A team of researchers has developed a method that improves semiconductor fibre optics, which holds significant advantages of silica-based fibre optics, paving the way for a new material structure for global transmission of data.

Silica-based fibre optics is the current technology used for transmitting nearly all digital data. However, silica—glass—fibres can only transmit electronic data converted to light data, requiring external electronic devices that are expensive and consume enormous amounts of electricity. Semiconductor fibres, in comparison, can transmit both light and electronic data and might also be able to complete the conversion from electrical to optical data on the fly during transmission, improving delivery speed, the researchers said.

Think of these conversions as exit ramps on the information superhighway, said Venkatraman Gopalan, professor of materials science and engineering, Pennsylvania State University. The fewer the exits the data takes, the faster the information travels. Call it "fly-by optoelectronics," he said.

Penn_Venkatraman-Gopalan (cr) Figure 1: Venkatraman Gopalan, professor of materials science and engineering, Pennsylvania State University. (Source: Penn State)

In 2006, researchers, led by John Badding, professor of chemistry, physics and materials science and engineering, first developed silicon fibres by embedding silicon and other semiconductor materials into silica-fibre capillaries. The fibres, comprised of a series of crystals, were limited in their ability to transmit data because imperfections, such as grain boundaries at the surfaces where the many crystals within the fibre core bonded together, forced portions of the light to scatter, disrupting the transmission.

A method designed by Xiaoyu Ji, doctoral candidate in materials science and engineering, improves on the polycrystalline core of the fibre by melting a high-purity amorphous silicon core deposited inside a 1.7μm inner-diameter glass capillary using a scanning laser, allowing for formation of silicon single crystals that were more than 2,000 times as long as they were thick. This method transforms the core from a polycrystal with many imperfections to a single crystal with few imperfections that transmits light much more efficiently.

Penn_fibre-optics (cr) Figure 2: Amorphous silicon core inside a 1.7μm inner-diameter glass capillary. (Source: Penn State)

That process, detailed in a trio of articles published in ACS Photonics, Advanced Optical Materials and Applied Physics Letters, demonstrates a new methodology to improve data transfer by eliminating imperfections in the fibre core that can be made of various materials. Gopalan said equipment constraints kept the crystals from being longer.

Because of the ultra-small core, Ji was able to melt and refine the crystal structure of the core material at temperatures of about 398°C-498°C (750-930°F), lower than a typical fibre-drawing process for silicon core fibres. The lower temperatures and the short heating time that can be controlled by the laser power and the laser scanning speed also prevented the silica capillary, which has different thermal properties, from softening and contaminating the core.

"High purity is fundamentally important for high performance when dealing with materials designated for optical or electrical use," said Ji.

The important takeaway, said Gopalan, is that this new method lays out an approach on how a host of materials can be embedded into fibre optics and how voids and imperfections can be reduced to increase light-transfer efficiency.

"Glass technology has taken us this far," said Gopalan. "The ambitious idea that Badding and my group had about 10 years ago was that glass is great, but can we do more by using the numerous electronically and optically active materials other than plain glass. That's when we began trying to embed semiconductors into glass fibre."

Like fibre-optic cable, which took decades to become a reliable data-delivery device, decades of work is needed to create commercially viable, semiconductor fibre networks. It took 10 years for researchers to reach polycrystalline fibres to specifications that are far better, but are still not competitive with traditional fibre-optic cable.

"Xiaoyu has been able to start from nicely deposited amorphous silicon and germanium core and use a laser to crystallise them, so that the whole semiconductor fibre core is one nice single crystal with no boundaries," said Gopalan. "This improved light and electronic transfer. Now we can make some real devices, not just for communications, but also for endoscopy, imaging, fibre lasers and many more."