FIGURE 5.24 array Block scheme of a laser array used as a tunable laser. A possible scheme of a laser array used as tunable laser is sketched in Figure 5.24, where a free space propagation (FSP) region, similar to that used in array waveguides (AWGs), is used to collect the laser output and an SOA to compensate losses. In principle, this is the easiest way of realizing a tunable laser since it is simply a col- lection in a single chip of a set of well-known devices: DFB lasers, passive waveguides, an MEMS deflector or an FSP, in case of an output SOA to reinforce the signal. In practice, the real difficulty consists exactly in creating a chip where all these compo- nents are realized together. In particular, two kinds of implementation problems arise, design and fabrication problems. From a design point of view, thermal and electrical isolation of all the components is not a trivial problem, since the component footprint has to fit into a standard laser pack- age. Moreover, the design has to be SO accurate that all the individual lasers hit the cor- rect wavelength region SO that each laser can be tuned on the frequencies for which it is designed by simple thermal tuning. In case an MEMS deflector is used to focus the light emitted from the laser that is cur- rently switched on onto the output fiber, this element has to be realized with great care for reliability. As a matter of fact, laser interfaces are the elements with a smaller lifetime in a DWDM terminal even when fixed lasers are used. Further shortening of the laser life span is not acceptable from a system point of view. On the other hand, if an FSP element is used it could be difficult to have a sufficiently high output power without an amplifier at the output of the FSP. In this case, besides the need of a very low-noise and wide-spectrum amplifier, further integration problems could arise. However, laser arrays have been the first "tunable lasers" to hit the market and they still are a type of tunable lasers widely used in practical systems. 5.3 Semic