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.

/pdf/advances-in-terahertz-communications-accelerated-by-t151adrqkd.pdf

Advances in terahertz communications accelerated by photonics

This paper presents a novel digital feedforward carrier recovery algorithm for arbitrary M-ary quadrature amplitude modulation (M-QAM) constellations in an intradyne coherent optical receiver. The approach does not contain any feedback loop and is therefore highly tolerant against laser phase noise. This is crucial, especially for higher order QAM constellations, which inherently have a smaller phase noise tolerance due to the lower spacing between adjacent constellation points. In addition to the mathematical description of the proposed carrier recovery algorithm also a possible hardware-efficient implementation in a parallelized system is presented and the performance of the algorithm is evaluated by Monte Carlo simulations for square 4-QAM (QPSK), 16-QAM, 64-QAM, and 256-QAM. For the simulations ASE noise and laser phase noise are considered as well as analog-to-digital converter (ADC) and internal resolution effects. For a 1 dB penalty at BER = 10-3, linewidth times symbol duration products of 4.1 x 10-4 (4-QAM), 1.4 x 10-4 (16-QAM), 4.0 x 10-5 (64-QAM) and 8.0 x 10-6 (256-QAM) are tolerable.

Hardware-Efficient Coherent Digital Receiver Concept With Feedforward Carrier Recovery for $M$ -QAM Constellations

The increasing demand of unoccupied and unregulated bandwidth for wireless communication systems will inevitably lead to the extension of operation frequencies toward the lower THz frequency range. Higher carrier frequencies will allow for fast transmission of huge amounts of data as needed for new emerging applications. Despite the tremendous hurdles that have to be overcome with regard to sources and detectors, circuit and antenna technology and system architecture to realize ultrafast data transmission in a scenario with extensive transmission loss, a new area of research is beginning to form. In this article we give an overview of emerging technologies and system research that might lead to ubiquitous THz communication systems in the future.

A Review on Terahertz Communications Research

We present a high-speed and highly scalable silicon optical modulator based on the free carrier plasma dispersion effect. The fast refractive index modulation of the device is due to electric-field-induced carrier depletion in a Silicon-on-Insulator waveguide containing a reverse biased pn junction. To achieve high-speed performance, a travelling-wave design is used to allow co-propagation of electrical and optical signals along the waveguide. We demonstrate high-frequency modulator optical response with 3 dB bandwidth of ~20 GHz and data transmission up to 30 Gb/s. Such high-speed data transmission capability will enable silicon modulators to be one of the key building blocks for integrated silicon photonic chips for next generation communication networks as well as future high performance computing applications.

High-speed optical modulation based on carrier depletion in a silicon waveguide

This review is focused on the latest developments in continuous-wave (CW) photomixing for Terahertz (THz) generation. The first part of the paper explains the limiting factors for operation at high frequencies ∼ 1 THz, namely transit time or lifetime roll-off, antenna (R)-device (C) RC roll-off, current screening and blocking, and heat dissipation. We will present various realizations of both photoconductive and p-i-n diode–based photomixers to overcome these limitations, including perspectives on novel materials for high-power photomixers operating at telecom wavelengths (1550 nm). In addition to the classical approach of feeding current originating from a small semiconductor photomixer device to an antenna (antenna-based emitter, AE), an antennaless approach in which the active area itself radiates (large area emitter, LAE) is discussed in detail. Although we focus on CW photomixing, we briefly discuss recent results for LAEs under pulsed conditions. Record power levels of 1.5 mW average power and conversion efficiencies as high as 2 × 10−3 have been reached, about 2 orders of magnitude higher than those obtained with CW antenna-based emitters. The second part of the paper is devoted to applications for CW photomixers. We begin with a discussion of the development of novel THz optics. Special attention is paid to experiments exploiting the long coherence length of CW photomixers for coherent emission and detection of THz arrays. The long coherence length comes with an unprecedented narrow linewidth. This is of particular interest for spectroscopic applications, the field in which THz research has perhaps the highest impact. We point out that CW spectroscopy systems may potentially be more compact, cheaper, and more accurate than conventional pulsed systems. These features are attributed to telecom-wavelength compatibility, to excellent frequency resolution, and to their huge spectral density. The paper concludes with prototype experiments of THz wireless LAN applications. For future telecommunication systems, the limited bandwidth of photodiodes is inadequate for further upshifting carrier frequencies. This, however, will soon be required for increased data throughput. The implementation of telecom-wavelength compatible photomixing diodes for down-conversion of an optical carrier signal to a (sub-)THz RF signal will be required.

