Terahertz wireless communication is receiving great interest from researchers and industries, thanks to the new spectral windows between 0.1 and 1 THz offering opportunities for ultra-high-data-rate wireless transmission. Wavelength division multiplexing for wireless-over-fiber is foreseen as an enabling technique to support connection between base stations and a central station. This paper reviews architectures for photonic distribution and generation of multiband signals for sub- THz wireless communications, giving rates up to 100 Gb/s (20 Gb/s per band) using the full spectrum between 220 GHz and 280 GHz for downlink wireless transmission, and 10 Gb/s for uplink using on-off keying.
A comparative study of two different Photonic Integrated Circuits (PICs) structures for continuous-wave generation of millimeter-wave (MMW) signals is presented, each using a different approach. One approach is optical heterodyning, using an integrated dual-wavelength laser source based on Arrayed Waveguide Grating. The other is based on ModeLocked Laser Diodes (MLLDs). A novel building block -Multimode Interference Reflectors (MIRs) – is used to integrate on-chip both structures, without need of cleaved facets to define the laser cavity. This fact enables us to locate any of these structures at any location within the photonic chip. As will be shown, the MLLD structure provides a simple source for low frequencies. Higher frequencies are easier to achieve by optical heterodyne. Both types of structures have been fabricated on a generic foundry in a commercial MPW PIC technology.
The THz part of the spectrum (0.1 to 10 THz) has been gathering increasing interest over the past 10 years as it could
enable interesting new applications. Key to its exploitation has been the development of photonic based sources and
detectors. However there is a lack of room temperature operating devices for both sources and detectors. We propose the
use of uni-travelling carrier photodetectors (UTC-PDs) as both source and detectors for the range up to 2 THz.
Optical fibre transmission has enabled greatly increased transmission rates, with 10 Gb/s common in local area networks. End users find wireless access highly convenient, however limited spectrum availability at microwave frequencies results in per-user transmission rates which are limited to much lower values, 500 Mb/s for 5 GHz band IEEE 802.11ac, for example. Extending the high data-rate capacity of optical fibre transmission to wireless devices, requires greatly increased carrier frequencies. This paper will describe how photonic techniques can enable ultra-high capacity wireless data distribution and transmission using signals at millimetre-wave and TeraHertz (THz) frequencies.
Indium phosphide and associated epitaxially grown alloys is a material system of choice to make photonic integrated circuits for microwave to terahertz signal generation, processing and detection. Fabrication of laser emitters, high speed electro-optical modulators, passive waveguides and couplers, optical filters and high speed photodetectors is well mastered for discrete devices. But monolithic integration of them while maintaining good performances is a big challenge. We have demonstrated a fully integrated tunable heterodyne source designed for the generation and modulation of sub-Terahertz signals. This device is to be used for high data-rate wireless transmissions. DFB lasers, SOA amplifiers, passive waveguides, beam combiners, electro-optic modulators and high speed photodetectors have been integrated on the same InP-based platform. Millimeter wave generation at up to 120 GHz based on heterodyning the optical tones from two integrated lasers in an also integrated high bandwidth photodetector has been obtained.
In this presentation we discuss application of broadband terahertz (THz) time-domain spectroscopy for probing subwavelength
(micrometer) size objects. The problem of weak coupling between THz waves and sub-wavelength objects,
which limits the use of THz spectroscopy to large samples, is mitigated by employing surface plasmon waves. In one
implementation, THz surface waves, excited on a broad-band planar THz bow-tie antenna, are used to enhance the
interaction with a small particle placed on the antenna surface. The surface field distribution near the particle is mapped
with an integrated sub-wavelength aperture THz near-field probe. We demonstrate that imaging and probing of the subwavelength size dielectric particles (TiO<sub>2</sub> and SrTiO<sub>3</sub>) can be realized using the enhanced THz field between the antenna and the probe. We also discuss THz wave confinement using two sharp metallic needles. We demonstrate that in the near-field region of the needle tips, the electric field of THz pulses is concentrated to a volume smaller than (10 μm)<sup>3</sup> without limiting the THz pulse bandwidth. Application of both methods for high spatial resolution imaging and spectroscopy will be discussed.
A 0.2 A/W responsivity waveguide-uni-travelling carrier photodiode with a -3 dB electrical frequency
response > 108 GHz is demonstrated. Up to -5 dBm electrical power at 110 GHz, and 28 mA
photocurrent (DC excitation) were detected. The photodiode was also integrated with an antenna to
permit a record breaking emission of up to 148 μW at 457 GHz and 25 μW at 914 GHz.