Digitally Tunable Dispersion Controller Using Low-Loss Silicon Nitride Multimode Waveguide Gratings

A digitally tunable dispersion controller (DTDC) using low-loss silicon nitride multimode waveguide gratings is developed. The architecture of the DTDC is based on N stages of cascaded chirped multimode waveguide gratings (CMWGs) and (N + 1) 2 × 2 Mach-Zehnder switches (MZSs). The nth stage incorporates m = 2 ^n-1 CMWGs, resulting in a total of M = 2 ^N - 1 CMWGs across the entire structure. The overall dispersion is determined by the collective states of all the MZSs. The DTDC enables a digital tuning range of dispersion from 0 to (2 ^N - 1) D ₀ with a minimum step of D ₀, where D ₀ is defined as the dispersion provided by a single CMWG. A DTDC designed with N = 5 and M = 31 is fabricated with 400-nm-thick silicon nitride via a standardized 8-inch foundry process, providing an average dispersion D ₀ of approximately 1.60 ps/nm per single 4-mm-long CMWG over a 26-nm-wide bandwidth. The total dispersion can be digitally tuned from 0 to 49.72 ps/nm by controlling the MZSs appropriately, and the corresponding group delay can be tuned from 0 to 1304 ps. The group delay ripple (GDR) accounts for as low as 1.14% of the total delay, with the propagation loss estimated to be 0.22 dB/cm.