Researchers at the University of Oxford have developed an optical line-of-sight LAN using CMOS transmitters and receivers and resonant cavity LEDs.
The system, which offers an alternative to Wi-Fi in security-sensitive environments and for buildings such as hospitals, currently operates at 155Mbit/s.
Dr Dominic O’Brien said the key challenge was in designing a detector with a large effective area. “Big detectors often have quite low bandwidth because their capacitance interacts with the impedance of the amplifier – which is a problem with fibre optic systems too,” said O’Brien.
“To make a big detector but keep it low capacitance and high bandwidth you can pixelate it, so it’s like having 20 receive antennas in parallel, and you then add the signal together.” In practise the detectors have a capacitance around five times lower than most commercially available examples.
In order to connect the receptors with their amplifiers in a scalable way, O’Brien’s group used flip chip bonding. Each detector is connected to a preamplifier within a custom CMOS receiver IC (not shown), so the circuitry for an individual channel lies under a single detector.
In the seven-element demonstration shown here, each detector is 500µm across, and the array is fabricated from epitaxially grown InGaAsP. Each of these receiver channels is designed to operate at 310Mb/s with a sensitivity of -30dBm at 980nm.
The optical detector array was built to be compatible with a flip-chip process. Each element is sensitive to radiation from 980nm to beyond 1500nm
The system uses digital CMOS to keeps costs low, with optical in-car networking a possible eventual aim. “Target prices for those [systems] mean that CMOS is being used for the analogue bits, with reasonable performance,” said O’Brien. “We’ve just about finished testing the array components – we hope to do that by the end of the year.”