 waveguide, SO that the entire cavity appears at the end to be a curve. The resulting more practical scheme of the external cavity laser is shown in Figure 5.23. Beside tunability, one of the most interesting features of the high-feedback external-cav- ity laser is its narrow linewidth. By extending the optical cavity length, the spontaneous recombination phase fluctuation in the laser linewidth can be dramatically reduced. Since resonance oscillations are damped by the small reflectivity of the internal-gain chip facet, the linewidth can be considered Lorentian with an FWHM given by (5.23) Telecommunication Networks Laser front mirror Cavity Laser diode Etalon filter Pigtailing Tunable mirror Etalon filter transmittance Voltage Locked to ITU channel GRID control (50 GHz/100 GHz/200 FIGURE 5.23 Practical implementation of an external cavity laser based on a liquid crystal mirror and on a curved cavity gain chip. where P is the power in the mode Rsp is the number of spontaneous emission photons in the mode g is the local gain Avf is the free running linewidth of the gain chip The linewidth enhancement factor of the gain chip is indicated as usual with a. The total cavity loss is indicated with at = - where a is the optical propa- gation loss. It is not difficult to derive from Equation 5.23 substituting realistic values to the param- eters that the external cavity reduces the linewidth of the gain chip of a factor that can be something like 5 or 7 SO that external-cavity lasers with a linewidth of 250 or 500kHz are used as sources in DWDM systems. 5.2.4.3 Laser Arrays Laser arrays, where each laser in the array operates at a particular wavelength, are an alternative to tunable lasers. These arrays incorporate a combiner element, which makes it possible to couple the output to a single fiber. If each laser in the array can be tuned by an amount exceeding the wavelength differ- ence between the array elements, a very wide total tuning range can be achieved. The DFB laser diode array might be the most promising confi