ULTRALOW SIDELOBE-LEVEL INTEGRATED ACOUSTOOPTIC TUNABLE FILTERS USINGTAPERED-GAP SURFACE-ACOUSTIC-WAVE DIRECTIONAL-COUPLERS

We propose a new weighted-coupling scheme using tapered-gap surface ac oustic wave directional couplers for realization of ultralow sidelobe- level integrated acoustooptic tunable filters (IAOTF's). Appropriate d esign analysis has been carried out for 30-mm-long filters operating a t an optical wavelength of 1.55 mum in an X-cut Y-propagating LiNbO3 s ubstrate. New synthesized weighting functions have been used for the i mprovement of sidelobe level suppression over existing single-stage fi lters by as much as 27 dB. The -20 dB mainlobe width of the resulting IAOTF's varies from 2.7 to 3.9 nm only for the worst sidelobe levels r anging from -30.6 to -44.7 dB, respectively. It has also been shown th at further suppression of sidelobe levels by 3-9 dB is possible if the filter is underdriven at 80% mode-conversion efficiency.