There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

Jasmonates: biosynthesis, perception, signal transduction and action in plant stress response, growth and development. An update to the 2007 review in Annals of Botany

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

Wave propagation and group velocity

A wireless communication system with a maximum data rate of 100 Gbit s−1 over 20 m is demonstrated using a carrier frequency of 237.5 GHz. The photonic schemes used to generate the signal carrier and local oscillator are described, as is the fast photodetector used as a mixer for data extraction.

Wireless sub-THz communication system with high data rate

An all-optical space-time coding technique for polarization diversity transmissions is proposed and experimentally demonstrated for the first time. Based on the wavelength-shift-free optical phase conjugation technique, the Alamouti-type polarization-time coding is realized in the optical domain without additional electro-optic modulation circuits.

First All-optical Alamouti Coding Demonstration for Polarization Diversity Transmissions via Optical Phase Conjugation

Demodulation scheme based on multilabel eigenvalue-domain neural network for a 4096-ary eigenvalue-modulated signal is demonstrated experimentally. Successful demodulation with a 2.5 dB power margin compared with multiclass single-label classification is achieved at 10.7 Gb/s. © 2022 The Author(s)

Eigenvalue-domain Neural Network Receiver for 4096-ary Eigenvalue-modulated Signal

We experimentally demonstrate the transmission of a 4096-ary eigenvalue-modulated signal using 12 triangular-lattice-shaped eigenvalues. A 50-km transmission with a BER < 3.8 × 10−3 at 11.25 Gb/s is achieved successfully.

4096-ary Eigenvalue Modulation Using 12 Triangular-lattice-shaped Eigenvalues

Feasibility of a high-capacity spatially-multiplexed free-space optical (FSO) communications system utilizing small-footprint spatial diversity transceivers based on multicore fiber and 2-D photodetector array is investigated. With the aid of MIMO-DSP, the micrometer-scale spatial diversity transceivers enable 37-Gbps 2x4 MIMO-FSO transmission over strongly correlated optical MIMO channels.

Spatial Diversity Gain of Micrometer-scale MIMO FSO Transceivers utilizing Multicore Fiber and 2-D Photodetector Array

High-speed intensity-modulation direct-detection (IM/DD) fiber optical transmission system is of great interest to the applications of datacenter interconnects (DCI) as well as edge/access networks. However, when the fiber reach is beyond 40 km such as ER or ZR, the power fading induced by chromatic dispersion (CD) in C-band, S-band and even edge-wavelengths of O-band becomes a severe issue with the ever-increasing signal data rate. In this work, we focus on an ultra-low complexity joint optical-electrical feedforward equalization (OE-FFE) scheme to mitigate the CD distortion. We establish a comprehensive theory to jointly model the IM/DD channel, optical circuit and the digital equalizer, which offers analytical bit-error ratio (BER) expressions of pulse amplitude modulation (PAM)-2 and PAM4 formats and guides co-optimization of optical and electrical parts. As proofs of concept, we experimentally demonstrate C-band 60 Gb/s PAM2 over 85.8 km single-mode fiber (SMF) and up to 112 Gb/s PAM4 over 50 km or 61.3 km SMF, enabled by OE-FFE with only a 1-tap optical delay line and lean digital equalizers. For 112 Gb/s PAM4 over 50 km, BER satisfying 7% hard-decision FEC limit is achieved using a symbol-spaced simplified polynomial FFE with only 39 multiplications per PAM-4 symbol. For 60 Gb/s PAM-2, we achieved 85.8 km transmission with symbol-spaced 25-tap linear FFE and KP4 FEC-compliant BER. In addition, investigations on the parameters of the optical delay line are also carried out. The analytical and experimental results show the great potential of proposed OE-FFE approach for CD-constrained beyond-100G/λ ER and beyond-50G/λ ZR IM/DD systems.

Analysis and Demonstration of Low-Complexity Joint Optical-Electrical Feedforward Equalization (OE-FFE) For Dispersion-Limited High-Speed IM/DD Transmission

Yuki Yoshida

Papers

First All-optical Alamouti Coding Demonstration for Polarization Diversity Transmissions via Optical Phase Conjugation

Eigenvalue-domain Neural Network Receiver for 4096-ary Eigenvalue-modulated Signal

4096-ary Eigenvalue Modulation Using 12 Triangular-lattice-shaped Eigenvalues

Spatial Diversity Gain of Micrometer-scale MIMO FSO Transceivers utilizing Multicore Fiber and 2-D Photodetector Array

Analysis and Demonstration of Low-Complexity Joint Optical-Electrical Feedforward Equalization (OE-FFE) For Dispersion-Limited High-Speed IM/DD Transmission