Fibres with a filtering effect: twisted photonic crystal fibres suppress specific optical wavelengths
03 Sep 2012
A simple longitudinal twist converts certain microstructured optical fibres into filters. Researchers at the Max Planck Institute for the Science of Light in Erlangen have measured this effect in detail in photonic crystal fibres and found a theory to explain it. The results of their research will allow new applications in optical communications and the construction of lasers, sensors and light amplifiers.
Glass fibres are used generally to transport light over long distances – for example, fast data transmission over the internet. Photonic crystal fibres (PCF) are a novel variant of such optical fibres, currently used mainly in basic research. Their cross-section is reminiscent of a honeycomb: tiny hollow tubes surrounding the core run along the circular fibre. They ensure that the light only travels in the core where it is transported with low loss.
The transmission behaviour of photonic crystal fibres changes significantly if they are twisted around their longitudinal axis – the transmission of certain wavelengths becomes much poorer. The optical fibre becomes spiral and works like a filter. The behaviour can be controlled very easily through the twist: with a stronger twist, the dips in transmission shift towards longer wavelengths.
The team of researchers working in Erlangen led by Philip Russell has studied this effect in detail. They secured one end of a PCF and used a motor to rotate the other accurately around its axis while scanning a carbon dioxide laser along to fibre to heat and soften the glass.
Light from a supercontinuum source, able to emit light almost equally over a wide range of wavelengths, was then launched into the twisted fibre core and an optical spectrum analyser used to measure the transmission spectrum, i.e., to find out which wavelengths are suppressed particularly effectively.
In the experiment, the transmission in the wavelength range from 400 to 1000 nanometres dipped clearly at four points which, as expected, shifted towards longer wavelength when the PCF was more tightly twisted.
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