| Summary: | Integrated optical delay lines (ODLs) featuring fast and continuous tunability are important building blocks in microwave photonic signal processing systems. In this work, we propose and demonstrate an ultra-fast tunable ODL based on cascaded silicon microdisk resonators (MDRs). In the proposed integrated ODL, a specially designed MDR is used as the basic delay element, which has an additional slab waveguide to surround the disk and the bus waveguide aiming to suppress higher-order whispering gallery modes (WGMs) and to support the incorporation of a lateral PN junction for electrical tunability. By exploiting the free-carrier dispersion (FCD) effect on silicon, the proposed ODL can be tuned with an ultrahigh speed. Multiple MDRs are connected in series to increase the time delay provided by the ODL. As a demonstration, integrated ODLs with 5 and 10 MDRs are designed and fabricated. The experimental results show that the fabricated ODL provides an ultra-short response time of 989 ps, a low power consumption below 0.58 mW, and a small footprint of 0.13 mm<sup>2</sup>. The proposed integrated ODL holds great advantages including ultra-high tuning speed, low power consumption and ultra-compact footprint, which is potential to be widely used in next-generation phased array radar and high-speed wireless communication systems.
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