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Three-dimensional tunable true time delay line

We propose a novel three-dimensional (3-D) continuously tunable delay line based on high performance 3-D tunable directional couplers (DCs) driven by graphene heaters. The device includes a continuously tunable delay line section and 2 bit discretely tunable delay line section, the former can be considered as 1 bit discretely tunable delay line structure, the latter includes two delay units and three switch units. We set the maximum delay tuning range of the continuously tunable delay line section equal to incremental delay step ∆t of discretely tunable delay line section, so that the tuning is continuous over the entire delay range. The use of the graphene electrode by placing in direct contact with the upper waveguide core combined with the air slots formed on both sides of the graphene electrode can improve the heating efficiency. The proposed delay line has an optimal dimension of 25 mm × 6.8 mm (length × width). The simulation results show crosstalks are less than -21.6 dB over the whole C-band and less than ~-45.2 dB (OFF state) at 1550 nm. The delay line is capable of operation for continuously tuning and the delay time is linearly dependent on K. And the continuously tuning delay range is from 0 to 200 ps over minimum 3 dB bandwidth of <10 GHz. The minimum 3 dB bandwidth of our proposed device is four times larger than that of a continuously tunable delay line constructed with a typical 2×2 asymmetric MZI.