More power with less loss

MIT researchers have developed a photonic bandgap fibre that conducts an intense stream of laser light that would melt traditional fibre optic materials.

MIT researchers have developed a photonic bandgap fibre, which has a hollow core surrounded by a highly confining reflective surface. The fibre conducts an intense stream of laser light that would melt traditional fibre optic materials.

‘Due to the efficient confinement of light in the hollow core, we are able to utilise materials that would normally be damaged under such intense illumination conditions,’ said Yoel Fink, assistant professor of materials science and engineering.

To create the fibre, the researchers identified a pair of materials that have very different optical properties yet soften at the same temperature.

These materials are layered in alternating thicknesses to create a hollow pre-form – a scaled-up version of the final fibre. When the pre-form is fed into a furnace and drawn into a fibre, the layers reduce in thickness to micrometer dimensions, resulting in a mirror that confines light to the hollow core.

The transmission window is determined by the layer thickness and thus can be scaled to target a wide range of wavelengths.

Tens of meters of fibre with ‘transmission losses … orders of magnitude lower than those of the constituent materials’ demonstrate that low attenuation can be achieved through structural design rather than high-transparency material selection, according to the researchers.

The researchers chose to concentrate on the transmission of 10.6-micron light along the fibre because there are ‘no good fibres at this wavelength, and yet very strong, low-cost lasers exist at this wavelength that may be useful for a variety of applications,’ said Shandon D. Hart, a graduate student in the Department of Materials Science and Engineering.

The new fibre would allow a carbon dioxide laser’s high power to be transmitted over longer distances than are possible today.

Possible applications include medical treatments that necessitate high-power delivery, such as surgery or facilitating the breakup of kidney stones, and medical diagnosis requiring broad-band infrared transmission such as detecting cancerous cells with spectroscopy. For manufacturing and materials processing, the fibre may, in the future, transmit sufficient laser light to cut metal. Another potential spectroscopic application involves the construction of a fibre-optic sensor.

‘The significance of this work is that it clearly demonstrates a key attribute of photonic bandgap fibers, namely the ability to achieve lower losses than their index-guided counterparts,’ said John D. Joannopoulos, Francis Wright Davis Professor of Physics at MIT.