Optical wireless communication (OWC) is emerging as a key technology that can complement radio-frequency (RF) based communication systems, adding hundreds of THz of licence-free bandwidth. Standard visible-wavelength light-emitting diodes (LEDs), mostly based on gallium nitride technology, can support Gigabit per second (Gbps) wireless data rates which can be deployed in lighting-compatible systems, supporting the development of LiFi (light fidelity).

Custom micro-scale (<100 µm diameter) LEDs, known as micro-LEDs, emerging rapidly as the basis of new forms of electronic visual display technology, have been shown to exhibit an order of magnitude higher modulation bandwidth than the standard chip-size LEDs, and thus to be capable of supporting very high wireless data rates of up to ~10 Gbps. Hence exploration of the data communications capability of custom micro-LEDs for OWC has emerged as a key topic in research.

The OWC spectrum extends to ultraviolet wavelengths which are also accessible to gallium nitride based LED technology. Al(In)GaN-based LEDs have been demonstrated to operate across much of the UV-A, UV-B and UV-C regions of the spectrum. These developments naturally raise the possibility of UV-based OWC systems taking advantage of the distinctive properties of UV light.

Short wavelengths are much more strongly scattered by air than visible light, meaning UV communications should support non-line-of-sight links that can divert around obstacles such as buildings. Furthermore, in the UV-B and UV-C regions of the UV spectrum there is little background noise, largely due to the Earth's upper atmosphere absorbing solar radiation in these wavelength bands. This allows deep UV communications to operate in a near noise-free environment.

A research team from the University of Strathclyde has combined the motivations of exploring UV OWC technology and the capabilities of micro-LEDs, to advance the state-of-the-art in UV communications. UV-emitting micro-LEDs have previously been shown to support data rates of up to 2 Gbps, lower than at visible wavelengths primarily because of the relatively low output power of these micro-LEDs. The team reports the use of Wavelength Division Multiplexing (WDM) and custom UV micro-LED devices as an approach to overcome this issue.

They increase the wireless data rate up to a total of 10 Gbps by the simultaneous transmission of data from three micro-LEDs, emitting in the UV-A, UV-B and UV-C regions of the spectrum, respectively. LED-based WDM has been little explored in the UV region, so their work extends this concept into a new wavelength band. Furthermore, they combine this approach with record data rate performance at each of the respective wavelengths, taking full advantage of the capabilities of the micro-LED device format. Their research results are published in Photonics Research, Volume 10, No. 2, 2022 (D.M. Maclure, et al. 10 Gbps wavelength division multiplexing using UV-A, UV-B, and UV-C micro-LEDs[J]. Photonics Research, 2022, 10(2): 02000516).

In their OWC system, the emission from each micro-LED was collected using lenses and combined into a common optical path towards a receiver using dichroic mirrors. Optical bandpass filters allowed the receiver to distinguish each channel. An Orthogonal Frequency Division Multiplexing (OFDM) modulation scheme allowed data to be efficiently transmitted from each micro-LED, with data rates of 4.17, 3.02 and 3.13 Gbps being achieved from the UV-A, -B and -C micro-LEDs, respectively.

In their work, not only does each of these micro-LEDs individually exhibit a record data rate for a UV-emitting LED, it is also the first time that WDM using LEDs or micro-LEDs has been demonstrated across the UV-A, -B and -C spectrum. This work shows the enormous potential of UV optical wireless communications, as well as demonstrating how micro-LEDs and WDM can enable very high data rates. It opens the way to deploying multi-wavelength UV micro-LED communication systems in a variety of environments including in atmosphere and in space.

Professor Martin Dawson of the University of Strathclyde, who led the research team, commented:

"These are very exciting research results. In close collaboration with our colleague Professor Harald Haas and his team, we have been exploring micro-LEDs for optical wireless communications for over a decade, but the ultraviolet region of the spectrum is still relatively unexplored. To be able to introduce WDM into this spectral region and as a result achieve very high aggregated data rates above 10Gbps opens the door to new communications systems with a wide range of application potential, both terrestrial and in space."

Future research directions opened up by this work include the possibility of combining more of these discrete UV wavelengths in WDM, studying the longer distance OWC performance of UV micro-LEDs, extending the studies to non-line-of-sight communications and continuing to improve the performance of the micro-LEDs across all UV wavelengths and especially in the deep ultraviolet.

Experimental setup for UV WDM communications using UV-A, UV-B, and UV-C micro-LEDs. The inset shows UV micro-LEDs in operation.