Apodized grating silicon waveguides for tunable optical delay lines

Modern optical communication systems require tunable optical delay lines for synchronization of data packages. Chirped fiber grating are typically used as a dispersive element for this application. Fiber-based schemes, however, suffer from bulkiness, slow response and/or low time resolution. Integrated photonics on silicon is one alternative approach. Several more advanced device architectures have been subsequently demonstrated on silicon, including ring-resonator and photonic crystal (PhC) line-defect waveguide types. The coauthors of this paper have recently proposed a novel class of tunable optical delay lines based on apodized grating waveguides. It was shown that high bit-rate operation is achievable by cascading two complementary (inward and outward) apodized gratings. In this paper, we have experimentally demonstrated our proposed approach and studied performance of the fabricated delay lines for optical telecommunication applications. The bit-rate was extracted from broadening of transform-limited input pulses of the measured dispersion of the delay lines. Characterization of the compact delay lines shows that the single grating devices offer true-time delays as long as 164 ps, tuning range of similar to 81 ps, and a minimum bit rate of similar to 15 Gb/s. The cascaded scheme offer true-time delays of 82 ps and tuning range of 32 ps and can potentially operate at bit rates as high as 107 Gb/s.