed the work done by other research groups in the field. This chapter is divided in two main sections: Section 2 is focused on novel applications of acousto-optic fiber devices based on flexural acoustic waves, and Section 3 is focused on applications of the acoustooptic devices based on the interaction of longitudinal acoustic waves with fiber Bragg gratings (FBG). Finally, our conclusions are shown in Section 4.
