Monolithically Integrated Tandem Waveguide-Type Acoustooptic Modulator Driven by Surface Acoustic Waves

A waveguide-type acoustooptic modulator (AOM) using coplanar AO coupling due to a surface acoustic wave (SAW), that is, Bragg diffraction, in a tapered crossed-channel proton-exchanged (PE) optical waveguide on a 128°-rotated Y-cut LiNbO3 substrate for an optical wavelength of 1.55 µm has been proposed. In this study, a monolithically integrated tandem waveguide-type AOM driven by SAW was designed and fabricated. The structure considered was a 2×4 optical switch, in which the input ports of two second 1×2 switches were connected to the two output ports of the first 2×2 switch on the same substrate. An interdigital transducer (IDT) with a period length of 32 µm and an overlap length of 2 mm was fabricated in the first and second stages of the AO interaction region. Diffraction efficiency was measured at a driving frequency of approximately 120 MHz. A diffraction efficiency of approximately 90% was obtained for each stage. When both stages were driven at the same frequency, a peak diffraction efficiency of 63% was obtained. Furthermore, the optical frequency shifts for the sum of two driving frequencies and the difference frequency ranging from DC to 5 MHz were observed.