mic of the external cavity laser can be studied. In particular the following property is derived. The laser operation is strongly depen- dent on the so-called feedback parameter defined as where a is the line- width enhancement factor of the gain chip. If us < 1, the laser is in a regime of weak feedback, the laser cavity coincides with the chip cavity, and the feedback is a perturbation for the laser working. In this regime the rate equations can be solved using the perturbation theory where the "small" parameter is K. If us < 1 but K << 0.1, different regimes alternate with stable and unstable regions where, depending on the cavity length and feedback intensity, there is a strong mode hopping or a single-mode operation. However, when single-mode operation is achieved, the laser behavior in this region is quite unstable and changes in environmental parameters or aging can completely change the laser emission. Moreover, laser tuning is difficult due to this instability. Thus this region is difficult to exploit practically. If us > 1 SO that K > 0.1, a stable operation regime of high injection is reached. The laser cavity is de facto the extended cavity (i.e., the chip plus the external cavity) and the pres- ence of the chip internal facet can be considered a small perturbation to the extended laser working. In this regime the laser is quite stable, its linewidth is sensibly reduced with respect to the value of the free running chip laser, and tuning can be operated by changing the cavity length without altering laser stability and without provoking mode hopping. This is the regime in which practical external cavity tunable lasers work. Considering from now on only the strong feedback regime, the laser output power can be evaluated once the exact laser structure is known. Considering in particular the struc- ture in Figure 5.21, the output power is given by (5.21) where n is the chip quantum efficiency E is the dielectric constant L is the length of the gain chip T is the total loss internal to the external cavity