Low-Loss Calibration-Free 2 × 2 Mach-Zehnder Switches With Varied-Width Multimode-Interference Couplers

Large-scale photonic switches are essential components for telecommunications and interconnects, incorporating many cascaded 2 × 2 optical switches. To keep the excess loss of the entire switch low, it is crucial to develop ultralow-loss 2 × 2 optical switches by introducing new coupler designs. In this paper, a low-loss calibration-free 2 × 2 Mach-Zehnder switch (MZS) designed with varied-width multimode-interference (MMI) couplers is proposed and fabricated. In this design, the MMI coupler has a varied-width MMI region and two multimode output waveguides. The MMI coupler optimized by the particle swarm optimization (PSO) algorithm has an ultralow excess loss < 0.053 dB and a suppressed higher-order mode excitation < −26 dB across the C-band (1530–1565 nm). Such a varied-width MMI coupler with two multimode output waveguides can be used compatibly for the MZS with widened phase shifters, so that the random phase imbalance between the two MZS arms can be suppressed effectively, which thus is promising for calibration-free operation. For the present MZS fabricated with standard 180-nm silicon photonic foundry processes, the excess loss is ∼0.5 dB across the C-band and the extinction ratio is ∼25 dB without calibration.