ERLANGEN, Germany, Aug. 10, 2012 — Simple longitudinal twists that convert photonic crystal fibers (PCFs) into filters were measured and theorized upon; the results suggest new applications in optical communications and will enable the construction of sensors, light amplifiers and lasers.
Glass fibers typically are used to transport light over long distances — for example, fast data transmission over the Internet. PCFs are a novel variant of such optical fibers and are currently used for basic research. Their cross section is similar to that of a honeycomb: tiny hollow tubes surrounding the core run along the circular fiber. They ensure that light travels only in the core, where it is transported with low loss.
Transmission behavior within these PCFs changes significantly if they are twisted around their longitudinal axis — the transmission of certain wavelengths becomes much poorer. The optical fiber becomes spiral and works like a filter. Its behavior can be controlled very easily through the twist: With a strong twist, the dips in transmissions shift toward longer wavelengths.
Now Philip Russell and scientists at Max Planck Institute for the Science of Light are studying 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 the fiber to heat and soften the glass. A supercontinuum source was used to launch light into the twisted fiber core, and an optical spectrum analyzer measured the transmission spectrum to evaluate which wavelengths were effectively suppressed.
Structure of a photonic crystal fiber (PCF). (© Wong et al, Science
2012, doi: 10.1126/science.1223824)
The scientists observed that the transmission in the 400- to 1000-nm wavelength range dipped clearly at four points, which, as expected, shifted toward longer wavelengths when the PCF was more tightly twisted. They also found good agreement with their simulations.
"Earlier studies explained the filter with a kind of lattice effect," Russell said. "However, the wavelengths of the transmission minima would have had to increase with the length of the twist cycle. Our measurements and simulations show that exactly the opposite must be the case."
The filter effect is analogous to the whispering gallery phenomenon discovered in 1878 by the English physicist John William Strutt (Lord Rayleigh). He noticed that sound was guided in a circular path around the dome of St. Paul's Cathedral in London. This same effect also exists in optics, for example when light bounces around inside a glass microsphere many times, forming a high-quality resonance at certain optical wavelengths.
The effect is similar to what happens to wavelengths that are filtered out in the twisted PCFs: Orbital resonances appear in the honeycomb cladding, causing power to drain away laterally from the core instead of flowing straight ahead, so that very little of it arrives at the other end.
"With a sensitive camera, it would be possible to see the side of the fibers glowing in the colors which are particularly strongly suppressed," Russell said.
He anticipates interesting technical applications for the effect.
"What is particularly attractive about it is that we can twist the PCFs almost any way we want after they have been made. That means, for example, that we have a lot of flexibility in making filters for specific wavelengths."
These components play an important role in many areas — for optical data transfer as well as for sensors, fiber lasers and optical amplifiers. It is also possible to vary the twist along the fibers, allowing many different filters to be created. This makes it possible to modify the linear and nonlinear responses of the fibers and, as a result, influence two important parameters for generating a supercontinuum.
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