Adiabatic Coupler With Nonlinearly Tapered Mode-Evolution Region

In this work, we propose a time-efficient method to analyze and design the adiabatic couplers (ACs) on the silicon-on-insulator platform. By analyzing the mode-evolution region of ACs, we derive the boundary conditions and necessary constraints on the tapering functions to achieve optimized performance. Taking these conditions into consideration, we choose three common types of functions for the mode-evolution region. Based on the width and separation of the constituent waveguide pair, the performance of ACs with different tapering functions is compared. The compared ACs were fabricated and measured. The splitting ratios (SRs) of the fabricated devices are characterized using unbalanced Mach-Zehnder interferometers. We analytically and experimentally prove that, for a designed 3-dB AC, a quadratic separation and exponentially varying width provides the least SR imbalance with the smallest footprint among the compared tapering methods. The extracted SRs of the designed 3-dB AC using such tapering method are between 47%/53% from 1500 nm to 1600 nm with a mode evolution length of 110 μm. Using this tapering method, we also experimentally demonstrate imbalanced ACs with SRs of 8%/92%, 12%/88%, 15%/85%, 23%/77%, 30%/70%, and 42%/58% measured at 1550 nm.