/pdf/tunable-continuous-wave-terahertz-photomixer-sources-and-rulvqaamg0.pdf

Tunable, continuous-wave Terahertz photomixer sources and applications

This paper describes the development of a InP-HEMT MMIC chipset for 120-GHz wireless applications. The transmitter chip includes a frequency doubler for carriers, an ASK modulator, an RF band-pass filter, and a power amplifier. The receiver chip includes a low-noise amplifier and an ASK demodulator. A back-to-back test of the chipset has shown it to be fully functional at 10-Gbit/s data rate with BER=e-12 at -45.7-dBm input power of the receiver chip. To our knowledge, this is the first report of the development of highly integrated MMIC chipset operating at 120 GHz for wireless data communication.

120-GHz Tx/Rx chipset for 10-Gbit/s wireless applications using 0.1 /spl mu/m-gate InP HEMTs

This paper evaluates reactively sputtered WSiN films as an annealing cap for GaAs substrate. WSiN film successfully suppresses As and Ga outdiffusion. The WSiN/SiO2 /SiN/substrate multilayer annealing cap reconfirms this. As and Ga accumulate in the WSiN/SiO2 interface after annealing at 800 °C for 20 min. The accumulated As peak concentration is estimated to be 1019 atoms/cm3 . X‐ray diffraction shows WSiN film remains in an amorphous phase, exhibiting no recrystallized grain boundary. Film stress only changes by one‐half before and after annealing. These characteristics are very different from other refractory metals such as WN and WSi which change from amorphous to crystalline phase after annealing. It is therefore concluded that reactively sputtered WSiN films prevent As and Ga outdiffusion and form a good annealing cap for GaAs.

Reactively sputtered WSiN film suppresses As and Ga outdiffusion

Electrical PMD equalizer ICs for 40-Gbit/s transmission were developed using InP/InGaAs HBT technology. In experiments using a PMD emulator, these ICs exhibit good compensation for DGD of over 20 psec at 40 Gbit/s.

Electrical PMD equalizer ICs for a 40-Gbit/s transmission

This paper describes the development of wavegude modules for 120-GHz wireless applications. The MMIC in the modules were fabricated using 0.1-mum-gate InP-HEMTs and coplanar waveguides. The transmitter (Tx) module contains a Tx MMIC and a multiplier MMIC for carrier generation. The Tx MMIC contains a frequency doubler, ASK modulator, and an amplifier. Output power of the Tx module is 0 dBm. The receiver (Rx) module contains a Rx MMIC with a low-noise amplifier and ASK demodulator. For high-power operation of transmitter, we developed a power amplifier (PA) waveguide module that attaches to the Tx module. This PA module contains one-chip PA MMIC, and has 13-dBm output power with associated gain of 10.5 dB at 125 GHz. A back-to-back test of the Tx/Rx modules with and without the PA module have shown it to be fully functional at 10-Gbit/s data rate with BER = 1e-10 at -34.5-dBm and -36.1-dBm input powers for the receiver, respectively

120-GHz Tx/Rx Waveguide Modules for 10-Gbit/s Wireless Link System

A laterally scaled-down ohmic structure and an InGaAs/InAs composite channel improve the dc and RF characteristics of InP-based HEMTs. We reduced the distance between the gate and ohmic metal to less than 100 nm and to form sub-100-nm-long gate simultaneously, and also introduced device passivation for future construction of subterahertz-band integrated circuits. A 50-nm-gate HEMT exhibiting extrinsic transconductance of 2.0 S/mm and extrinsic current gain cutoff frequency (fT) of 496 GHz was successfully fabricated with this technology. This is the first report of a transistor with both 500-GHz-class fT and large current drivability

Masami Tokumitsu

Papers

120-GHz Tx/Rx chipset for 10-Gbit/s wireless applications using 0.1 /spl mu/m-gate InP HEMTs

Reactively sputtered WSiN film suppresses As and Ga outdiffusion

Electrical PMD equalizer ICs for a 40-Gbit/s transmission

120-GHz Tx/Rx Waveguide Modules for 10-Gbit/s Wireless Link System

Lateral Scale Down of InGaAs/InAs Composite-Channel HEMTs With Tungsten-Based Tiered Ohmic Structure for 2-S/mm $g_{m}$ and 500-GHz $f_{T}$