Supercontinuum generation in optical fibers (the conversion of ultrashort pulses into broad spectrums of light) has helped to enable optical clocks that are accurate to within a second every million years, for which Roy Glauber, John Hall, and Theodor Hansch received the Nobel Prize in 2005. Since then, many researchers have generated supercontinua in optical fibers for coherence tomography, metrology and biomedical applications, despite the fact that the mechanism behind it has remained obscure. Now researchers at the University of Bath (England) claim to have explained the mechanism enabling supercontinuum generation.
"Creation and manipulation of supercontinua in photonic crystal fibers has been done in an ad-hoc way without [researchers] knowing exactly why [it happens]," said researcher Dmitry Skryabin. "Now we should be able to be much more precise when using them." Skryabin worked on the project with colleague Andrey Gorbach, both of whom are researchers at the Centre for Photonics and Photonic Materials of the Department of Physics at the University of Bath.
Supercontinua spectra, which range from near-ultraviolet to infrared (hundreds of times broader that the light driving the fiber) are in stark contrast to the monochromatic light generated by most lasers. Such broad spectral light could enable telecommunications signals to be encoded on many wavelengths simultaneously, multiplexing the amount of information able to be carried on a single fiber.
Other researchers have traced supercontinuum generation to optical solitons, self-reinforcing waves that maintain their shapes without dissipation over long distances. However, the precise mechanism by which solitons enable the creation of such broad spectra of light has been unclear. Skryabin and Gorbach now claim to have solved the mystery.
When short pulses of light are sent down the photonic crystal fibers that create supercontinua, the broadening of their wavelengths across the entire visible and infrared spectrum is caused by solitons that block the light pulses behind them, forcing them to shorten in wavelength and become progressively bluer, while the solitons themselves lengthen, becoming redder. This dual effect broadens the spectrum at both ends simultaneously, resulting in ultrabroad-bandwidth light the fibers emit that is characteristic of supercontinua.
The researchers are now investigating this interaction between light pulses and solitons, hoping to further explain the mechanism with analogies to the way that gravity acts on objects trapped inside a moving container, such as an elevator.