# Towards Large-Scale Fast Reprogramable SOA-Based Photonic Integrated Switch Circuits
## 2 Monolithically Integrated Fast Optical Switching  
Despite the considerable number of concepts and devices proposed and demonstrated over the years, only a relatively small number of integrated switches have scaled to tens of ports. Three-dimensional (3D) microelectromechanical systems (MEMS) have scaled up to hundreds of ports with low (\(\sim\)2 dB) insertion loss and very low power consumption [10]. However, commercially-deployed optical switching engines using MEMS technologies are unwieldy and too slow for future packet-based networking. It is therefore increasingly important to identify scalable optical switching engines which are capable of fast nanosecond-timescale reconfiguration. Photonic integrated circuits can offer an exciting and viable solution for creating a single-chip switching solution for high-capacity data routing. As a notable example, planar integrated MEMS have been deployed in combination with silicon photonics planar circuits to create up to 64 \(\times\) 64 photonic switch matrices [11]: An electrostatically actuated MEMS waveguide is moved to enable a directional coupling between the input and output buses. These waveguides are low-loss and broadband, with a compact footprint, although the actuation voltages of tens of volts are still relatively challenging and a sub-microsecond switching time is set by the intrinsically limiting mechanical movement. InP integrated photonics offers high performance amplifiers, switches, modulators, detectors, and de/multiplexers in the same wafer scale process. High speed switch elements have primarily used interferometric and gated switch elements that exploit the InP integrated photonics platform. Interferometers offer the potential for low-current and therefore low-power operation. Considerable work focusing on the use of directional couplers, Y-branch couplers, and Mach-Zehnder interferometers (MZI) has been done. The latter have been configured as 2 \(\times\) 2 elements in the larger monolithic, multistage 16 \(\times\) 16 and 32 \(\times\) 32 switch matrices on LiNbO\({}_{3}\)[12; 13; 14]. More recently, 8 \(\times\) 8 switch matrices have been fabricated on Silicon-on-Insulator [15]: Circuit level challenges have focused on improving the modest switch extinction ratios and relaxing the precision requirements for the voltage control. Improved crosstalk and power consumption performance have been achieved using either cascaded Mach-Zehnder interferometers [16] or by including a short semiconductor optical amplifier gate stage [17], to the expenses of a higher number of control electrodes. A different approach is provided by the notable example of the 8 \(\times\) 8 MOTOR circuit, that integrates eight wavelength converters and one cyclic router, offering the prospect of bandwidth reallocations at the optical layer [18]. However, this class of switch has proved more challenging to integrate. Semiconductor optical amplifier (SOA) gate switches have been more extensively deployed in system-level studies: Electrical control results are relatively simple and with low voltages; also, tolerance to current variations and excellent crosstalk suppression are typical characteristics [19]. Furthermore, SOA-based gate switches can enable passbands of several Terahertz, colorless and wavelength multiplexed routing [20]. System integrators have demonstrated large optical fabrics using multiple stages and planes of discrete components [21; 22]. These successes have led to a renewed interest in large-scale photonic integration as a route to more flexible and lossless routing systems to reduce system level complexity. As a consequence, in the last ten years, photonic integration technology has evolved rapidly to the point that a number of academic and industry research groups are making more and more complex integrated switching circuits. The first lossless 16 \(\times\) 16 optical switch matrices were demonstrated by [PERSON] et al. [23].