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DAtacommunications based on NanophotoniC Resonators

Final Report Summary - DANCER (DAtacommunications based on NanophotoniC Resonators)

The large-scale movement and storage of data has now become an essential component of society. The emergence of Google, Facebook, Amazon and so many others have changed the face of society and are now indispensable parts of everyday life. A key challenge of the 21st is to provide billions of people with the means to access, move and manipulate huge volumes of data. A significant fraction of the world’s economy is now based on the internet and is a significant area of future growth. The internet experience is now, somewhat invisibly, built around data-centers, huge warehouses of computers connected to one another. The effective and fast operation of these interconnects is crucial but huge amounts of energy is consumed in the process.
The energy consumption problems stem from the inefficient movement of data around and between computer chips. As clock speeds have increased and transistors have shrunk, the metal wires connecting them have become less and less efficient. Around 50-80% of a computers power consumption now occurs in these interconnects. Optical links are the solution to this problem. As optical waveguides do not suffer from resistive loss and do not require the line to be charge to the operating voltage, huge data transmission rates are possible. However, the current state of the art is still orders of magnitude away from the efficiencies (in terms of cost, power and volume) required.
The DANCER project has developed a new framework for optical interconnects that is based on the efficient coupling of silicon Photonic Crystal (PhC) Resonators to the outside world by means of polymer and dielectric waveguides positioned vertically about the PhC. These vertically coupled Photonic Crystals provide the ultimate confinement of light in space and time allowing orders of magnitude improvement relative to the state of the art, yet in a simpler system.
DANCER has used this approach to demonstrate a new family of hybrid Photonic Crystal lasers, that combine the best attributes of indium phosphide based amplifiers and silicon nanophotonics. Very good spectral purity was realised with excellent side more suppression ratios, (50dB+), which is comparable with Distributed Feedback Lasers that require more complex and costly fabrication Wall plug efficiencies of 8% have. The uniqueness of the PhC provides additional features that are not possible with monolithic lasers. For example, the Self tuning effect in PhCs was used to realise a laser, the output wavelength of which was constant even when the ambient temperature changed, with active cooling. The use of power hungry-thermoelectric coolers, widely used in today’s communication systems is thus avoided, giving important savings.
A fabrication process has been developed to produce vertically coupled photonic crystals with integrated pn junctions that is based on mix and match electron beam lithography and photolithography that is compatible with small-to-medium scale production. Frequency modulation of hybrid PhC lasers has been demonstrated via tuning of the PhC resonator using the pn junction. Using an optical filter that is external to the laser cavity, the frequency modulation was converted to intensity modulation. High extinction ratio modulation (9dB) was realised for very low voltages swings, less then 1 volt peak to peak. Operation at such low operating voltages is very promising as it potentially avoids, or at least minimises, the use of electronic modulator drivers, which consume a sizeable fraction of the power budget of the optical link. New coupled cavity photonic crystals have been demonstrated and used for efficient modulation (less then 0.5V swings) and low loss delay lines (15dB/ns).
A process has been developed to integrate photonics and electronics on bulk silicon wafers. The process was carefully designed so that transistor fabrication is completely unaffected by the steps needed to fabricate the photonic devices.