Fig. 1 CMOS-controlled color-tunable smart display. (a) Micro-LED chip, (b) I–V and L-I curves of a 72mm-diameter CMOS-bonded LED pixel, (c) CMOS driving board. These integrated LED/CMOS pixels have a modulation bandwidth of 100 MHz, thus providing simultaneously a wavelength-agile source for visible light communications.
The timeliness and ambition of this programme arises both from a unique opportunity and a profound challenge. The challenge is that communications technologies are increasingly struggling to meet demand. In terms of free-space, for example, the ubiquitous and highly successful RF wireless communications infrastructure is approaching a “looming spectral crisis” (as described by the US Federal Communications Commission). Future improvements in spectral efficiency using more advanced spatial multiplexing and coding techniques appear likely to be increasingly hard-won, and there is very little additional radio spectrum that is not already in use. Similar imperatives are being encountered in data communications, where 400G Ethernet systems are needed, and in interconnects where inter-chip communications bandwidths of 20Tb/s are needed for which there are currently no known solutions. In both free space and guided wave applications, the demand is not only for raw bandwidth but also for bandwidth density and the demand for this will continue to grow exponentially.
The opportunity comes from the huge potential for optical communications created by the global revolution in lighting. Efficient and robust light-emitting diode (LED) based “solid-state lighting”(SSL) is progressively replacing traditional incandescent and even fluorescent lamps and finding its way into new areas including signage, illumination, signalling, consumer electronics, building infrastructure, displays, clothing, avionics, automotive, submarine applications and medical prosthetics. This technology, which will be deployed in hundreds of billions of components and fixtures, has tended to be viewed so far primarily as a way to improve energy and spectral efficiency: these features are being explored widely in the two most promising embodiments of SSL, those of gallium nitride inorganic semiconductor LEDs (GaN LEDs), which cover UV-blue-green efficiently and achieve white light by colour conversion in phosphors, and organic light-emitting diodes (OLEDs) based on small molecule or polymeric organic semiconductors. It has been fairly widely appreciated that such sources, being “optoelectronic”, have the potential to be “smart” in terms of energy-efficiency and control functions, but what has been little understood is their potential also to transform communications.
Early-stage adoption of GaN LEDs and OLEDs in communications has been in the very limited sense of simple signalling applications, and in mobile phone displays and backlighting of liquid crystal displays. However, recent trends in both guided wave (polymer optical fibre (POF) and polymer backplane) and free-space combined illumination and communications, with GaN LEDs especially, raise the tremendous prospect of an entirely new form of high bandwidth communications infrastructure to complement, enhance, and in some circumstances supersede existing systems.
Vision and research objectives
The solid-state lighting revolution currently underway offers the prospect of an entirely new and ubiquitous intelligent communications infrastructure that integrates sensing, illumination, communications and control within energy-efficient visible light sources that can change colour and adapt to their environment. Users will thus be able to “see” the information exchange with each other, offering new types of “visibly secure” social and business networking, the ability to identify and localise areas of high bandwidth and many new forms of man/machine interaction. This new technology has, in our view, the potential to combine information display, lighting and high-bandwidth communications in a single system (Fig. 1), leading to “universal illumination sources”, the full implications of which promise to be particularly profound. Disruptive developments can be anticipated to result, in areas including machine-to-machine communications, smart homes and vehicles, computation and data centres, smart clothing, mobile communications, imaging systems, personal security and healthcare.
Fig. 2 Color emission patterns generated from a LED/CMOS smart display. The LED pixel emissions (spatial/spectral/intensity) are controlled by CMOS electronics
In this ambitious programme, we seek to make major strides in the development of these universal communications sources by exploring new conceptions in communications systems design where the spatial distribution and control of visible information channels can be exploited. Our vision is built on the unique capabilities of gallium nitride (GaN) solid-state lighting technology to achieve highly parallel (and ultra-high data density) visible wavelength optical communications for the very first time. It makes a central feature of the compatibility of GaN with CMOS, both in terms of sophisticated transmitter control functions and in arrayed and spectrally-matched custom receivers. This will open up entirely new vistas for embedded communications, creating systems with exceptional aggregated bandwidths and scalable bandwidth density, and very importantly bringing new prospects to combine data communications with “task lighting”, optical images and video (Fig. 2 shows color-tunable patterns generated from a LED/CMOS smart display).
In implementing this programme, we will explore the ultimate modulation limits of incoherent LED sources, implement sources capable of efficient, high-data density, multi-colour and multi-beam directional output, and develop advanced multi-channel and wavelength-selective CMOS receivers. We aim to build on these capabilities to develop new system concepts in displays and illumination that look beyond near-term trends and towards longer-term and more transformative implementation.