Team moves towards all-fibre optical comms network

A team from Southampton and Penn State Universities created a doped semiconductor junction in a microstructured optical fibre, enabling photodetection at high bandwidth.

‘This junction has a far higher performance than any electronic device ever integrated into a fibre before,’ Prof John Badding at Penn told The Engineer.

Fibre-optic cables are the backbone of internet and telephone communications worldwide. However, all these lines require external electronic devices to generate, amplify, receive and manipulate data.

Normally this is done with optical-electronic-optical (OEO) conversion, that in its entirety is expensive, complicated and uses large amounts of energy.

‘Coupling chips and fibres is often problematic… the size of a conventional fibre core is around 100 times smaller than a silicon waveguide,’ Badding explained.

In 2006, the UK-US team demonstrated a new optical fibre platform that could potentially handle electronics functions, as well as transmission, using a technique called chemical vapour deposition (CVD). It essentially involves creating specially fabricated pores in the optical fibres and putting down layers of semiconducting material.

‘The first paper was about the material deposition and here we’ve progressed to the stage where the chemistry is now understood to be good enough for doping, making sharp p-n junctions and really being able to integrate electronics in the fibre in a very robust way,’ Dr Peir Sazio of Southampton told The Engineer.

In the latest work the team was able to use the CVD approach to add layers of platinum and doped silicon to create a p-n junction, which acts as a photodetector at bandwidths of up to 3GHz.

‘It’s basically a reserve bias solar cell — a photodetector that detects light and converts it to an electronic signal,’ said Sazio.

The ability to create precisely doped semiconductors — a central plank of traditional chip-based electronics — could allow more complex devices in fibres, such as lasers for producing a signal, detectors for receiving it and modulators that separate wavelengths of light and disperse them through appropriate channels.

‘We don’t have to go off fibre onto a chip to do photodetection, we can embed semiconductor functionality directly inside the fibre. The fibre is no longer a passive photon transport medium, it becomes an actual active element — that’s the real driving motivation for what we’re doing,’ said Sazio.