 needed on each mirror to put it in place at the correct angle. Last thing to consider is the actuating mechanism, which can be either electrostatic or electromagnetic. Electrostatic forces involve the attraction forces of two oppositely charged plates. The key advantage of electrostatic actuation is that it is a very well-consolidated technology, applied in several fields in which MEMS are used like sensors or automotive and very robust and repeatable mechanisms. The disadvantages include nonlinearity in force versus voltage relationship, and requirement of high driving voltages to compensate for the low force potential. Electromagnetic actuation involves attraction between electromagnets with different polarity. The advantage of electromagnetic actuation is that it requires low driving volt- ages because it can generate large forces with high linearity. However, the isolation of the different actuators inside the same switch is not easy and it requires a very accurate design. Since MEMS technology is applied in several fields and MEMS are produced in billions in the world, there are several commercial optical switches based on this technology. A few reference characteristics are reported in Table 5.8 to get a feeling of what can be done in production devices. Telecommunication Networks TABLE 5.8 Characteristics of Practical MEMS-Based Optical Switches with Different Architectures Dimension 16 X 16 1 x 8 56 x56 Architecture Wavelength Range Third Fiber 1240~1640nm 1290-1330nm Window 1530-1570nm Insertion loss 2.0dB 2.2 dB Return loss 45 dB 50 dB Polarization-dependent loss 0.2dB 0.1 dB 0.15 dB Cross-talk -70 dB -75 dB -50 dB Switch speed 40 ms 30 ms Operating voltage 12 VDC 5 VDC 12 VDC Power consumption 700 mW 700 mW Operation temperature -5°C to 70°C 0°C~70°C -5°C to 70°C Size (LxWxH) 142 X 254 x 25 mm 76x93x9.5mm 100 X 80 X 15 mm 5.6.2 Liquid Crystals Optical Switches Another consolidated technology that finds application in large areas of the consumer market is that of liquid crystals, which has been used to