SUPPRESSION OF SIDELOBE LEVELS FOR GUIDED-WAVE ACOUSTOOPTIC TUNABLE FILTERS USING WEIGHTED COUPLING

Guided-wave acousto-optic tunable filters (AOTF's) possess the potenti al to be used as tunable wavelength filters and switches in fiber opti c wavelength-division-multiplexing systems because of their capabiliti es in providing very narrow bandwidth and very large tuning range. Muc h improvement in filter response, especially significant reduction or suppression of sidelobe levels, is still required, however. In this pa per, an analysis on guided-wave AOTF's that employ simple acousto-opti c (AO) weighted coupling techniques for sidelobe reduction, and the ca lculated and experimental results on a specific example which involves only variation of the width of a surface acoustic wave (SAW) slot wav eguide are presented. The calculations on single- and multi-stage AOTF 's consisting of an optical channel waveguide and a SAW slot waveguide in LiNbO3 substrate have shown that waveguide width weighting using g eneralized Hamming functions would provide significant improvement in sidelobe suppression. For example, using the square root of a generali zed Hamming function on the width of the SAW waveguide, a sidelobe lev el of -15 dB can be achieved as compared to that of -9 dB with the con ventional uniform coupling in a single-stage AOTF. Further improvement on sidelobe suppression can be accomplished by using multi-stage arra ngements. Calculated results together with the design, fabrication and measured performance characteristics of a single-stage AOTF that util izes a weighted-aperture SAW slot waveguide in YX-LiNbO3 substrate at the optical wavelength of 1.55 mum and the acoustic center frequency o f 175 MHz are reported. The measured sidelobe level was -13.6 dB and t he measured FWHM bandwidth was 26 angstrom as compared to the theoreti cal values of -15.0 dB and 15 angstrom, respectively. The RF drive pow er was measured to be 1.0 watt at a mode-conversion efficiency of 100% . Finally, the measured bandwidth of the SAW transducer was 37 MHz, su ggesting a tuning range of 325 nm for the optical wavelength centered at 1.55 mum.