Low-Loss C+L+S-Band 2 × 2 Thermo-Optic Mach-Zehnder Switches with Compact Fast Quasi-Adiabatic Couplers

Large-scale photonic switches are essential components for photonic circuits for classical and quantum applications. To develop high-performance large-scale photonic switches, we propose and demonstrate a low-loss, C+L+S-band (1460–1625 nm), and low-random-phase-imbalance 2 × 2 Mach-Zehnder switch (MZS) designed with fast quasi-adiabatic couplers (FACs). The FAC is designed to accelerate slow adiabatic evolutions by a tailored geometry tilt, and further optimized by the particle swarm optimization (PSO) algorithm. It features a simulated uniform splitting ratio that varies between 52.5%:47.5% and 47.9%:52.1% with a low excess loss of 0.020–0.054 dB across the C+L+S band. To achieve a large operation bandwidth of the MZS, 1-μm width phase shifters are implemented to reduce the wavelength dependence of the half-wave heating power
Q
_π
of the thermo-optic (TO) phase shifter. They also suppress random phase imbalance caused by the size variations in fabrication between the MZS arms. The simulated excess loss and extinction ratio of the present MZS are <0.13 dB and >22.6 dB across C+L+S band. Such an MZS has been fabricated with standard 180-nm silicon photonic foundry processes, featuring a low excess loss of ∼0.25 dB and an extinction ratio of >17 dB across the C+L+S band. Meanwhile, an extinction ratio of >20 dB is achieved across a slightly narrower bandwidth, 1470–1617 nm, with an excess loss of ∼0.2 dB. The MZS is an attractive option for scaling up N × N MZSs, which would be beneficial for large-scale and ultrahigh-capacity WDM and BDM telecom and interconnect applications, as well as on-chip near-infrared spectrometers.