Design of tapered-gap surface acoustic wave directional couplers for ultralow sidelobe-level integrated acousto-optic tunable filters

We propose a new weighted-coupling scheme using tapered-gap surface acoustic wave directional couplers for the realization of ultralow sidelobe-level integrated acousto-optic tunable filters (IAOTF). Appropriate design analysis has been carried our for 30 mm long filters operating at the optical wavelength of 1.55 μm in a X-cut Y-propagating LiNbO₃ substrate. New synthesized weighting functions have been used for the improvement of sidelobe level suppression over existing low-sidelobe single-stage filters by as much as 27 dB. The -20 dB mainlobe width of the resulting IAOTFs vary from 2.6 to 3.8 nm only for worst sidelobe levels ranging from -30.6 dB to -44.7 dB, respectively