Demonstration of a 4 × 4-port universal linear circuit

We present a silicon implementation of a 4 × 4-port universal linear optical circuit.Instead of predefining the exact functionality of a photonic circuit at design time, we demonstrate a simple generic silicon photonic circuit, combined with electronic control and software feedback, that can perform any linear operation between its four input ports and output ports.The circuit consists of a network of thermally tunable symmetric Mach-Zehnder interferometers with phase and amplitude control, in-circuit optical power monitors, and local software controlled feedback loops.The circuit can be configured using a training mechanism, which makes it self-adapt to implement the desired function.We use the circuit to demonstrate an adaptive, universal beam coupler, as well as a switch matrix.