ely pass from one waveguide to another simply because they are near, even without any contact between the waveguide cores. On the other hand, a sufficiently simple digital optics to push monolithic integration seems faraway, requiring substantial steps ahead before being sufficiently mature for the integration step. 5.5.3.3 Role of Packaging Unless very high-speed circuits are considered, packaging of very large-scale CMOS cir- cuits is a commodity that can be automated for large volumes through suitable packaging machines. Technology for Telecommunications: Integrated Optics and Microelectronics The situation for optics is completely different at least from two points of view. The first is related to the way in which the optical field enters into and exits from the package. Unless the component is designed to work in free space, fiber optics is the most natural way to interface the optical chip with the field. A standard monomode fiber has a core radius around 10 um, and coupling it with a waveguide is more and more difficult while the waveguide gets smaller and smaller. In order to achieve a good coupling, either a mode adaptor has to be realized, increasing both the package Bill of Materials and the amount of work to be done serially chip by chip, or complex structures (like multilevel tapers) have to be realized on the planar circuit SO that a great part of the circuit area is occupied by coupling structures more than by the component for which the circuit has been realized. In any case, active alignment is needed in both the cases to arrive at a coupling on the order of magnitude of 3 dB, an operation that is quite expensive. Besides the coupling problem, high-performance integrated optics has also a tempera- ture stabilization problem. Both in the III-V platform and in the silicon one, the main materials used to realize planar optical devices are quite sensitive to temperature; moreover, fabrication tolerance creates quite a spread on the characteristics of similar devices (another consequence of the analog