In the late nineteenth century, Heinrich Hertz demonstrated that the electromagnetic properties of materials are intimately related to their structure at the subwavelength scale by using wire grids with centimetre spacing to manipulate metre-long radio waves. More recently, the availability of nanometre-scale fabrication techniques has inspired scientists to investigate subwavelength-structured metamaterials with engineered optical properties at much shorter wavelengths, in the infrared and visible regions of the spectrum. Here we review how optical metamaterials are expected to enhance the performance of the next generation of integrated photonic devices, and explore some of the challenges encountered in the transition from concept demonstration to viable technology.

Subwavelength integrated photonics.

Optical modulators are at the heart of optical communication links. Ideally, they should feature low loss, low drive voltage, large bandwidth, high linearity, compact footprint and low manufacturing cost. Unfortunately, these criteria have been achieved only on separate occasions. Based on a silicon and lithium niobate hybrid integration platform, we demonstrate Mach–Zehnder modulators that simultaneously fulfil these criteria. The presented device exhibits an insertion loss of 2.5 dB, voltage–length product of 2.2 V cm in single-drive push–pull operation, high linearity, electro-optic bandwidth of at least 70 GHz and modulation rates up to 112 Gbit s−1. The high-performance modulator is realized by seamless integration of a high-contrast waveguide based on lithium niobate—a popular modulator material—with compact, low-loss silicon circuitry. The hybrid platform demonstrated here allows for the combination of ‘best-in-breed’ active and passive components, opening up new avenues for future high-speed, energy-efficient and cost-effective optical communication networks. Low-loss, high-speed and efficient optical modulators on a silicon platform are demonstrated.

High-performance hybrid silicon and lithium niobate Mach–Zehnder modulators for 100 Gbit s −1 and beyond

Research in photonic computing has flourished due to the proliferation of optoelectronic components on photonic integration platforms. Photonic integrated circuits have enabled ultrafast artificial neural networks, providing a framework for a new class of information processing machines. Algorithms running on such hardware have the potential to address the growing demand for machine learning and artificial intelligence in areas such as medical diagnosis, telecommunications, and high-performance and scientific computing. In parallel, the development of neuromorphic electronics has highlighted challenges in that domain, particularly related to processor latency. Neuromorphic photonics offers sub-nanosecond latencies, providing a complementary opportunity to extend the domain of artificial intelligence. Here, we review recent advances in integrated photonic neuromorphic systems, discuss current and future challenges, and outline the advances in science and technology needed to meet those challenges. Photonics offers an attractive platform for implementing neuromorphic computing due to its low latency, multiplexing capabilities and integrated on-chip technology.

Photonics for artificial intelligence and neuromorphic computing

Research in photonic computing has flourished due to the proliferation of optoelectronic components on photonic integration platforms. Photonic integrated circuits have enabled ultrafast artificial neural networks, providing a framework for a new class of information processing machines. Algorithms running on such hardware have the potential to address the growing demand for machine learning and artificial intelligence, in areas such as medical diagnosis, telecommunications, and high-performance and scientific computing. In parallel, the development of neuromorphic electronics has highlighted challenges in that domain, in particular, related to processor latency. Neuromorphic photonics offers sub-nanosecond latencies, providing a complementary opportunity to extend the domain of artificial intelligence. Here, we review recent advances in integrated photonic neuromorphic systems, discuss current and future challenges, and outline the advances in science and technology needed to meet those challenges.

Optical modulators are at the heart of optical communication links Ideally, they should feature low insertion loss, low drive voltage, large modulation bandwidth, high linearity, compact footprint and low manufacturing cost Unfortunately, these criteria have only been achieved on separate occasionsBased on a Silicon and Lithium Niobate hybrid integration platform, we demonstrate Mach-Zehnder modulators that simultaneously fulfill these criteria The presented device exhibits an insertion loss of 25 dB, voltage-length product of 22 Vcm, high linearity, electro-optic bandwidth of at least 70 GHz and modulation rates up to 112 Gbit/s The high-performance modulator is realized by seamless integration of high-contrast waveguide based on Lithium Niobate - the most mature modulator material - with compact, low-loss silicon circuits The hybrid platform demonstrated here allows for the combination of 'best-in-breed' active and passive components, opening up new avenues for enabling future high-speed, energy efficient and cost-effective optical communication networks

High-Performance Hybrid Silicon and Lithium Niobate Mach-Zehnder Modulators for 100 Gbit/s and Beyond

The design and characterization of a slow-wave series push-pull traveling wave silicon photonic modulator is presented. At 2 V and 4 V reverse bias, the measured −3 dB electro-optic bandwidth of the modulator with an active length of 4 mm are 38 GHz and 41 GHz, respectively. Open eye diagrams are observed up to bitrates of 60 Gbps without any form of signal processing, and up to 70 Gbps with passive signal processing to compensate for the test equipment. With the use of multi-level amplitude modulation formats and digital-signal-processing, the modulator is shown to operate below a hard-decision forward error-correction threshold of 3.8×10−3 at bitrates up to 112 Gbps over 2 km of single mode optical fiber using PAM-4, and over 5 km of optical fiber with PAM-8. Energy consumed solely by the modulator is also estimated for different modulation cases.

