Photonic integrated circuit (PIC)

A photonic integrated circuit (PIC) is a chip on which light travels in waveguides and is processed by components made in the same fabrication sequence: splitters and couplers, filters, phase shifters, modulators and photodetectors, and on some platforms lasers and amplifiers. It replaces an assembly of separate optics, each aligned by hand, with one part defined by lithography. The idea goes back to Miller's proposal of integrated optics in 1969; silicon became a photonic platform after Soref and Bennett worked out how free carriers change its refractive index (1987), which gave it a way to modulate light. The components are set in size by the wavelength, micrometers for a bend and up to millimeters for a modulator, so a PIC holds hundreds or thousands of components where an electronic chip holds billions of transistors, and most PICs work beside electronic chips that drive their modulators and amplify their detector signals.

Building blocks

• Waveguides and bends: strip and rib waveguides (strip vs rib), whose propagation loss and bend loss set how long and how tightly folded a circuit can be.
• Splitters and couplers: Y-branches, multimode interference couplers and directional couplers.
• Filters: ring resonators, Mach-Zehnder interferometers, Bragg gratings and arrayed waveguide gratings, which multiplex and demultiplex WDM channels.
• Phase shifters and modulators: thermo-optic phase shifters for slow tuning; Mach-Zehnder, ring and electro-absorption modulators for data.
• Detectors: germanium photodiodes on silicon, InGaAs photodiodes on indium phosphide.
• Light sources and gain: lasers and semiconductor optical amplifiers, native on indium phosphide and added to silicon by bonding or attaching III-V material.
• Interfaces: grating and edge couplers to fiber, and polarization splitter-rotators where the fiber's polarization is not controlled.

Platforms

Silicon on insulator confines light most tightly. A 500 × 220 nm strip waveguide carries a mode 0.46 µm across (mode field diameter) and bends with negligible radiation at a 2 µm radius, so circuits are dense. The platform is processed in CMOS fabs, modulates light through the carrier-induced index change (Plasma Dispersion Calculator), detects it with germanium grown on the silicon, and is transparent only beyond 1.1 µm, the band edge of silicon. It has no laser of its own, and its strip waveguides lose more than those of the other passive platforms, because the strong index contrast makes sidewall roughness scatter strongly.

Silicon nitride confines light less, needs larger bends, from tens of micrometers for thick cores to millimeters for the thinnest, and loses much less: thin-core waveguides have reached less than 0.1 dB/m (Bauters and colleagues), and nitride rings an intrinsic Q above ${10}^8$. It is transparent from the visible into the mid-infrared and serves delay lines, filters, microcombs, biosensors and circuits for visible light.

Indium phosphide is the platform on which lasers, amplifiers, electro-absorption and Mach-Zehnder modulators and photodetectors can all be made in one epitaxial stack, for wavelengths of 1.2 to 1.7 µm. A single InP transmitter chip carried 40 channels, 1.6 Tbit/s in total, in 2006 (Nagarajan and colleagues), and generic foundry processes now offer standard InP building blocks (Smit and colleagues). Its wafers are smaller and more expensive than silicon's, and it needs dedicated III-V fabs.

Thin-film lithium niobate, a crystal film a few hundred nanometers thick bonded onto oxide, keeps the Pockels effect that bulk lithium niobate modulators use while confining the light tightly enough to bring the electrodes close to it, and its modulators have been driven at CMOS-compatible voltages (Wang and colleagues). Heterogeneous integration bonds III-V material onto silicon or silicon nitride wafers and makes lasers, amplifiers or detectors in it, and hybrid integration attaches finished III-V chips, for example by flip-chip bonding, so that circuits in the passive platforms get their light sources.

