Dispersion-compensating fiber (DCF), which has a large negative dispersion coefficient, is used in long-haul telecommunications networks to counterbalance positive dispersion in the rest of the network; this prevents the signal from being degraded. Engineers at NTT Corp. (Ibaraki, Japan) have created a dual-concentric-core optical fiber with the largest-yet-reported negative dispersion of 13,200 ps/nm/km; the silica fiber has six air holes around a small solid core, with an additional high-refractive-index ring-shaped core surrounding and touching the air holes. The structure is much simpler than photonic-crystal fibers that have been designed for large negative dispersion.
The central core had a diameter of 2.6 µm; the air holes were 0.76 times this diameter. The inner and outer diameters of the ring were 4.2 and 6.0 times the diameter of the core. The refractive indices of the central core and the ring were 1.6% and 0.32%, respectively—higher than the index of the cladding. At a 1555 nm wavelength, the negative dispersion of the fiber varied as a function of input polarization from -8340 to -13,200 ps/nm/km due to fluctuations in the fiber-drawing conditions.
Contact Taiji Sakamoto at [email protected].