Design, analysis, and transmission system performance of a 41 GHz silicon photonic modulator.

We first optimize the design and compare the performance of thermo-optic phase-shifters based on TiN metal and N++ doped silicon, in the same SOI process. The designs don’t require special material processing, show negligible loss, and have very stable power consumption. The optimum TiN design has a switching powerPπ=21.4 mW and a time constantτ=5.6 µs, whereasPπ=22.8 mW andτ=2.2 µs for the best N++ Si design, enabling 2.5x faster switching compared to the metal heater. Doped-Si-based heaters are therefore the most practical and efficient on standard SOI. In addition, to optimize the layout density of highly integrated dies, we experimentally characterize internal and external thermal crosstalk for tunable Mach-Zehnder interferometers (MZIs) based on both heater designs for various power, distances, and etching patterns. Deep trenches are the best structures not involving special fabrication techniques to mitigate heat leakage affecting phase-sensitive devices close to heaters. Given the numerous applications of thermal tuners, this work is relevant to almost all silicon photonics designers.

Optimization of thermo-optic phase-shifter design and mitigation of thermal crosstalk on the SOI platform.

We present a silicon photonic traveling-wave Mach–Zehnder modulator operating near 1550 nm with a 3-dB bandwidth of 35 GHz. A detailed analysis of traveling-wave electrode impedance, microwave loss, and phase velocity is presented. Small- and large-signal characterization of the device validates the design methodology. We further investigate the performance of the device in a short-reach transmission system. We report a successful 112-Gb/s transmission of four-level pulse amplitude modulation over 5 km of SMF using 2.2 ${\rm V}_{{\rm p} - {\rm p}} $ drive voltage. Digital signal processing is applied at the transmitter and receiver. 56-GBaud PAM-4 and 64-Gb/s PAM-2 transmission is demonstrated below a pre-FEC hard decision threshold of $4.4 \times 10^{-3}$ .

A Low-Voltage 35-GHz Silicon Photonic Modulator-Enabled 112-Gb/s Transmission System

We report on the design and characterization of focusing-curved subwavelength grating couplers for ultra-broadband silicon photonics optical interfaces. With implementation of waveguide dispersion engineered subwavelength structures, an ultra-wide 1-dB bandwidth of over 100 nm (largest reported to date) near 1550 nm is experimentally achieved for transverse-electric polarized light. By tapering the subwavelength structures, back reflection is effectively suppressed and grating coupling efficiency is increased to −4.7 dB. A compact device footprint of 40 µm × 20 µm is realized by curving the gratings in a focusing scheme.

Focusing-curved subwavelength grating couplers for ultra-broadband silicon photonics optical interfaces

We report on the design and characterization of a silicon-on-insulator traveling-wave multi-electrode Mach–Zehnder modulator (MZM). The 2-bit electro-optic (EO) digital-to-analog converter is formed by dividing a series push–pull MZM into two segments, one for each bit. The EO bandwidth of the longer segment of the MZM is measured to be 48 GHz at 0 V reverse bias. We operate the device at speeds up to 50 GBd to create a four-level pulse amplitude modulation signal, and thus generating 100 Gb/s on a single wavelength without signal processing at the transmitter or the receiver. The pre-forward error correction (FEC) bit error rate is estimated to be lower than the hard-decision FEC threshold of $3.8 \,\, \times \,\, 10^{-3}$ over 1 km of standard single-mode fiber, and thus leading to error-free transmission at 100 Gb/s.

David Patel

Papers

Design, analysis, and transmission system performance of a 41 GHz silicon photonic modulator.

Optimization of thermo-optic phase-shifter design and mitigation of thermal crosstalk on the SOI platform.

A Low-Voltage 35-GHz Silicon Photonic Modulator-Enabled 112-Gb/s Transmission System

Focusing-curved subwavelength grating couplers for ultra-broadband silicon photonics optical interfaces

Silicon Photonic Segmented Modulator-Based Electro-Optic DAC for 100 Gb/s PAM-4 Generation