Functional characteristics of optical waves in transverse acousto-optic waveguide and active coupler using SAW

Collinear waveguide-type A-O devices use collinear interaction with mode coupling based on Bragg condition between optical waves and SAW. Collinear A-O devices of waveguide-type show excellent performance for wavelength selective filtering with narrow bandwidths. However, interaction time in collinear A-O devices is several micro seconds for a few mm waveguide device length. A-O devices of optical waveguides using transverse A-O interaction where SAW propagates transversely to optical wave propagation direction, yield high speed functional characteristics because SAW propagation lengths are a few μm and interaction time is several nano seconds. In this paper, fundamental dynamic characteristics of transverse A-O interaction are studied. Refractive indexes induced by A-O effects of SAW are shown by sine functions in the transverse x direction. Wave field characteristics in inhomogeneous media of periodic structures for transverse directions are discussed by Mathieu functions. Dispersion characteristics of A-O eigen modes are studied for several wavelengths of optical waves and SAW. Mode coupling, modulation and switching in transverse A-O waveguide devices controlled by SAW are shown. Transverse A-O devices as couplers and separators are also discussed.

I. Introduction

Optical characteristics and application processing systems of collinear optical switching device using acousto-optic (A-O) effects, have been studied about optical dielectric waveguides on LiNbO₃ crystal substrates [1]–[3]. Waveguide-type A-O devices use collinear interaction and mode coupling based on Bragg condition between optical waves and SAW both propagating in same axial directions [4]–[6]. However, in these collinear A-O devices, since SAW propagation speed is very slow, interaction time is several micro seconds for waveguide device length of more than 10mm. In A-O devices of optical waveguides using transverse A-O interaction, where SAW propagates transversely to optical wave propagation direction, SAW propagation lengths needed for A-O interaction may become and interaction time may be several nano seconds [7]–[9]. Ultra high speed switching and modulation can be accomplished, in transverse A-O waveguide devices