Contents I. Introduction
Silicon photonics is expected as a basic technology for next-generation optical communication networks. The development of advanced optical integrated circuits is progressing through the realization of passive devices such as various wavelength filters, resonators, and diffraction gratings, high-speed optical modulators, germanium photodiodes and hybrid lasers [1]–[8]. Among various silicon photonics optical integrated circuits, large-scale integrated optical switches are used in the fields of short-range optical communication such as data centers [9]–[11], quantum computing [12], [13], and sensing [14], [15]. There is a demand for compact, low power consumption, and high-speed optical switches. An optical switch using the carrier plasma effect of silicon (Si) is advantageous in terms of high speed, but because the amount of change in the refractive index due to the carrier plasma effect is small, a long phase shifter of several hundred μm or more is necessary [16], [17]. Furthermore, optical loss due to carrier plasma absorption is also large. On the other hand, a thermo-optical switch that utilizes the large thermo-optic coefficient of Si is very small because it can be configured with a very short phase shifter of about several tens of μm [18]–[20]. It has been reported that a thermo-optical switch utilizing the resonance phenomenon in a ring resonator can operate at high speed and low power consumption [21]. However, because the resonance phenomenon has a strong wavelength dependence, the resonance type optical switch cannot be used in a broadband system. Mach–Zehnder interferometer (MZI)-type optical switches with low loss and wide wavelength band are key devices in large-scale integrated optical switches system. We have previously developed a high-speed phase shifter by directly injecting current through a Si optical waveguide using multimode interference (MMI) and achieved high-speed switching operation of several microseconds [22], [23]. In this research, the previous MMI phase shifter is improved, and a new MZI optical switch using asymmetrical doping and integrated heat sink is proposed. The fabricated optical switch has characteristics that exceed those of previous optical switches in terms of operating speed, compactness, and power consumption.
