Fibre Characterization and Measurements

Abstract

Propagation of light in free-space can not be reliably used due to the significant wave diffraction and to the dependence of the attenuation on atmospheric conditions. Therefore, it is to use the guided propagation of light by using a convenient medium: the optical fibre. The story of optical fibres started in 1963 with the first realization of a semiconductor laser diode emitting at 0.8 μm. In 1970, these laser diodes became stable, powerfull and reliable enough to start the first studies on optical fibres. From 100 to 1000 dB/km in glasses, the fibre attenuation at 0.85 μm then decreased to 20 dB/km in 1970 and down to 0.5 dB/km in 1973. At the beginning of the eighties the theoretical attenuation limit of 0.15 dB/km at 1.55 μm was almost reached. In the middle of the eighties, optical fibres could easily be tailored with common attenuation values of 0.35 dB/km at 1.3 μm and 0.2 dB/km at 1.55 μm. As a consequence, in 1988, the first transatlantic link was realized with 6000 km of single-mode fibre using a signal rate of 280 MBit/s at 1.3 μm. To take advantage of the lowest attenuation, the interest was to work at 1.55 μm. However, the fibre dispersion is greater at 1.55 μm than at 1.3 μm: 20 ps/(nm.km) compared with a maximum of 5 ps/(nm.km). So the dispersion became the limiting factor in high speed transmission. Therefore, new kinds of fibre were designed with a dispersion lower than 5 ps/(nm.km) at 1.55 μm. An instance for their use is the transatlantic link planned in 1996 that will use dispersion shifted fibres supporting a rate of 5 GBit/s at 1.55 μm. Such high performance evidences the need for accurate attenuation and dispersion measurements. The characterization of those parameters will be described in two different sections of this article. If we consider the practical use of fibre, the fibre to fibre coupling losses become the actual limiting factor. That points out the need for the fibre geometrical characteristics measurements that are presented first in this article. Finally, single-mode fibre measurements, cut-off wavelength and mode field diameter, will be presented. Due to their specific status and role, calibration laboratories acting in the optical fibre domain, must use test methods which comply with the international standards given in the references [1 to 6], to issue certificates of calibration for their customers. Therefore, this article presents results obtained by these standard methods.