Getting light on and off the chip

A silicon strip mode 0.46 µm across meets the 10.4 µm mode of standard fiber with a mismatch loss of about 20 dB (mode mismatch loss), so PICs couple to fiber through grating couplers, which in their standard uniform form lose 3 to 6 dB each, or through edge couplers whose inverse tapers expand the mode to a few micrometers (spot-size converter). Grating couplers can sit anywhere on the surface, which lets every die be measured before the wafer is diced (wafer-level test); Fiber-to-Chip Coupling for Silicon Photonics compares the methods. Fiber attachment, temperature control and the electrical connections are often a large part of the cost of turning a PIC into a product, and co-packaged optics places PICs beside switch chips to shorten the electrical path.

Design and fabrication

PICs are laid out against a foundry's process design kit, which carries the layer stack, the design rules and a library of characterized components, and multi-project wafer runs share the cost of a mask set among many designs. Because the effective index of a high-contrast waveguide depends on its width and thickness at the nanometer level, fabrication variation shifts the wavelengths of resonant and interferometric devices, and silicon circuits are usually trimmed with heaters in operation. Standard measurements of the finished circuit are described in Active Fiber Alignment to Surface Grating Couplers, Waveguide Propagation Loss by the Cutback Method and Loaded and Intrinsic Q Factor Extraction from Ring Resonator Transmission Spectra.

Uses

• Data-center transceivers: silicon PICs with modulators and germanium detectors in pluggable modules and in co-packaged optics.
• Coherent transceivers: silicon or InP PICs that combine I/Q modulators with the 90° optical hybrid and balanced detectors of a coherent receiver.
• Lidar and beam steering: optical phased arrays steer a beam without moving parts; a silicon array of 4,096 antennas was shown in 2013 (Sun and colleagues).
• Sensing, quantum and computing: nitride and silicon biosensors, quantum photonic circuits, and photonic processors for neuromorphic and linear-algebra computing.

Common questions

What is a photonic integrated circuit?

A chip on which light is guided in waveguides and processed by components made in the same fabrication process: splitters, filters, modulators and detectors, and on some platforms lasers and amplifiers. It replaces an assembly of separate optical parts with one part made by lithography.

How is a PIC different from an electronic integrated circuit?

It routes and processes light rather than electrical current, and its components are sized by the wavelength, micrometers to millimeters, so it holds hundreds or thousands of components rather than billions of transistors. Most PICs work beside electronic chips that drive their modulators and amplify their detector signals.

What materials are photonic integrated circuits made of?

Mostly silicon on insulator, silicon nitride, indium phosphide and thin-film lithium niobate. Silicon offers dense circuits made in CMOS fabs, silicon nitride the lowest loss, indium phosphide lasers and amplifiers on the same chip, and lithium niobate fast Pockels-effect modulators; heterogeneous and hybrid integration combine them.

What is silicon photonics?

The making of PICs in silicon on insulator with the tools and fabs of CMOS electronics. Silicon has no laser of its own, so silicon photonic chips take their light from bonded or attached III-V lasers, or from a laser outside the chip.

References: S. E. Miller, "Integrated optics: an introduction," Bell System Technical Journal 48, 2059 (1969). R. A. Soref and B. R. Bennett, "Electrooptical effects in silicon," IEEE Journal of Quantum Electronics 23, 123 (1987). L. Chrostowski and M. Hochberg, Silicon Photonics Design (Cambridge University Press, 2015). M. Smit et al., "An introduction to InP-based generic integration technology," Semiconductor Science and Technology 29, 083001 (2014). J. F. Bauters et al., "Planar waveguides with less than 0.1 dB/m propagation loss fabricated with wafer bonding," Optics Express 19, 24090 (2011). R. Nagarajan et al., "Single-chip 40-channel InP transmitter photonic integrated circuit capable of aggregate data rate of 1.6 Tbit/s," Electronics Letters 42, 771 (2006). C. Wang et al., "Integrated lithium niobate electro-optic modulators operating at CMOS-compatible voltages," Nature 562, 101 (2018). J. Sun et al., "Large-scale nanophotonic phased array," Nature 493, 195 (2013).