ble by the laser above the noise plateau. Since in a real laser the reflectivity of the external mirror will be on the order of 0.5 to allow sufficient feedback and sensible output optical power, it can be put at approximately Rs/(1 - Rs) 22 1, SO that the aforementioned condition becomes (5.18) Equation 5.18 states an important property of external cavity lasers: the shorter the cavity (i.e., the smaller the Ts), the more sensitive is the laser to small amounts of feedback. Thus, if the feedback has to strongly influence the laser behavior it needs to design a short cavity laser. Moreover, it is to be noted that Rsp/r is practically proportional to the laser linewidth Av (compare Equation 5.8), but for a factor on the order of 1. This factor, due to the condi- tion > appearing in Equation 5.18 can be neglected and the condition can also be written as RE > (AVT) showing that the purer the laser emission (i.e., the smaller the Av), the more the laser is sensible to reflections. In order to study an external cavity semiconductor laser, the rate equations have to be written taking into account the electrical field composing the optical wave and not the optical energy, since the feedback has to be represented in terms of the electrical field. Neglecting the space dependence of the variables and writing the linearly polarized electrical field as E(t) = A(t) exp[iwot+o(t)], the external cavity rate equations may be written as follows: (5.19) where Ti is the round trip time of the gain chip inside the laser and A'(t) = (K/Tj) cos[d(t)]A(t - T) is the intensity of the delayed light injected from the external cavity to the active cavity. The overall ratio between the emitted and the reinjected intensity is indicated by K and the phase coupling angle is given by the following formula: Telecommunication Networks v(t)=wot+(t)-t-) (5.20) In Equation 5.19 also the noise terms have been neglected, since we are interested in the analysis of the deterministic behavior of the laser. From the solution of the rate equations the dyna