"A general goal in fiberoptic communications is to carry many distinct channels at once so as to increase the transmission data rate. Wavelength-division multiplexing (WDM) achieves this by using many separate diode-laser sources, each operating at distinct wavelengths. Currently, most WDM systems operate at a channel spacing of 50-100 GHz (0.4-0.8 nm), while each individual laser has an instantaneous linewidth an order of magnitude smaller. Novel systems are approaching 25-GHz channel spacing and 2.5-GHz linewidth. Clearly, there is a sizable amount of wasted bandwidth. The reason for this waste comes primarily from the need to maintain distinct channels over a long period of time, taking into account such things as drift of the diode-laser line centers due to thermal and mechanical effects.
Of course, the desire is to continue decreasing the channel spacing while at the same time decreasing the individual source linewidths. A number of solutions have been proposed for WDM; some of them involve optical feedback into a diode laser with such elements as diffraction gratings, etalons, and prisms. These optical elements act as the spectral tuning object through coupled resonators. Spectral frequencies away from the tuned line center show a poor coupling of the return signal into the waveguide structure of the laser. The result is a narrowing of the linewidth of the ultimate output of the laser. ..."