Electro-optic modulators translate high-speed electronic signals into the optical domain and are critical components in modern telecommunication networks1,2 and microwave-photonic systems3,4. They are also expected to be building blocks for emerging applications such as quantum photonics5,6 and non-reciprocal optics7,8. All of these applications require chip-scale electro-optic modulators that operate at voltages compatible with complementary metal–oxide–semiconductor (CMOS) technology, have ultra-high electro-optic bandwidths and feature very low optical losses. Integrated modulator platforms based on materials such as silicon, indium phosphide or polymers have not yet been able to meet these requirements simultaneously because of the intrinsic limitations of the materials used. On the other hand, lithium niobate electro-optic modulators, the workhorse of the optoelectronic industry for decades9, have been challenging to integrate on-chip because of difficulties in microstructuring lithium niobate. The current generation of lithium niobate modulators are bulky, expensive, limited in bandwidth and require high drive voltages, and thus are unable to reach the full potential of the material. Here we overcome these limitations and demonstrate monolithically integrated lithium niobate electro-optic modulators that feature a CMOS-compatible drive voltage, support data rates up to 210 gigabits per second and show an on-chip optical loss of less than 0.5 decibels. We achieve this by engineering the microwave and photonic circuits to achieve high electro-optical efficiencies, ultra-low optical losses and group-velocity matching simultaneously. Our scalable modulator devices could provide cost-effective, low-power and ultra-high-speed solutions for next-generation optical communication networks and microwave photonic systems. Furthermore, our approach could lead to large-scale ultra-low-loss photonic circuits that are reconfigurable on a picosecond timescale, enabling a wide range of quantum and classical applications5,10,11 including feed-forward photonic quantum computation. Chip-scale lithium niobate electro-optic modulators that rapidly convert electrical to optical signals and use CMOS-compatible voltages could prove useful in optical communication networks, microwave photonic systems and photonic computation.

Integrated lithium niobate electro-optic modulators operating at CMOS-compatible voltages

Recent advances in photonic integration have propelled microwave photonic technologies to new heights. The ability to interface hybrid material platforms to enhance light–matter interactions has led to the development of ultra-small and high-bandwidth electro-optic modulators, low-noise frequency synthesizers and chip signal processors with orders-of-magnitude enhanced spectral resolution. On the other hand, the maturity of high-volume semiconductor processing has finally enabled the complete integration of light sources, modulators and detectors in a single microwave photonic processor chip and has ushered the creation of a complex signal processor with multifunctionality and reconfigurability similar to electronic devices. Here, we review these recent advances and discuss the impact of these new frontiers for short- and long-term applications in communications and information processing. We also take a look at the future perspectives at the intersection of integrated microwave photonics and other fields including quantum and neuromorphic photonics. This Review discusses recent advances of microwave photonic technologies and their applications in communications and information processing, as well as their potential implementations in quantum and neuromorphic photonics.

Integrated microwave photonics

Optics offers unique opportunities for reducing energy in information processing and communications while simultaneously resolving the problem of interconnect bandwidth density inside machines. Such energy dissipation overall is now at environmentally significant levels; the source of that dissipation is progressively shifting from logic operations to interconnect energies. Without the prospect of substantial reduction in energy per bit communicated, we cannot continue the exponential growth of our use of information. The physics of optics and optoelectronics fundamentally addresses both interconnect energy and bandwidth density, and optics may be the only scalable solution to such problems. Here we summarize the corresponding background, status, opportunities, and research directions for optoelectronic technology and novel optics, including subfemtojoule devices in waveguide and novel two-dimensional (2-D) array optical systems. We compare different approaches to low-energy optoelectronic output devices and their scaling, including lasers, modulators and LEDs, optical confinement approaches (such as resonators) to enhance effects, and the benefits of different material choices, including 2-D materials and other quantum-confined structures. With such optoelectronic energy reductions, and the elimination of line charging dissipation by the use optical connections, the next major interconnect dissipations are in the electronic circuits for receiver amplifiers, timing recovery, and multiplexing. We show we can address these through the integration of photodetectors to reduce or eliminate receiver circuit energies, free-space optics to eliminate the need for timing and multiplexing circuits (while also solving bandwidth density problems), and using optics generally to save power by running large synchronous systems. One target concept is interconnects from ∼1 cm to ∼10 m that have the same energy (∼10 fJ/bit) and simplicity as local electrical wires on chip.

Attojoule Optoelectronics for Low-Energy Information Processing and Communications

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.

We report membrane lasers on high-thermal-conductivity SiC exhibiting flat small-signal responses with 3-dB bandwidths of 98 and 84 GHz at 25 and $55^{\circ}\mathrm{C}$, respectively, due to the high relaxation oscillation frequency and photon-photon resonance frequency. The laser is capable of semi-cooled 128-Gbit/s NRZ transmission.

Semi-cooled 128-Gbit/s NRZ operation of directly modulated membrane lasers on SiC substrate

We designed and fabricated lateral-current-injection (LCI) Fabry-Perot (FP) lasers for the 1.3-µm wavelength region. The devices were fabricated using selective doping by means of thermal diffusion and ion implantation to make pn junctions. To increase the optical confinement factor in the active region, we used an InGaAlAs-based 20-quantum-well structure. The device exhibited a threshold current of 16 mA and differential efficiency of 47% for a 400-µm long and 0.5-µm wide cavity. We also measured the relative intensity noise (RIN) spectrum to obtain parameters for the direct modulation. The LCI FP laser provided both high output power and high relaxation oscillation frequency.

Design and fabrication of directly-modulated 1.3-µm lateral-current-injection lasers

Directly modulated lasers are key components for constructing low-power-consumption optical links at low cost. By employing photonic crystal (PhC) cavities, cavity length can be reduced to wavelength scale; therefore, PhC lasers can be used as transmitters for optical links inside boards and chips.

Directly modulated photonic crystal lasers for extremely short optical links

Heterogeneous integration of a DFB laser on SiO 2 /Si substrate is demonstrated by combining direct bonding and epitaxial regrowth techniques. Energy-efficient direct modulation is achieved by reducing cavity length and increasing optical confinement.

On-silicon integration of compact and energy-efficient DFB laser with 40-Gbit/s direct modulation

We report a coherent driver modulator sub-assembly for 100-Gbaud class transmitter. The sub-assembly employs a codesigned high-speed driver IC and optical modulator, where the designs for the characteristic impedance between the driver IC and IQ modulator and for the RF pads are optimized. The EO bandwidth is over 67 GHz. This response is sufficient for 100-Gbaud-class operation even if we consider RF losses caused by a package, PCB, and DAC. We also demonstrated QPSK and 16-QAM at the speed of 128 and 112 Gbaud, respectively.

Takuro Fujii

Papers

Semi-cooled 128-Gbit/s NRZ operation of directly modulated membrane lasers on SiC substrate

Design and fabrication of directly-modulated 1.3-µm lateral-current-injection lasers

Directly modulated photonic crystal lasers for extremely short optical links

On-silicon integration of compact and energy-efficient DFB laser with 40-Gbit/s direct modulation

Recent advances in ultra-high-bandwidth coherent driver modulator