 figure, the laser threshold, the dependence of the laser behavior on tempera- ture, and the saturation of the cavity gain are evident, SO that at each temperature a maxi- mum emitted power exists at the edge of saturation. Above the maximum emitted power, the gain saturation drives a decrease of the emitted power while increasing the driving current. When emitting in continuous wave (CW) mode, a semiconductor laser is affected both by phase and by intensity noise due to the amplified spontaneous emission into the laser cavity and to the fluctuations of the carrier density. The noise characteristics can be obtained from the rate equations with a perturbation approach, at least as far as the noise is small [7]. In particular, substituting the variables of the rate equations the following expressions N =No+8N =So+8S (5.5) 0=00+80 where No, So, and are the stationary solutions of the rate equations without noise, and lin- earizing with respect to the noise terms, the noise statistical characteristics can be derived. In particular, the linearized equations assume a simpler form by introducing the vector X = (S, N, 0) and the matrix = Tos INS INN TON ISN Tso toto ayGn (5.6) Output power (mW) FIGURE 5.6 Typical emitted power versus current characteristic of a DFB laser. Current (mA) Technology for Telecommunications: Integrated Optics and Microelectronics In order to evidence the macroscopic noise terms in the expression of the emitted field, it can be written as (5.7) where r(t) and Q(t) are the relative intensity noise (RIN) and the phase noise I is the end mirror transmittance Tc is the cavity round trip time that allows us to pass from internal energy to emitted power y is the phase contribution of the extraction mirror The power spectral density of the RIN is given by (5.8) where it is not difficult to recognize the typical behavior of a resonance, whose natural frequency is S2 = and whose dumping rate is (TNN+I'ss). The observation that the laser behavior is characterized by a dumped resonance is very general, no