Microwave photonics (MWP) is an emerging field in which radio frequency (RF) signals are generated, distributed, processed and analyzed using the strength of photonic techniques. It is a technology that enables various functionalities which are not feasible to achieve only in the microwave domain. A particular aspect that recently gains significant interests is the use of photonic integrated circuit (PIC) technology in the MWP field for enhanced functionalities and robustness as well as the reduction of size, weight, cost and power consumption. This article reviews the recent advances in this emerging field which is dubbed as integrated microwave photonics. Key integrated MWP technologies are reviewed and the prospective of the field is discussed.

Integrated microwave photonics

Flexibility to support mesh topologies, dynamic capacity allocation, and automated network control and light path setup are key elements in the design of next-generation optical transport networks. To realize these capabilities, reconfigurable optical add/drop multiplexers with dynamic add/drop structures, embedded control planes, and lightpath characterization are required. This article presents the architectures and various ROADM implementations including colorless, directionless, and contentionless add/drop structures. The effect of scaling bit rates beyond 100 Gb/s on ROADM architectures is reviewed including providing variable channel bandwidth depending on bit rate. Automated provisioning and restoration using the GMPLS control plane and optical measurement approaches for lightpaths are also discussed.

Flexible architectures for optical transport nodes and networks

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

Abstract The emergence of silicon photonics over the past two decades has established silicon as a preferred substrate platform for photonic integration. While most silicon-based photonic components have so far been realized in the near-infrared (near-IR) telecommunication bands, the mid-infrared (mid-IR, 2–20-μm wavelength) band presents a significant growth opportunity for integrated photonics. In this review, we offer our perspective on the burgeoning field of mid-IR integrated photonics on silicon. A comprehensive survey on the state-of-the-art of key photonic devices such as waveguides, light sources, modulators, and detectors is presented. Furthermore, on-chip spectroscopic chemical sensing is quantitatively analyzed as an example of mid-IR photonic system integration based on these basic building blocks, and the constituent component choices are discussed and contrasted in the context of system performance and integration technologies.

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Mid-infrared integrated photonics on silicon: a perspective

Oxide electronic materials provide a plethora of possible applications and offer ample opportunity for scientists to probe into some of the exciting and intriguing phenomena exhibited by oxide systems and oxide interfaces. In addition to the already diverse spectrum of properties, the nanoscale form of oxides provides a new dimension of hitherto unknown phenomena due to the increased surface-to-volume ratio.

Oxide electronic materials are becoming increasingly important in a wide range of applications including transparent electronics, optoelectronics, magnetoelectronics, photonics, spintronics, thermoelectrics, piezoelectrics, power harvesting, hydrogen storage and environmental waste management. Synthesis and fabrication of these materials, as well as processing into particular device structures to suit a specific application is still a challenge. Further, characterization of these materials to understand the tunability of their properties and the novel properties that evolve due to their nanostructured nature is another facet of the challenge. The research related to the oxide electronic field is at an impressionable stage, and this has motivated us to contribute with a roadmap on 'oxide electronic materials and oxide interfaces'.

This roadmap envisages the potential applications of oxide materials in cutting edge technologies and focuses on the necessary advances required to implement these materials, including both conventional and novel techniques for the synthesis, characterization, processing and fabrication of nanostructured oxides and oxide-based devices. The contents of this roadmap will highlight the functional and correlated properties of oxides in bulk, nano, thin film, multilayer and heterostructure forms, as well as the theoretical considerations behind both present and future applications in many technologically important areas as pointed out by Venkatesan.

The contributions in this roadmap span several thematic groups which are represented by the following authors: novel field effect transistors and bipolar devices by Fortunato, Grundmann, Boschker, Rao, and Rogers; energy conversion and saving by Zaban, Weidenkaff, and Murakami; new opportunities of photonics by Fompeyrine, and Zuniga-Perez; multiferroic materials including novel phenomena by Ramesh, Spaldin, Mertig, Lorenz, Srinivasan, and Prellier; and concepts for topological oxide electronics by Kawasaki, Pentcheva, and Gegenwart. Finally, Miletto Granozio presents the European action 'towards oxide-based electronics' which develops an oxide electronics roadmap with emphasis on future nonvolatile memories and the required technologies.

