 (e.g., focusing lenses) R is the reflectivity of the cleaved chip facet The aforementioned equations for power output indicate that, for a given injection cur- rent, the external-cavity laser generally has a somewhat lower power output than that of the solitary diode laser and, moreover, has higher threshold current, as shown in Figure 5.22. As far as tunability is concerned, the emitted frequency is given, with a very good approximation, by the simple equation Technology for Telecommunications: Integrated Optics and Microelectronics Optical power (mW) Gain chip Weak feedback Strong feedback FIGURE 5.22 Modification of the current optical power characteristic of an FP laser as a consequence of the antireflection coating of a facet and Current (mA) the addition of an external cavity. (5.22) where k is an integer. Also in this case, among all the possible oscillating modes, generally only one experiences spontaneous oscillations, that is, the mode having the greater net cavity gain. Different schemes have been proposed to change the external cavity length, but in prac- tice two have been adopted in commercial products: the use of a wavelength-selective liquid crystal mirror that can be tuned by the applied voltage and a mechanically movable grating mounted on a micromachining electromechanical switch (MEMS) actuator. In both cases the selectivity of the tunable mirror does not assure stable single-mode operation. On the other hand, a tunable laser for application in DWDM transmission does not need to be continuously tunable; on the contrary it has to be restricted to the International Association for Standardization in Telecommunications (ITU-T) DWDM wavelengths. Taking into account such a restriction, sufficient wavelength selectivity can be achieved by inserting a solid-state etalon in the laser cavity, whose FSR coincides with the ITU-T grid spacing. In order to have good control of the optical signal reinjection and to avoid the creation of spurious cavities, special gain chips are produced with a curve-active