Photonic integrated circuits

The first five articles of this series described light as a wave and showed that two waves from one laser can add to more light or to less, depending on the difference of their phases. A photonic integrated circuit (PIC) puts this to work on a chip. It guides light along paths of its own and acts on it: it splits the light, delays it, recombines it, switches it between paths and measures it. Where an electronic circuit carries currents in metal wires, a photonic circuit carries light in waveguides, narrow strips of transparent material. This article gives an overview of these circuits, what they are used for and where their limits lie, and of the rest of the series, which builds one of them from its parts.

Circuits for light

Most photonic circuits are built from a small set of parts. Waveguides carry the light, couplers divide it between waveguides or join it, and phase shifters delay the light in one waveguide relative to another [1]Bogaerts et al. 2020, p. 208. Detectors at the end of a circuit convert light into an electrical current, and modulators convert an electrical signal into changes of the light. Together these parts form circuits with hundreds or thousands of components on one chip [2]Chrostowski & Hochberg 2015, Fig. 1.4, p. 8.

This series is about silicon photonics: photonic circuits made of silicon on silicon wafers, in the factories of the electronics industry. The processes cannot be used unchanged, since they were developed for transistors, but they have been adapted to make photonic components [2]Chrostowski & Hochberg 2015, §1.2, p. 5. Photonic circuits need much coarser structures than processors: processes of the 90 nm generation suffice [2]Chrostowski & Hochberg 2015, §1.2, p. 5. Even the most advanced process that makes photonic components and transistors on the same wafer uses the 45 nm generation, several generations behind the FinFET processes of today’s processors [3]Shekhar et al. 2024, p. 8.

Where photonic circuits are used

The largest application is data communication [2]Chrostowski & Hochberg 2015, §1.3, p. 7. A transceiver is a module that converts electrical data into light for an optical fibre and converts the light arriving from another fibre back into electrical data. Photonic circuits form the core of many transceivers. In data centres, links between racks and between buildings are optical, while copper cables still connect the components within a rack over distances below about 10 m [4]Stone et al. 2023, §1.5.3, p. 28. Links built with silicon photonics reach 0.5–2 km [4]Stone et al. 2023, p. 11, and the traffic these networks carry has doubled about every two years [4]Stone et al. 2023, §1.1, p. 1.

Light suits communication because many independent signals can share one fibre. Each signal is carried by light of a different wavelength, and the wavelengths are combined in one fibre and separated again at its end, so that a single fibre carries terabits per second while no electronic circuit has to run at that rate [2]Chrostowski & Hochberg 2015, §1.3.1, p. 9.

Beyond communication, photonic circuits are studied and built for switching light between fibres [5]Cheng et al. 2023, §8.5, for sensors of chemical and biological substances, for LIDAR and for optical gyroscopes [2]Chrostowski & Hochberg 2015, §1.3, pp. 7–8, and for computing with light [6]Shen et al. 2017 [1]Bogaerts et al. 2020.

What photonic circuits do not do

Photonic circuits do not replace electronic ones. Electrons interact strongly with each other, which is what makes a transistor, a switch controlled by another electrical signal, possible. Photons, except under special conditions, do not interact, which is why many signals can travel through one fibre without disturbing each other [2]Chrostowski & Hochberg 2015, §1.3.1, pp. 8–9. The same property makes it hard to build a switch for light that is controlled by light. Current systems are therefore hybrid: electronics computes and controls, photonics carries data and performs linear operations on light.

Photonic links are also not free. In 2023 an optical link in a data centre cost about eleven times as much as a copper cable per unit of bandwidth [4]Stone et al. 2023, §1.5.3, pp. 28–29. In the data centres built for AI, the processors inside a rack are still connected by copper over a few metres, and cost remains the main barrier to using optics there [7]Torrijos-Morán & Pérez-López 2026, pp. 2–3.

A sense of scale

A silicon waveguide is 500 nm wide and 220 nm high [2]Chrostowski & Hochberg 2015, Fig. 3.14, p. 61. The light used on such chips has a wavelength of about 1550 nm in vacuum; inside the waveguide its field repeats every 0.63 µm, a little more than the waveguide’s width. A switch built from two couplers and a heater, the circuit this series ends with, is a few hundred micrometres long. Figure 1 shows the three scales.

Large circuits contain many such elements. A switch that connects any of 64 inputs to any of 64 outputs has been built from 352 of these switches, 1328 waveguide crossings and 726 electrical contacts on a chip of 21.7 mm × 9.6 mm [5]Cheng et al. 2023, §8.5.1.2, p. 279.

The rest of this series

The remaining articles build one circuit, the Mach-Zehnder interferometer (MZI), a switch made of two splitters and a phase shifter. It is one of the basic elements of silicon photonics, and meshes of MZIs are the basis of programmable photonic circuits [1]Bogaerts et al. 2020, Fig. 1, p. 208. The MZI is interference, as described in the article §Interference, made controllable on a chip. The articles introduce its parts one at a time:

1. §Waveguides: how light stays on a chip.
2. §Phase shifters: how a circuit sets the phase of light.
3. §Splitters and combiners: how light is divided and joined.
4. §The Mach-Zehnder interferometer: the switch built from these parts.
5. §Circuits as matrices: the formalism for any such circuit.
6. §The MZI in practice: applications, and what limits real devices.

Each article can be read on its own and links to the articles it builds on. The main text uses the mathematics of a first university year; derivations are collected at the end of each article under How do we model it.