Embedding electronics into optical fibres

By depositing semiconductors and metals inside the long, thin channels of a microstructured optical fibre, researchers at Pennsylvania State University and the University of Southampton in the UK have built high-quality electronic devices within the fibres.
Unlike previous methods to integrate devices inside fibre, the new technique works with many different types of semiconductor materials. The researchers, led by John Badding, a chemistry professor at Penn State, can also precisely dope the materials to create n-type and p-type semiconductors, which are butted together to make the junctions that form the basis of electronic devices.

The advance, reported in Nature Photonics, could simplify and streamline fibre-optic telecommunications, spectroscopy equipment, and remote-sensing tools. ''We're integrating a semiconductor junction with performance and characteristics that have never been possible in fibre,'' Badding says.
In fibre-optic systems, a silica fibre carries digital data encoded onto light pulses. On either end, however, semiconductor devices manipulate the light. One side has lasers that create light pulses and modulators that encode data, while the other side has detectors that convert the optical data to electrical signals.

Coupling light between a hair-thin fibre and the light-guiding structure on a semiconductor chip is not easy, Badding says. Moving some of the chip-based devices into the fibre eliminates that coupling and makes the optic-electronic interface smoother, he says.
What's more, adds Pier Sazio, an optoelectronics researcher at the University of Southampton, embedding some electronic components inside the fibres makes the remaining electronic components of the system simpler and cheaper. ''This is a new way of thinking about what you can do with an optical fibre,'' Sazio says. ''Here the fiber becomes more of a device. You'll never remove the electronic interface, but [making] it more seamless has advantages.''
Badding and Sazio start with pre-made microstructured fibres, also called photonic crystal fibres, which have arrays of nanometre-scale pores along their length. They can block some of these pores using epoxy glue, leaving desired ones open for making devices.

Then they pump in a gas containing silicon, germanium, or platinum precursors at a high pressure. To dope these semiconductor materials, the researchers mix boron or phosphorus compounds into the gas. Heating the fibre creates a ring-shaped layer of crystalline material coating the surface of the pore.
The researchers deposit semiconductor and platinum layers one at a time to form concentric rings that act as device junctions. Badding says the process works because the pores are so narrow that the precursor molecules easily attach to their surface. ''The precursor molecules are so close together that if they weren't in a tiny hole they would just find each other and make a powder,'' he says.
The researchers have already made high-speed optical detectors that work at up to 3 GHz frequencies. They say that the technique could be used to make lasers and modulators as well.
Other research groups have tried to incorporate electronics inside optical fibers by drawing the fibre out from cylindrical starting materials - either silica or polymer - that are embedded with semiconductor wires.

This method is limited to using semiconductors that melt at the same temperatures as the fiber material, Badding says, and it does not allow for doping or making well-defined junctions.
Badding and his colleagues have been able to deposit compound semiconductors such as zinc selenide, which is used in lasers and light-emitting diodes. They're now working on using yet other materials and refining the devices.