 design optical switches for routing applications [43]. The scheme of Figure 5.36 shows the operational principle of an elementary 2x2 liquid crystal switch. An input polarization diversity system divides the x and y linear polarizations among two branches of the device. On each branch the light passes through a liquid crystal cell and a polarization splitter, which is deflected or not depending on the orientation of the liquid crystal that is fixed by the applied voltage [46]. After the switching the two polarizations are put together again by a polarization splitter. When liquid crystals started to be used in optical switches their performances were hardly limited by a highly temperature-dependent loss and a very slow switch time. From Out 1 Out 2 FIGURE 5.36 Scheme of a polarization diversity 2x2 optical switch based on two liquid crystal cells (LQ1 and LQ2), each of which work on one of the polarizations (x and y) after their separation via a polarization beam splitter. Technology for Telecommunications: Integrated Optics and Microelectronics that date, however, liquid crystal cells have gone several steps ahead and now we can real- ize cells with a switch time on the order of 300 us and a loss on the order of 1.2 dB per cell. These cells also retain the advantage of liquid crystals to consume a low power and to incorporate easily other functionalities inside the switch like broadcasting (obtained by switching polarization of 45°), selective attenuation used to equalize the switch output, and other possible features. On the other hand, liquid crystal-based devices are necessarily built on micro-optics; thus they scale with difficulty to include thousands of elementary cells, like MEMS do. For this reason, while liquid crystals are for sure competitive when small switches are considered, it is difficult for large switches to be fabricated with this technology. 5.6.3 Wavelength-Selective Switches An important application of both LC (liquid crystals) and MEMS is in wavelength selective switches (WSSs), a type of