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Weighted coupling for strong sidelobe supprkion of integrated acoustooptical mode converters in LiNbOs using acoustical directional couplers has been studied theoretically and experimentally. A parameter free model for the propagation of surface acoustic waves in guiding structures has been developed based on a step-like variation of the acoustic velocity. Comparisons of theoretical results with experimental ones for acoustic waveguides and directional coupler structures confirm the applicability of the model. A coupled mode description of the acousto-optical polarization conversion in converters with acoustical directional couplers has been developed and applied to several tapered acoustical directional couplers. The model reveals that the conversion characteristics are usually strongly asymmetric. If the directional coupler is appropriately designed, a sidelobe suppression of about 30 dB can be achieved. First experimental results with tapered directional couplers confirm within some limits the theoretical predictions. I. I NTRODUCTI ON UNABLE wavelength filters and wavelength selective T switches are key components for a variety of applications especially in wavelength division multiplexed (WDM) optical communication systems and for optical instrumentation. In particular, integrated acousto-optical filters and switches [ 11, [2] in LiNbOs are very attractive as they offer a broad tuning range [3], [4], a fast tuning speed [5] and especially their unique property of simultaneous filtering capability [6].These devices consist of a combination of collinear acoustooptical polarization converters and polarizing elements, e.g., polarization splitters andor polarizers. The principle of operation of the acousto-optical polarization converters is based on an interaction of guided optical fields of orthogonal polarization and a surface acoustic wave (SAW). Via this interaction a polarization conversion is achieved, if the phase-matching condition is fulfilled. It requires that the difference of the wave vectors of the optical waves is compensated by the SAW wave vector. Usually, the mode conversion is achieved with a homogeneous coupling strength along the interaction length due to a (nearly) constant SAW intensity, either guided in an acoustical waveguide or even unguided. In these cases the resultant conversion characteristics, i.e., the power of the converted optical wave as function of the optical Wavelength, are sinc2-IEEE Log Number 9409167. 0733-8724/95$04like functions with sidelobes theoretically -9.7 dB below the maximum conversion leading to a significant contribution to crosstalk especially in dense WDM systems.To reduce the sidelobes two different approaches have been investigated: cascading of several devices [7]-[ 101 and using a weighted coupling technique [11]-[16]. The first approach requires more complex devices and it is not directly applicable to switch structures. Moreover, cross talk due to coherent channel interference [17], [ 181, which occurs if simultaneous filtering is applied, is not suppres...

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