This Review covers the state-of-the-art technologies on photonics-based terahertz communications, which are compared with competing technologies based on electronics and free-space optical communications. Future prospects and challenges are also discussed. Almost 15 years have passed since the initial demonstrations of terahertz (THz) wireless communications were made using both pulsed and continuous waves. THz technologies are attracting great interest and are expected to meet the ever-increasing demand for high-capacity wireless communications. Here, we review the latest trends in THz communications research, focusing on how photonics technologies have played a key role in the development of first-age THz communication systems. We also provide a comparison with other competitive technologies, such as THz transceivers enabled by electronic devices as well as free-space lightwave communications.

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Advances in terahertz communications accelerated by photonics

Wavelength multiplexers, demultiplexers and routers based on optical phased arrays play a key role in multiwavelength telecommunication links and networks. In this paper, a detailed description of phased-array operation and design is presented and an overview is given of the most important applications.

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PHASAR-based WDM-devices: Principles, design and applications

Emerging applications based on optical beams carrying orbital angular momentum (OAM) will probably require photonic integrated devices and circuits for miniaturization, improved performance, and enhanced functionality. We demonstrate silicon-integrated optical vortex emitters, using angular gratings to extract light confined in whispering gallery modes with high OAM into free-space beams with well-controlled amounts of OAM. The smallest device has a radius of 3.9 micrometers. Experimental characterization confirms the theoretical prediction that the emitted beams carry exactly defined and adjustable OAM. Fabrication of integrated arrays and demonstration of simultaneous emission of multiple identical optical vortices provide the potential for large-scale integration of optical vortex emitters on complementary metal-oxide–semiconductor compatible silicon chips for wide-ranging applications.

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Integrated Compact Optical Vortex Beam Emitters

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

Photonic integrated circuits (PICs) are considered as the way to make photonic systems or subsystems cheap and ubiquitous. PICs still are several orders of magnitude more expensive than their microelectronic counterparts, which has restricted their application to a few niche markets. Recently, a novel approach in photonic integration is emerging which will reduce the R&D and prototyping costs and the throughput time of PICs by more than an order of magnitude. It will bring the application of PICs that integrate complex and advanced photonic functionality on a single chip within reach for a large number of small and larger companies and initiate a breakthrough in the application of Photonic ICs. The paper explains the concept of generic photonic integration technology using the technology developed by the COBRA research institute of TU Eindhoven as an example, and it describes the current status and prospects of generic InP-based integration technology.

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An introduction to InP-based generic integration technology

A review of the complexity development of InP-based Photonic ICs is given. Similarities and differences between photonic and microelectronic integration technology are discussed and a vision of the development of photonic integration in the coming decade is given.

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Moore's law in photonics

Similarities and differences between photonic and microelectronic integration technology are discussed and a vision of the development of InP-based photonic integration in the coming decade is given.

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Generic foundry model for InP-based photonics

We report all-optical switching due to state filling in quantum dots (QDs) within a Mach–Zehnder interferometric switch (MZI). The MZI was fabricated using InGaAsP/InP waveguides containing a single layer of InAs/InP QDs. A 1530–1570 nm probe beam is switched by optical excitation of one MZI arm. By exciting below the InGaAsP band gap, we prove that the refractive index nonlinearity is entirely due to the QDs. The switching efficiency is 5 rad/(μW absorbed power), corresponding to a 6 fJ switching energy. Probe wavelength insensitivity was obtained using a broad size distribution of QDs.

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All-optical switching due to state filling in quantum dots

A new photonic integration technique is presented, which enables the use of indium-phosphide-based membranes on top of silicon chips. This can provide the electronic chips (complementary metal-oxide semiconductor (CMOS)) with an added optical layer (indium-phosphide membrane on silicon (IMOS)) for resolving the communication bottleneck. Very small passive devices have been realised, with performances comparable to other membrane devices (propagation loss 7 dB/cm, negligible bending loss for micron size radii, 3 dB splitter with 0.6 dB excess loss, resonator with Q-factor of 15.500). Also, a new passive device is introduced, a 4.12 micron long polarisation converter which in simulations promises broadband performance and tolerant fabrication. Finally, an active/passive regrowth technique is investigated for submicron active regions within an otherwise mostly passive membrane. A good morphology is obtained around the interfaces between the active and passive regions. The processing involved did not damage the materials severely, so that light emission in micro-PL measurements was found. However, an increasing blue shift with decreasing size occurred, due to quantum well intermixing. Optimising the design and the processing can take care of this. Taken together, the results presented here show that it is feasible to realise extremely small passive and active devices in a photonic circuit in an InP membrane.

Photonic integration in indium-phosphide membranes on silicon

A uni-traveling carrier photodetector (UTC-PD), heterogeneously integrated on silicon, is demonstrated. It is fabricated in an InP-based photonic membrane bonded on a silicon wafer, using a novel double-sided processing scheme. A very high 3 dB bandwidth of beyond 67 GHz is obtained, together with a responsivity of 0.7 A/W at 1.55 μm wavelength. In addition, open eye diagrams at 54 Gb/s are observed. These results promise high speed applications using a novel full-functionality photonic platform on silicon.

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van der Jjgm Jos Tol

Papers

Moore's law in photonics

Generic foundry model for InP-based photonics

All-optical switching due to state filling in quantum dots

Photonic integration in indium-phosphide membranes on silicon

High-bandwidth uni-traveling carrier waveguide photodetector on an InP-membrane-on-silicon platform