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

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

Femtosecond-laser micromachining (also known as inscription or writing) has been developed as one of the most efficient techniques for direct three-dimensional microfabrication of transparent optical materials. In integrated photonics, by using direct writing of femtosecond/ultrafast laser pulses, optical waveguides can be produced in a wide variety of optical materials. With diverse parameters, the formed waveguides may possess different configurations. This paper focuses on crystalline dielectric materials, and is a review of the state-of-the-art in the fabrication, characterization and applications of femtosecond-laser micromachined waveguiding structures in optical crystals and ceramics. A brief outlook is presented by focusing on a few potential spotlights.

Optical waveguides in crystalline dielectric materials produced by femtosecond-laser micromachining

We report the reversible transfer of photon-photon entanglement, generated by means of spontaneous parametric down-conversion, into entanglement between a photon and a collective atomic excitation in a thulium-doped lithium niobate waveguide cooled to 3 K.

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Broadband waveguide quantum memory for entangled photons

Lithium niobate on insulator (LNOI) technology is revolutionizing the lithium niobate industry, enabling higher performance, lower cost and entirely new devices and applications. The availability of LNOI wafers has sparked significant interest in the platform for integrated optical applications, as LNOI offers the attractive material properties of lithium niobate, while also offering the stronger optical confinement and a high optical element integration density that has driven the success of more mature silicon and silicon nitride (SiN) photonics platforms. Due to some similarities between LNOI and SiN, established techniques and standards can readily be adapted to the LNOI platform including a significant array of interface approaches, device designs and also heterogeneous integration techniques for laser sources and photodetectors. In this contribution, we review the latest developments in this platform, examine where further development is necessary to achieve more functionalities in LNOI integrated optical circuits and make a few suggestions of interesting applications that could be realized in this platform. © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Status and Potential of Lithium Niobate on Insulator (LNOI) for Photonic Integrated Circuits

The state-of-the-art of high-refractive-index-contrast single-crystalline thin lithium niobate (LiNbO3) films as a new platform for high-density integrated optics is reviewed. Sub-micrometer thin LiNbO3 films are obtained by “ion-slicing”. They can be bonded by two different techniques to a low-index substrate to obtain “lithium niobate on insulator” (LNOI) even as wafer of 3” diameter. Different micro- and nano-structuring techniques have been used to successfully develop micro-photonic devices. To be specific, the fabrication and characterization of LNOI photonic wires with cross-section < 1 µm2, periodically poled LNOI photonic wires for second harmonic generation, electro-optically tunable microring resonators, free standing microrings for hybrid integration, and photonic crystal structures are described.

Lithium niobate on insulator (LNOI) for micro-photonic devices

Lithium niobate offers incredible versatility as a substrate for integrated
 optics. Researchers have developed an array of new optical devices based on this
 material, including waveguide structures, electro-optical wavelength filters and
 polarization controllers, lasers with remarkable properties, nonlinear frequency
 converters of exceptional efficiency, ultrafast all-optical signal processing devices
 and single photon sources.

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Integrated Optical Devices in Lithium Niobate

The fabrication of LiNbO3 ridge waveguides etched by a mixture of HF and HNO3 using chromium (Cr) stripes as masks is reported. Smooth etched surfaces are obtained by adding some ethanol into the etchant. Under-etching is nearly avoided by annealing the sample with the Cr masks before the wet etching process. Low-loss monomode ridge guides with a height of up to 8 mum and a width between 4.5 and 7.0 mum are demonstrated. As an example, the propagation losses in a 6.5-mum-wide and 8-mum-high structure are 0.3 dB/cm for transverse-electric and 0.9 dB/cm for transverse-magnetic polarization, respectively, at 1.55-mum wavelength

Lithium Niobate Ridge Waveguides Fabricated by Wet Etching

LN photonic wires of cross-section dimensions down to 1 x 0.73 microm2 were fabricated by Ar milling of a single-crystalline LiNbO3 (LN) film bonded to a SiO2/LiNbO3 substrate. Mode intensity distributions, propagation losses, and group indices of refraction were measured at 1.55 microm wavelength and compared with simulation results. Moreover, effective mode indices and end face reflectivities were numerically evaluated. The waveguide of 1 microm top width is the smallest LN photonic wire reported to date; it has a mode size of approximately 0.4 microm2 (0.5 microm2) only and propagation losses of 9.9 dB/cm (12.9 dB/cm) for qTM (qTE) polarization.

Lithium niobate photonic wires

The development of a novel self-aligned photolithographic process is reported enabling selective Ti-deposition on the surface of wet etched ridges on lithium niobate. Thus local diffusion doping of the ridges becomes possible to fabricate optical waveguides. They have surprisingly low propagation losses down to 0.05 dB/cm for TE-polarization (at ∼1.55 μm wavelength). The fabrication procedure of locally doped ridge guides in LN and their optical properties are presented in detail.

Hui Hu

Papers

Lithium niobate on insulator (LNOI) for micro-photonic devices

Integrated Optical Devices in Lithium Niobate

Lithium Niobate Ridge Waveguides Fabricated by Wet Etching

Lithium niobate photonic wires

Low-loss ridge waveguides on lithium niobate fabricated by local diffusion doping with titanium