FEATURE : PHOTONIC INTEGRATION
GRAPHENE MIGHT GET INTO AN ENABLING ROLE WITH A UNIQUE SELLING POINT, IF WE SUCCEED IN MAINTAINING ITS INHERENT PROPERTIES DURING THE DEVICE INTEGRATION PROCESS
AMO produces graphene devices on six-inch silicon-on-insulator wafers in its pilot line
g to 120km. ‘We detected a negative penalty for most of the span length, meaning the signal was
at that distance beter than at zero kilometres,’ D’Errico said. Te main limit was due to a saturation effect above a certain voltage level. If commercialised, this result could simplify network architectures, D’Errico notes, and replace digital signal processing components used to compensate for chromatic dispersion.
Make a switch Graphene could also enable faster optical switching. Switching in reconfigurable optical add-drop multiplexers (ROADMs) ‘cannot go below tens of milliseconds’, D’Errico said. Graphene put on top of micro-ring resonators can switch status in picoseconds, the Ericsson team has found. ‘When you apply voltage, the graphene absorption can be tuned,’ D’Errico said. ‘When you increase voltage, you reduce graphene absorption, the microring resonator couples light from one waveguide to another.’ Implementing the switch in Ericsson-brand 100Gb/s transceivers doubled their interconnection flexibility, allowing two transponders to receive the information without errors. Integrating high-performance graphene
onto silicon like this not only improves device performance, it will ‘enable a fundamentally new generation of microchips,’ Schall said. ‘Increasing data throughput is very important for scaling data centre or communication infrastructure. Te benefit of graphene is the potential mass-production of very high-speed devices that surpass the current state-of-the-art for silicon. We see integrated silicon photonics and electronics, but all of them seem to have certain limitations regarding mass production.’ Schall stresses that a key challenge is
developing a scalable transfer process to put graphene onto silicon. ‘Tis is required, not only to come to market-relevant production capacity, but also to have enough capacity to develop
12 FiBRE SYSTEMS n Issue 26 n Winter 2020
reproducible and stable fabrication technology,’ he said. In terms of device design, Schall wants to optimise parameters like parasitic resistance and capacitance to their ultimate limit, as driving voltages at high data rates, in particular, are currently very limited. ‘Graphene has to be of excellent quality to obtain minimum insertion loss,’ he said.
Questions all the way down AMO has already demonstrated that graphene ‘is a perfect material for electro-absorption modulators, for example, the extinction to insertion loss ratio is by far superior to silicon,’ Schall added. ‘Te question is, what is the maximum speed that can be obtained with such a graphene modulator?’ AMO researchers are now answering that question in new firm Black Semiconductor. Tis aims to take recent findings ‘to the next level, and offer graphene photonic products soon,’ Schall said. Tis builds on a 2017 demonstration
from Graphenea and AMO of graphene photodetectors with bandwidth exceeding 75GHz, fabricated on a six-inch wafer process line. Modulators and photodetectors are the two main component types currently atracting interest, according to Zurutuza. ‘But non-linear optical switches, lasers, saturable absorbers, it has potential in all those devices,’ she observed. ‘Photodetectors are probably closer to market than other components, but in all of them, graphene makes sense. Also, currently the modulator and photodetector are manufactured separately and assembled in the chip, while with graphene everything will be fabricated at the same time on the chip.’ Te market for modulators and
photodetectors will be similar, Zurutuza suggests. For commercialisation of modulators and photodetectors in telecommunications, 12-inch wafers will be required and ‘the larger the wafer, the more complicated things become’, she said. She stresses that this will slow down
the time to market, so graphene products in other applications are nearer realisation. ‘Te requirements in optical communications are maybe stricter,’ Zurutuza said. ‘You need very high charge carrier mobility, and it’s also going to take longer to do it at a larger scale.’
Ahead of the curve Looking more broadly, graphene promises features that can’t be realised with conventional technologies, Templ stressed. ‘A 1:1 application of conventional device- and system-concepts bears the risk that we miss a number of interesting options,’ he said. ‘Te hierarchy and structure of the networks requires combining and extracting data traffic to/from segments with higher bundled traffic and distribute by means of ramified network segments with lower traffic down to the individual users and homes. New-material-based components can’t change that. But said components can significantly change how we utilise and exploit such networks by improving on the dynamics, flexibility, performance and efficiency.’ Meanwhile, applying graphene to well-
established device concepts could unleash features beyond the high charge carrier mobility that supports extremely high bandwidths and bitrates. In particular, graphene-based plasmonic optical modulators and detectors might open the way to highly integrated devices enabling communication with bitrates of several Tb/s. Such networks could enable a ‘deep communication experience of entirely different quality to what we have today,’ Templ said. D’Errico is similarly excited by graphene’s
prospects, but recognises the challenges faced in delivering them. ‘Since I started with graphene, every successive step was more brilliant than the previous one,’ he said. ‘At every step, we answer some questions – but we also ask an enormous amount of new questions.’ n
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