The growing maturity of integrated photonic technology makes it possible to build increasingly large and complex photonic circuits on the surface of a chip. Today, most of these circuits are designed for a specific application. However, the increase in complexity creates an opportunity for a generation of photonic circuits that can be programmed in software for a wide variety of functions through a mesh of on-chip waveguides, tunable beam couplers and optical phase shifters. Here we discuss the state of this emerging technology, not just the recent developments in photonic building blocks and circuit architectures, but also the higher levels in the technology stack for the electronic control and programming strategies. We also cover the various possible applications in linear matrix operations, quantum information processing and microwave photonics and examine how these generic chips can accelerate the development of future photonic circuits by providing a higher-level platform for prototyping novel optical functionalities without the need for custom chip fabrication.

We demonstrate a time-multiplexed row-column addressing scheme to drive thermo-optic phase shifters in a silicon photonic circuit. By integrating a diode in series with the heater, we can connect N × M heaters in an matrix topology to N row and M column lines. The heaters are digitally driven with pulse-width modulation, and time-multiplexed over different channels. This makes it possible to drive the circuit without digital-to-analog converters, and using only M + N wires. We demonstrate this concept with a 1 × 16 power splitter tree with 15 thermo-optic phase shifters that are controlled in a 3 × 5 matrix, connected through 8 bond pads. This technique is especially useful in silicon photonic circuits with many tuners but limited space for electrical connections.Index Terms-Optical phase shifters, optical waveguide components, silicon on insulator technology digital control, time division multiplexing, thermooptic effects, integrated optoelectronics.

Low-power and compact phase shifters are crucial for large photonic circuits, both to cope with variability and to create programmable waveguide circuits scaling to thousands of tuning elements. This work demonstrates a liquid crystal phase shifter where there is a lateral silicon electrode "rail" on one side of the waveguide core. Using this architecture, a strong quasi-static electric field E actuation can be applied over the gap, which is filled with liquid crystal cladding material, with modest voltages. Because the mode is largely confined in the waveguide, optical losses are limited, compared to earlier experiments with slot waveguides. The liquid crystal is deposited locally on three different device variations using inkjet printing. The local deposition avoids unwanted interference of the liquid crystal with other optical components such as grating couplers. Measurements show similar trends as simulations of the liquid crystal orientations. For one device with a length of 50 µm, a phase shift of almost 0.9π is shown at 10 VRMS. We also discuss the challenges with this first demonstration of this phase shifter geometry using a silicon side-rail as an electrode.

Programmable photonics configure the flow of light on a photonic chip using software and electrical actuators. Such flexible chips are useful for prototyping new applications. We discuss the underlying technologies for hardware and software.