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Silicon-photonic matrix switches and control technologies to accelerate switching speed

Optical devices based on silicon photonic technology are very important as supporting current and future communication systems in terms of their scale of integration and productivity. Although various devices have been proposed and realized using silicon photonics technology, this paper describes an optical path switch combining a large number of 2×2-unit-switches loaded with electrical heaters on a Mach-Zehnder interferometer (MZI). We have been developed 8×8 and 32×32 optical matrix switches, which employ a path-independent-insertion-loss (PILOSS) configuration that has a feature of the same device count on the any path of the connection. The PILOSS structure has a feature of Strictly-nonblocking, which can connect any input port to any output port without changing existing connections when making a new connection. Each heater of the optical switch is driven by pulse-width-modulation (PWM) generated by a Field Programmable Gate Array (FPGA). The calibration of the pulse width according to the 2×2-unit switch state (Cross or Bar) is performed on all of the MZIs in advance, and the values are stored in a table of the FPGA. Separately, Cross / Bar state tables corresponding to the connection pattern of the optical input port and output port are prepared, and the pulse width switching according to the state table is simultaneously performed based on a switching command from the upper layer controller. It takes a certain amount of time to change the heater temperature of the MZI arm for switching. However, it is possible to shorten the switching time by applying a signal named “Turbo pulse” for a short term at the transition. When the temperature is raised by the heater, the switching can be speeded-up by applying a continuous high-level voltage to the heater temporarily. Even in the case of the temperature drops, the switching time can be accelerated by applying a continuous high-level voltage to the heater on the other arm of the MZI. The switching time, which was actually 30μs without Turbo pulse, could be reduced to 2.5 μs with Turbo pulse. The fabricated silicon photonics switch chip was mounted to the chip-carrier, assembled on a printed circuit board, and housed in a 19-inch wide 1-rack unit (RU) height blade. In addition to measuring the various characteristics of the device in our laboratory, it has also been installed and operated at the telecommunication carrier's collocation space to confirm long-term operation in the field. This shows that the technology related to the large-scale silicon photonics devices shown here can be adopted to practical use.