System Combines Optical and Terahertz Signals at 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.

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