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A team of UK researchers working at the University of Leeds, supported by colleagues from University College London (UCL) and others, has demonstrated a wireless line-of-sight Free Space Optical (FSO) communication link pushing data speeds of 4Gbps (Gigabits per second) using Terahertz (THz) frequencies; something that has previously been extremely difficult to harness.
Past FSO links have already demonstrated low latency (<1 ms) data rates reaching several hundred Gigabits per second (Gbit/s) to Terabits per second (Tbit/s), albeit using different spectrum bands. However, FSO links in the visible and near-infrared spectrum do present some challenges due to their very short wavelengths, which are more susceptible to “Rayleigh scattering” in the atmosphere and require very stringent beam-pointing alignment criteria to maintain acceptable link loss as frequency increases.
The ability to shift FSO links into the THz frequencies, which are located in the “gap” between microwave and infrared radiation, could potentially avoid some of the above pitfalls. The catch is that working with the THz band is difficult. The fastest THz FSO system to date achieved a data rate of just 20Mbps and that required the use of a “cryogenically cooled quantum well detector” (i.e. not exactly practical for casual commercial applications).
Suffice to say that establishing a THz FSO communication link capable of multi-gigabit-per-second data rates remains a “significant challenge“, but the UoL team have clearly made some significant progress in this field.
In this work, we combine some of the most promising technologies in this frequency band and demonstrate a THz FSO communication link with a 4 Gbit/s data rate using a QCL transmitter and a room-temperature Schottky barrier diode as a receiver. The THz QCL carrier frequency is dominated by a 2.4 THz mode and is operated in a closed-cycle cryocooler.
Rather than employing a separate modulator, we show that it is possible to modulate the QCL emission directly over a wide bandwidth without significant distortion or excessive modulation power requirements. We investigate encoding data in NRZ-OOK and pulse-amplitude-modulation-4 (PAM-4) format signals at a free-space transmission distance of 0.5 m
Granted, a distance of 0.5 metres might not sound brilliant, but this is only an early lab test and distance may be less of an issue for some of its potential applications. Past studies have operated more at distances of 5m to 7.5m, albeit delivering speeds between just 1Mbps and 115kbps respectively – extremely slow.
The ability to send data at multi-Gigabit speeds using the THz band could have benefits, such as via cable-free connections (e.g. between server racks) inside data centres. Not to mention the potential for high-capacity inter-satellite links, which would avoid the usual signal losses caused by Earth’s atmosphere. Other potential areas could include secure point-to-point communication in sectors such as defence, finance and healthcare.
Dr Elumalai said: “The main challenge was tuning the terahertz quantum cascade laser to act like a clean, high-speed transmitter that would transmit the signal without distortion. Once we achieved that, we were able to push the laser to transmit high data rates and reliably recover the information.
The researchers are now working to extend the system beyond simple on/off signalling to more advanced data modulation schemes. With further development of higher-power devices, the approach could support even greater data rates (potentially up to 1 Terabits per second), while helping to relieve pressure on increasingly congested wireless frequency bands.