Sunrise Luciol - Glossary

Luciol
Many people have asked us what is the meaning of Luciol.
"Luciole" is the french word for firefly. We adopted this name to show our commitment to be your source of light, which will light up your dark fiber, and help you find out its properties. As you all know, french is a beautiful although complicated language... Words often have extra letters, which are not pronounced. This is precisely the case with "luciole", where the final "e" is not pronounced. So we dropped this "e" to enable our non french-speaking customers to pronounce our name right!

Dispersion
To understand the importance of this effect, we need to step back briefly in time. Till a few years ago, an optical fibre was mainly considered as a pipe, which permitted the transmission of light pulses, encoding the information. The primary factor characterising the fibres was therefore the attenuation. To increase the transmission rate, one had to bring these pulses closer and closer together. A new parameter entered, dispersion, characterising the spread of these light pulses during the propagation. A pulse, which was short at the beginning, could spread out during the propagation, and get mixed with the preceding or following one, thus causing errors in the transmission. The two components of dispersion are: Polarisation Mode Dispersion, or PMD; and Chromatic Dispersion or CD.

Polarisation Mode Dispersion
The first component of the dispersion is Polarisation Mode Dispersion, caused by the fact that, even in a so-called singlemode fiber, there are in fact two propagation modes allowed, with two orthogonal polarisations. Each of these modes may propagate at a slightly different velocity, leading to pulse spreading.

Chromatic Dispersion
The second component of the dispersion is chromatic dispersion, caused by the fact that a light pulse is not monochromatic, i.e. it contains several colours. This chromatic dispersion has become an important factor, limiting the growth of transmission rates. In recent systems, the influence of chromatic dispersion can be eliminated by using dispersion compensators. In general, these compensators are simply fibres with opposite chromatic dispersion: the frequency, which propagated faster in the original fibre, propagates slower in the compensator, and vice versa.

Chromatic dispersion coefficient
The chromatic dispersion coefficient, D, is the parameter, which is usually used to characterize the chromatic dispersion of the fibre. It is generally given in ps/nm.km. For example, a value of D = 4 ps/nm.km means that two light pulses,separated by 1 nm in wavelength, would arrive with a relative delay of 4 ps after propagating through 1 km of fibre. More importantly, it also means that a pulse with a linewidth of , say 0.1 nm, would also spread out by 4 ps after a 10 km propagation.

Photon-counting
In contrast to more usual technologies, the major advantage of photon counting is its extreme sensitivity. Indeed, the photon, or quantum of light, is the smallest amount of light, which can be detected. A standard detector, as used in all telecom applications, can only detect a much larger amount of light, corresponding to hundreds of photons. Photon counting can thus be useful for all types of measurements, which are restricted by the amount of light available.