ctions of a grid of light paths, either as waveguides or as free space beams as is shown in Figure 5.33. In this case the mirrors act as 2x2 switches of a so-called cross-bar matrix (see Section 7.2.1). As an example this can be realized via a mirror that can be lifted and released to deflect or not the light beam, as is shown in Figure 5.34. As evident from the switch architecture, this 2D approach requires N2 mirrors to form an N X N switch. Alternative approaches will be dis- cussed in Section 7.2.1; however, in general, decreasing the number of mirrors causes a more complex interconnection layout that in waveguide circuits is a price that is often hard to pay. A 3D or analog MEMS has mirrors that can rotate about two axes. Light can be redi- rected precisely in space to multiple angles-at least as many as the number of inputs. Since each elementary mirror of this switch has much more than two states, it is expected that a much smaller number of mirrors is needed. As a matter of fact, depending on the exact switch design the number of mirrors varies between N and 2N. The majority of the commercial 3D MEMS use two sets of N mirrors (total of 2N mirrors) to minimize insertion loss as is shown in Figure 5.35; if only N mirrors were used, the port count will be limited by insertion loss that results from a finite acceptance angle of fibers/ lens on the input or on the output side. Waveguide MEMs mirror FIGURE 5.33 Scheme of an MEMS-based cross-bar optical switch. Out-1 Out-n Technology for Telecommunications: Integrated Optics and Microelectronics FIGURE 5.34 Pictorial representation of an MEMS movable mirror for integration in a cross-bar optical switch. ......... 000000000 Input Output FIGURE 5.35 fibers fibers Scheme of a 3D MEMS-based optical switch. This architecture can be scaled to thousands by thousands of ports with high unifor- mity in losses. Inevitably the control of the mirrors, being analog and not digital as in the 2D case, is much more complex than in the 2D case and a continuous analog feedback is