Single-mode sources

During transmission on an optical fiber, the different wavelengths propagate at different speeds: this is the effect of chromatic dispersion. If we use pulses emitted by a multimode laser (emitting at several wavelengths) such as a Fabry-Perot laser, these will widen during transmission and overlap, so that the maximum distances for correct transmission will only be a few hundred meters. To overcome this drawback, lasers emitting at a single wavelength have been developed since the 1980s. The principle is to use wavelength-selective mirrors, i.e. mirrors that reflect only a very narrow spectral range. The most common of these lasers are Distributed FeedBack Lasers (DFB). The mirror is distributed along the cavity: a grating is etched above...