Demonstrations of soliton propagation, pulse compression, and visible-supercontinuum generation all require optical fibers with anomalous dispersion-in other words, a dispersion greater than zero. Because material dispersion of conventional solid-silica fibers is negative at wavelengths below 1300 nm, it was thought that only difficult-to-fabricate photonic-crystal or holey fibers could be used for these applications. But now, researchers at OFS Laboratories (Somerset, NJ) have developed what they believe to be the first all-solid, silica-based, index-guided fiber that achieves anomalous dispersion at a variety of wavelengths below 1300 nm.
The secret, say the researchers, is exploiting the strong positive waveguide dispersion of the LP02 mode for a specially designed high-order-mode (HOM) fiber--waveguide dispersion for the LP01 mode in this fiber is negative. The researchers fabricated special fibers containing long-period gratings that efficiently couple an incoming LP01 mode and the converted LP02 mode with identical group velocities, achieving an anomalous dispersion of +60 ps/nm km at 1080 nm. This all-silica HOM fiber module was then used as the positive-dispersion element to enable mode-locking in an ytterbium-based femtosecond fiber ring laser. Contact Siddharth Ramachandran at [email protected].