Up to 100 km of 1310-nm single-mode fiber could be used to carry 1550-nm signals by adding just 1 km of a new fiber developed by researchers at the Institute for Research in Optical and Microwave Communications (IRCOM) in Limoges, France. The new waveguide has a negative chromatic dispersion of 1800 ps/(nm.km), higher than any other for the design wavelength. The theoretical model used to design the fiber suggests that this dispersion could be optimized even further.
Switching to 1550-nm transmission is desirable because it would allow the use of devices such as erbium-doped fiber amplifiers. Unfortunately, the 1310-nm fiber already in the ground produces chromatic dispersion at that wavelength. IRCOM researchers have suggested that the best way to compensate for this dispersion is to use fibers passively, and they have discovered an efficient way of doing so.
The dispersion-compen- sating fiber (DCF) consists of two concentric, asymmetric cores (see figure) and works because it contains two wavelength-dependent supermodes. For wavelengths below the design, waveguiding happens almost entirely in the inner core, whereas for those above the design, confinement takes place in the outer core. For the second supermode, the opposite situation occurs. In each case, it is only at the design wavelength-here 1550 nm-that coupling between the two cores takes place and strong dispersion (negative and positive, respectively) occurs.
The preform for the fiber was created through modified chemical-vapor deposition, and the main difficulty was the tight tolerances necessary for correct operation. Just a 5% difference in the core radius or index difference could cause the design wavelength to be out by up to 100 nm for the central core and 50 nm for the outer core. Some variation from the preform design, made during fabrication, was compensated for in choosing the drawing diameter.
One issue that concerned researchers was exciting the correct supermode. In fact, it turned out to be extremely difficult not to do so. The chromatic dispersion, measured many times and in many configurations, was in good agreement with the theoretical model, and the full-width-half-maximum of the dispersion curve of the bier was measured at 24 nm. For more information, contact J.-L. Auguste at email@example.com.
J.-L. Auguste et al., Elec. Lett. 36 (20), pp. 1689-1691 (Sept. 28, 2000).
Sunny Bains is a scientist and journalist based in London, England.
The refractive index profile of the IRCOM dispersion-compensation fiber must meet strict tolerances in order to work at the 1550-nm design wavelength.