OPTICAL FIBER: Large-effective-area fiber minimizes nonlinearities

ROCHESTER, NY--In long-haul telecommunications lines, signal degradation caused by accumulated dispersion and nonlinear effects limits repeater-to-repeater transmission distances and system bit rate. Engineers can compensate for dispersion in existing systems by adding negative dispersion fiber that allows such transmission lines to operate at 10 Gbit/s. Nonlinear effects persist, however, further degrading system performance as data rates increase. A group at Corning Inc. (Corning, NY) has addressed this problem by inventing a large-effective-area fiber (LEAF) that minimizes fiber-induced nonlinearities, appreciably extending the repeater-to-repeater transmission distance for long-haul systems. Project leader Yanming Liu reported the work at the recent OSA Annual Meeting (paper TuO1).

Nonlinear effects in optical fiber can attenuate and distort the temporal profiles of signals; at high bit rates, signal degradation due to nonlinear effects becomes pronounced. While nonlinear degradation accumulates with transmission distance, it is inversely proportional to effective fiber area, a relationship that offers a means for improving performance of high-speed systems.

Using outside vapor deposition, the Corning group has fabricated two large-area fibers with different index profiles. The first fiber profile incorporates a triangular index core, with an outer enhanced-index ring. The triangular core shifts the dispersion wavelength to 1550 nm. The outer ring modifies the modal distribution of light propagating through the fiber, yielding an effective area of about 80 µm², compared to an effective area of 50 µm² for conventional dispersion-shifted fiber.

The index profile in the second design consists of two off-axis rings, with the inner ring causing the same dispersion wavelength shift as the triangular profile. The dual-ring profile produces a flat-top field distribution, so the effective area of this fiber is several percent larger than that of the triangular-core profile.

The use of large-effective-area fiber can extend transmission-line repeater spacings by more than 20%. By minimizing the number of repeaters required for a given transmission line, the fiber opens the door to more-economical and reliable systems that can carry more data channels at a higher bit rate per channel before the onset of nonlinear signal degradation.