System Combines Optical and Terahertz Signals at 400 GHz

Jack Browne | Microwaves and RF

http://mwrf.com/systems/system-combines-optical-and-terahertz-signals-400-ghz

Data-hungry applications are steadily consuming wireless bandwidth, to the point where network managers are eying available bandwidth at millimeter-wave and even terahertz frequencies. To that end, researchers based in Lyngby, Denmark and Cambridge, England have surveyed efforts at developing terahertz wireless-communications systems and evaluated various methods of designing  terahertz-frequency communications links for high-data-rate applications.

The team learned that links can be assembled completely from electrical components using electromagnetic (EM) energy or from a combination of electrical and optoelectronic technologies. Since higher data rates have been achieved with the latter approach, the researchers propose an optoelectronics terahertz wireless communications system operating in the 400-GHz band; it uses optical signals in a 12.5-GHz ultradense wavelength division multiplexing (UD-WDM) grid.

The research and system development were performed by Xianbin Yu from Zhejiang University (Hangzhou, China) and DTU Fotonik (Department of Photonics Engineering, Technical University of Denmark), along with Rameez Asif of the University of Cambridge  and a team consisting of Molly Piels, Darko Zibar, Michael Galili, Toshio Morioka, Peter Jepsen, and Leif Oxenlowe (also from DTU Fotonik). The terahertz carriers are generated by heterodyne photomixing of free-running optical sources—in this case, a 100-kHz continuous-wave (CW) laser array with frequency stability of ±12.5 GHz and power stability of ±0.003 over 24 h.

This generation of millimeter-wave and terahertz signals is transparent to modulation sources already being used in WDM optical communications systems. The researchers demonstrated the compatibility of their system with optical networks by using spectrally efficient optical Nyquist channels with a quadrature-phase-shift-keying (QPSK) modulation format, as used for commercial 100 Gigabit Ethernet applications.

For testing, a wireless propagation distance was fixed at 50 cm, with path loss of less than 2 dB achieved under optimum conditions. Downconversion was to intermediate-frequency (IF) channels in the 20-GHz band. The researchers achieved aggregated data rates to 60 Gb/s with their system, and showed the potential for a terahertz-frequency communications link that combines optical and EM signals. Current limits in photodiode responsivity and terahertz-frequency amplifiers and antennas limit the practical application of such a system. However, as engineering efforts lead to more components at terahertz frequencies, this is an attractive communications system for short-distance, high-data-rate applications.

See: “400-GHz Wireless Transmission of 60-Gb/s Nyquist-QPSK Signals Using UTC-PD and Heterodyne Mixer,” IEEE Transactions on Terahertz Science and Technology, Vol. 6, No. 6, November 2016, p. 765.

Record-fast terahertz electro-absorption modulation in quantum dots supports switching rates of at least 3 THz

In an effort led by DTU Fotonik, a group of researchers from 5 different European universities and companies demonstrated a record-fast THz-rate electro-absorption modulator. Such an ultrafast electro-absorption modulator will have a primary application in the wireless data communication systems capable of supporting Terabit-per-second (Tbit/s) data rates.

Application of THz electro-absorption modulator in an ultra-high-speed wireless data communication system. Measurements on an emulated communication channel with the data rate of 88 Gbit/s, and capacity of 0.5 Tbit/s
In an effort lead by DTU Fotonik, a group of researchers from 5 different European universities and companies demonstrated a record-fast THz-rate electro-absorption modulator. Such an ultrafast electro-absorption modulator will have a primary application in the wireless data communication systems capable of supporting Terabit-per-second (Tbit/s) data rates.
Semiconductor quantum dots were used as the switching medium, in which the optical absorption strength was changed by the ultrafast THz signals incident form a free space. Quantum dots (QDs) are the smallest-possible semiconductor elements, only consisting of 10s of thousands of individual atoms, and they can only accomodate up to 2 electron-hole pairs in the lowest energy state. It is this tiny size of the QDs, that allowed the researchers to achieve the unprecedented fast speeds at which the optical absorption in the QDs could be manipulated.
In the experiment, recently described in an article in Applied Physics Letters [1], the researchers managed to encode a THz signal with the spectrum covering the range of 0.2 – 3 THz, onto an optical signal with the wavelength of 1040 nm resonant with the quantum dot absorption. The full 3 THz bandiwdth of a THz signal was encoded onto an optical signal, with individual encoded temporal features being as fast as only 460 fs. It was demonstrated, that such an electro-absorption modulator can be used as a receiver for the THz-range wireless data links supporting the data rates of at least 0.5 Tbit/s.
This project was led by assoc. prof. Dmitry Turchinovich (DTU Fotonik). Matthias Hoffmann (Univ. Hamburg), Boris Monozon (Marine Tech. Univ., St. Petersburg), Daniil Livshits (Innolume GmbH), and Edik Rafailov (Univ. Dundee) participated in the project.
This work is based on DTU Fotonik’s invention (D.Turchinovich, patent pending PCT/DK2010/050200, 05 August 2009), and a DKK 750,000 grant from the Danish Proof of Concept Foundation was recently awarded to Dmitry Turchinovich for further development of this device.
For more information please contact assoc. prof. Dmitry Turchinovich (dmtu@fotonik.dtu.dk)
[1] M.C.Hoffmann, B.S.Monozon, D.Livshits, E.U.Rafailov, and D.Turchinovich, “Terahertz electro-absorption effect enabling femtosecond all-optical switching in semiconductor quantum dots”, Appl. Phys. Lett. 97, 231108 (2010).

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