In summary, we do hope that this oxide roadmap appears as an interesting up-to-date snapshot on one of the most exciting and active areas of solid state physics, materials science, and chemistry, which even after many years of very successful development shows in short intervals novel insights and achievements.

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The 2016 oxide electronic materials and oxide interfaces roadmap

A platform for the realization of tightly-confined lithium niobate photonic devices and circuits on silicon substrates is reported based on wafer bonding and selective oxidation of refractory metals. The heterogeneous photonic platform is employed to demonstrate high-performance lithium niobate microring optical resonators and Mach-Zehnder optical modulators. A quality factor of ~7.2 × 104 is measured in the microresonators, and a half-wave voltage-length product of 4 V.cm and an extinction ratio of 20 dB is measured in the modulators.

Heterogeneous lithium niobate photonics on silicon substrates

Silicon-on-nitride ridge waveguides are demonstrated and characterized at mid- and near-infrared optical wavelengths. Silicon-on-nitride thin films were achieved by bonding a silicon handling die to a silicon-on-insulator die coated with a low-stress silicon nitride layer. Subsequent removal of the silicon-on-insulator substrate results in a thin film of silicon on a nitride bottom cladding, readily available for waveguide fabrication. At the mid-infrared wavelength of 3.39 μm, the fabricated waveguides have a propagation loss of 5.2 ± 0.6 dB/cm and 5.1 ± 0.6 dB/cm for the transverse-electric and transverse-magnetic modes, respectively.

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Silicon-on-nitride waveguides for mid- and near-infrared integrated photonics

Electronically tunable optical true-time delay lines are proposed. The devices utilize the combination of apodised gratings and the free-carrier plasma effect to tune the enhanced delay of silicon waveguides at a fixed wavelength. Three variations of the proposed scheme are studied and compared. The compact and integrable devices can achieve tuning ranges as high as ~660 ps with a loss of < 2.2 dB when operated in the reflection mode of the gratings. A delay of ~40 ps with a loss of < 10 dB and an estimated operation bit rate of ~20 Gb/s can be achieved.

Electronically tunable silicon photonic delay lines

Suspended silicon-membrane ridge waveguides are fabricated and characterized on a single-material photonic device platform. By using direct bonding, a thin layer of silicon is fused to a bulk silicon substrate, which is patterned with narrow trenches. Waveguides are etched on the resulting suspended membranes and are characterized at mid- and near-infrared wavelengths. Transverse magnetic-mode propagation losses of 2.8 ± 0.5 and 5.6 ± 0.3 dB/cm at 3.39 and 1.53 μm wavelengths are measured, respectively. This all-silicon optical platform is capable of continuous low-loss waveguiding from wavelengths of 1.2–8.5 μm, enabling numerous applications in frequency conversion and spectral analysis.

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High-contrast, all-silicon waveguiding platform for ultra-broadband mid-infrared photonics

Second-harmonic generation is demonstrated using grating-assisted quasi-phase matching, based on waveguide-width modulation or mode-shape modulation. Applicable to any thin-film integrated second-order nonlinear waveguide, the technique is demonstrated in compact lithium niobate ridge waveguides. Fabricated devices are characterized with pulsed-pumping in the near-infrared, showing second-harmonic generation at a signal wavelength of 784 nm and propagation loss of 1 dB/cm.

Saeed Khan

Papers

Heterogeneous lithium niobate photonics on silicon substrates

Silicon-on-nitride waveguides for mid- and near-infrared integrated photonics

Electronically tunable silicon photonic delay lines

High-contrast, all-silicon waveguiding platform for ultra-broadband mid-infrared photonics

Second-harmonic generation in single-mode integrated waveguides based on mode-shape modulation