 lasers when tuning is driven by the change of the optical cavity length is due to the fact that in integrated devices this is pos- sible only by injecting carriers and changing the refraction index. The situation is completely different in external cavity lasers, where the external part of the cavity can change its length with physical means in order to realize sufficient varia- tions for very wide tuning. The principal scheme of an external cavity laser is shown in Figure 5.21: after reducing the reflectivity of the cleaved active chip facet, part of the emitted radiation is reintroduced in the cavity of an FP semiconductor laser from an external etalon SO as to realize a two- cavity system: the FP cavity inside the chip and the external cavity. The effect of the feedback on the original laser emission can be analyzed by introducing two characteristic factors of the external cavity: 1. The field phase shift = WOT experienced during a single external cavity round trip, where WO is the field angular frequency and T = 2Lc/c is the cavity round trip, L being the cavity length. 2. The feedback strength K, defined as the amount of optical power reinjected into the laser. The feedback strength can be evaluated starting from the reflectivity RF of the external mirror and Rs of the facet of the chip obtaining Antireflection Inter-cavity coated optics facet Cleaved facet External mirror Laser diode Output fiber External cavity FIGURE 5.21 Schematic view of an external cavity laser. Technology for Telecommunications: Integrated Optics and Microelectronics (5.17) where Ts is the laser cavity round trip time. Naturally the external cavity will influence the laser behavior only if the feedback is not too weak. In order to state a qualitative condition that will help us to understand the order of magnitude of the parameters, let us state that the feedback is relevant for the laser dynam- ics only if it is more powerful than the spontaneous emission. This seems a reasonable condition since it requires that the feedback be visi