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.

A high-efficiency InGaAsP Mach-Zehnder modulator is integrated with hydrogen-free deuterated silicon nitride (SiN:D) waveguide circuits on a Si substrate. A thin InP-based layer provides a high optical confinement factor of around 50% and enables easy optical coupling to the SiN:D waveguides, which are fabricated by a low-temperature backend process. The fabricated Mach-Zehnder modulator with a 300-μm-long phase shifter shows a VπL of 0.4 Vcm, insertion loss of ~4.5 dB, and an error-free operation for 40-Gbit/s non-return-to-zero signal.

Membrane InGaAsP Mach-Zehnder modulator with SiN:D waveguides on Si platform

Controlling gain and loss of coupled optical cavities can induce non-Hermitian degeneracies of eigenstates, called exceptional points (EPs). Various unconventional phenomena around EPs have been reported, and are expected to incorporate extra functionalities into photonic devices. The eigenmode exactly under EP degeneracy is also predicted to exhibit enhanced radiation. However, such responses have yet to be observed in on-chip lasers because of both the limited controllability of their gain and loss and the lifting of degeneracy by pump-induced cavity detuning. Here, we report, to the best of our knowledge, the first non-Hermitian nanophotonic platform based on two electrically pumped photonic crystal lasers and its spontaneous emission at EP degeneracy. Systematically tuned and independent current injection to our wavelength-scale active heterostructure cavities enables us to demonstrate the clear EP phase transition of their spontaneous emission, accompanied with the spectral coalescence of coupled modes and reversed pump dependence of the intensity. Furthermore, we find experimentally and confirm theoretically the peculiar squared Lorentzian emission spectrum very near the exact EP, which indicates a four-fold enhancement of the photonic local density of states induced purely by the degeneracy. Our results open a new pathway to engineer the light–matter interaction by non-Hermiticity and explore larger reconfigurable laser arrays for further non-Hermitian features and physics.

Observing exceptional point degeneracy of radiation with electrically pumped photonic crystal coupled-nanocavity lasers

We report on space-division multiplexing (SDM) transmissions of up to 400 Gb/s over a homogeneous four-core fiber using discrete multitone (DMT) modulation for intra-datacenter applications and 200/400 GbE links. The transmission system is enabled by a compact, SDM-channel scalable, multi-core fiber (MCF) pluggable, and energy-efficient SDM transmitter composed of a 4-channel 1.3-μm membrane directly modulated laser (DML) array-on-silicon integrated with a fiber-bundle type fan-in with loss of less than 1 dB and negligible optical and electrical crosstalk (XT). By simultaneously modulating all four lasers, 200-Gb/s (50-Gb/s/channel) and 400-Gb/s (100-Gb/s/channel) are achieved over a 425-m MCF link for KP4 and soft-decision forward error correction, respectively, with a power reduction of ~6× compared with conventional DML transmitters. Additionally, numerical simulations based on a detailed rate-equations model predict the feasibility of the system for MCF transmissions up to 10 km even with moderate levels of XT.

400-Gb/s DMT-SDM Transmission Based on Membrane DML-Array-on-Silicon

Large-scale photonic integrated circuits (PICs) are essential for reducing the cost and power consumption of optical networks. Selective doping by means of thermal diffusion and ion implantation is suitable for fabricating PICs because it enables doping into desired regions without crystal growth. In this paper, we report the fabrication of a lateral-p-i-n-diode-structure electro-absorption modulator (EAM) integrated with a DFB (EADFB) laser. Owing to the lateral p-i-n structure, we are able to use the broadening of the exciton absorption peak that occurs when the electric field is applied parallel to the quantum well. By comparing the calculated absorption spectrum change for parallel and vertical electric fields, we found that it occurs with a low electrical field, although the amount of absorption coefficient change is small for the parallel one. Thus, in our design, we make the EAM length larger than that of the conventional vertical electric field EAM. However, because of the low parasitic capacitance of the lateral EAM, we can expect to achieve high-speed operation. With a fabricated device with a 200-μm-long EAM, we have achieved 50-Gb/s operation with clear eye opening. We have also achieved a side-mode suppression ratio of more than 50 dB by using a surface grating. These results indicate that our lateral EADFB laser is very promising as an integrated light source for large-scale PICs and that our fabrication process based on selective doping is suitable for them.

High-Speed Modulation of Lateral p-i-n Diode Structure Electro-Absorption Modulator Integrated With DFB Laser

Increasing demand for higher data rates in data centers and high-performance computing systems require optical interconnects that support more than 100 Gbps-per-lane. Meanwhile, as optics are packed ever closer to Ethernet switches and electronic processors, both operating temperatures and power consumptions increase, resulting in increasing operational and environmental costs. In this work we present our recent results on a two-channel energy-efficient directly-modulated membrane laser array on SiO2/Si with ∼60-GHz 3-dB bandwidth, that can support both 100 Gbps-per-lane modulations as well as very small form-factors and power consumptions. The extension to 60 GHz bandwidths denotes a ∼26.3% increase compared to previous works, and it was achieved based on an optimized distributed-reflector laser design for maximizing the photon-photon resonance effect. Based on the fabricated two-channel DML array, 200 Gbps (2×112-Gbps NRZ) with laser operating energy-per-bit cost of less than 0.3 pJ/bit over 2-km transmissions, and the feasibility of 400 Gbps (2×200-Gbps PAM-4) transmissions are demonstrated. Finally, the temperature dependence of the PPR effect and its impact on the E-O response have been studied both experimentally and with numerical simulations for temperatures up to 75 °C for the first time.

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Takuro Fujii

Papers

Membrane InGaAsP Mach-Zehnder modulator with SiN:D waveguides on Si platform

Observing exceptional point degeneracy of radiation with electrically pumped photonic crystal coupled-nanocavity lasers

400-Gb/s DMT-SDM Transmission Based on Membrane DML-Array-on-Silicon

High-Speed Modulation of Lateral p-i-n Diode Structure Electro-Absorption Modulator Integrated With DFB Laser

60 GHz Bandwidth Directly Modulated Membrane III-V Lasers on SiO2/Si