ange state, whatever the state they are occupying (it is said that they have to be latching). On the contrary, routing switches are designed to provide signal routing, either to find the right path in the network or to protect the signal from a failure. Here we will talk mainly about routing switches technologies, even if a few of them are applied also to semi-static switches. Technology for Telecommunications: Integrated Optics and Microelectronics There are a plethora of different technologies that have been used to design optical switches, which can be classified as follows [43]: MEMS: involving arrays of micromirrors that can deflect an optical signal to the appropriate direction Piezoelectric beam steering involving piezoelectric ceramics providing enhanced optical switching characteristics Microfluidic methods involving the intersection of two waveguides SO that light is deflected from one to the other when a liquid micro-bubble is created Liquid crystals that rotate polarized light either 0° or 90° depending on the applied electric field to support switching Thermal methods that vary the index of refraction in one leg of an interferometer to switch the signal on or off Acousto-optic methods that change the refractive index as a result of strain induced by an acoustic field to deflect light SOA amplifiers used as gates in an integrated III-V waveguide matrix Naturally the only way to review in detail all the aforementioned technologies is to devote an entire book to optical switches, and it is not the scope here. Thus, we will concentrate on a few technologies, those allowing us to better underline achievements and challenges in designing optical switches for telecommunications 5.6.1 Micromachining Electromechanical Switches (MEMS) MEMS fabrication techniques utilize the mature fabrication technology of CMOS- integrated circuits (see next section). The fact that silicon is the primary substrate mate- rial used in the integrated circuit and that it also exhibits excellent mechanical properties make it the mo