1. Introduction . As one of the most important components in photonic integrated circuits (PICs), integrated optical true-time delay lines (OTTDLs) have broad applications in optical signal processing such as optical beamforming networks (OBFNs) [ 1 – 13 ], microwave photonic filters [ 14 – 16 ], and optical computing [ 17 , 18 ], due to their compact size, low cost, and wide bandwidth. Several structures, including Mach-Zehnder interferometers (MZIs) [ 19 – 23 ], micro-ring...