by James J. Refi
Almost since the invention of singlemode fibre, different types have existed. In the early 1980s, dispersion-unshifted fibres had matched clad and depressed clad refractive-index profiles. In the late 1980s, cut-off shifted (more commonly known as “dispersion-shifted” fibre) and dispersion-flattened designs were introduced, which were then followed in the 1990s by various flavours of non-zero dispersion-shifted singlemode fibres. Then came “extended band” dispersion-unshifted fibres (“zero water peak” fibre). More recently, wideband non-zero dispersion-shifted fibres were born. Because these fibres may get connected to one another, and because the various fibres possess different optical characteristics, the International Electrotechnical Commission (IEC) has published a guideline indicating items that should be taken into account when intermixing them.
Published in late 2005, IEC 62000, “Single-mode Fibre Compatibility Guidelines,” was prepared by SC86A/WG1 under the guidance of project leader Daniele Cuomo of Prysmian. The document addresses the effect of connecting fibres that have different (1) cut-off wavelengths, (2) mode field diameters, (3) chromatic dispersion coefficients and slopes, (4) polarisation-mode dispersion, and (5) nonlinear effects. The guideline examines these differences from the aspect of their effect on system performance and splicing.
From the aspect of system performance, the dispersion characteristics of various singlemode fibres can actually be used to advantage. For example, fibres having opposite signs of the dispersion coefficient at a given wavelength can be combined to bring the total link dispersion to near zero-so called “dispersion management.” And fibres having different effective areas can be combined to minimise the overall impact of nonlinear effects. This can be accomplished by placing fibres with large effective areas close to the launch source where optical power levels are high, followed by using fibres with smaller effective areas that have smaller dispersion slopes and higher Raman gain efficiency closer to the receiver where power levels are low.
James J. Refi represents Chromis Fibreoptics in the standards arena and is secretary of IEC SC86A/WG1. He can be reached at firstname.lastname@example.org.