Photonic crystal fiber yields near-IR solitons

During a postdeadline talk at the annual meeting in November of the IEEE Lasers and Electro-Optics Society (LEOS; San Francisco, CA), researchers from the University of Bath (Bath, England) reported the generation of both white-light supercontinua and near-infrared solitons in a photonic crystal fiber. Constructed of a long thread of silica glass with a periodic array of air holes running down its length, the fiber forms a two-dimensional photonic crystal that is periodic across the transverse plane of the fiber but uniform in the normal direction.

By filling in the central hole, the researchers created a "defect" in the crystal structure with a high index of refraction that consequently guided light by total internal reflection, similar to the core of an optical fiber. A key factor in constructing these photonic crystal fibers is relatively large hole diameters (d) compared to the pitch or distance between the holes (L), according to William Wadsworth, lead author of the paper presented at the meeting.

"In early fibers we were working with a very small [value of] d over L, 0.1 and less. And that gives interesting effects such as endlessly single-mode fiber (single-mode for all wavelengths) and large-mode-area fibers," he said. "But if you make the holes very big, with d over L greater than 0.5 (currently we are using 0.8), then the structure becomes very dispersive and you can modify the dispersion of the guided mode greatly."

Making use of these properties allowed the Bath researchers in the laboratory of Philip Russell to produce solitons at wavelengths below 1.28 µm, when the silica in the optical fiber was no longer anomalous. The high ratio of diameter to pitch allowed them to make the dispersion while the fiber itself remained single-mode.

"We shifted the group velocity dispersion (GVD) zero [point] from 1300 nm down to the 740- to 800-nm range," Wadsworth said. "This is partly a theoretical prediction that was made years ago. Then advances in the technology of making fibers enabled us to produce big holes at small pitches."

Solitons with a characteristic length slightly over 1 m were observed at 850 nm upon pumping them with 200-fs pulses at energies below 0.07 nJ, from a modelocked Ti:sapphire laser. Two octaves of a white-light supercontinuum—from below 400 to above 1600 nm—were observed over a distance of approximately 2 m when the input pulse energy was increased to 2 nJ, at least three orders of magnitude below conventional energy input requirements (see figure).

The fact that measured values of GVD were very close to calculated values indicates that the photonic crystal fiber structure should facilitate design of fibers with desirable dispersion properties, Wadsworth said. "It enables you to produce solitons at short wavelengths which you normally couldn't do at all," he said.

Immediate applications would allow use of solitons for long-distance optical communication in the 1.3-µm band, as well as production of solitons in the multipurpose Ti:sapphire tuning range. Another new design potential would be to tailor group velocity dispersion strength, and consequently soliton length, at particular wavelengths such as 1500 nm.

In the area of white-light continua for applications such as spectroscopy and wavelength metrology, the low input energy requirement should also prove beneficial. "That means you need only use a laser oscillator," Wadsworth said. "You don't need an oscillator and amplifier to give you enough energy for white-light continuous spectra."