Research: Optical fibre design cuts insertion loss

A team of researchers at the A*STAR Singapore Institute of Manufacturing Technology has proposed a fibre structure that uses a metal core to increase the efficiency of optical fibres. According to the team, optical fibres are rapidly replacing electrical wires as the primary medium for sending digital information over long distances. With the recent solution, the limits the total strength of the optical signal received at the far end can be removed, cutting insertion loss.

Xia Yu and her team based the design of their device on a new type of photonic-crystal fibre (PCF). PCFs include spaces in the glass that run along the entire length of the fibre. When examined in cross-section, these 'air holes' have a honeycomb-like arrangement. Reflection of the incoming light at the numerous air-glass interfaces confines the light to the centre of the fibre.

In a theoretical modelling study, Yu and co-workers showed that the addition of a metal wire through the centre of the fibre improved device efficiency. The incoming light couples to the electrons in the metal wire, and this forms matter-light hybrid particles called surface plasmons, Yu stated.

The researchers simulated the optical confinement of infrared light in a structure with air holes 2?m in diameter. They then divided the light-carrying core of the fibre with a silver wire. Their calculations indicated that the best design has a hole-to-hole distance of 4?m: it enabled a coupling efficiency of nearly 82 per cent.

Another important way to characterise fibre performance is to determine the distance light must travel along the structure before it couples properly with the fibre—the shorter the better. Compared with conventional designs, this metal-core approach enabled a one order of magnitude reduction in this coupling length.

Such a device could be used as an optical switch or to combine or separate optical signals at different wavelengths—a procedure called multiplexing—which is vital for maximising the amount of information optical signals can carry.

"The next step is to fabricate the device," noted Yu. "The structure can be easily realised by replacing the centre air hole with a metal wire during fibre production, or by pumping molten metal into the centre air hole post fabrication."