hin each fiber input on one port onto a fiber on an output port. Attempts to create an optical switch include the use of pop-up and other mirrors, etching of planar arrays, and the use of intersecting wave- guides. above the substrate. This permits a voltage to be applied to circuitry on the substrate, which in effect pulls on the side of a mirror, tilting the mirror at an angle commensurate with the applied voltage to reflect an input beam to an output port. Because this method of switching is elec- trically driven, it is faster than the MEMS method, which is an electro- mechanical method. A third design being tested employs pop-up mirrors. This design is referred to as a digital MEMS switch since each mirror can be placed in one of two possible positions, either flat along the surface or in a pop- up position. In the pop-up position the mirror intercepts a light beam and reflects it to an applicable output port. Although this switching method is less likely to misdirect a beam of light, the design mechanism is more complex. A fourth method of optical switching being considered involves the use of two intersecting sets of parallel waveguides set in a solid sub- strate. Similar to the composition of an optical fiber core, the waveguide layers have a higher refractive index than does the surrounding material; consequently, the light flows within the waveguide. This method also involves placement of a tiny set of liquid-filled holes at intersecting points. When the holes are filled with liquid, light passes through the Chapter Six intersection. However, draining the hole results in the formation of a bubble at the intersection, causing a change to the refractive index, which reflects light into the intersecting waveguide. This type of switch is referred to as a microbubble switch. Although it may be several years until the deployment of optical switches is commonplace, it is probably a given that one or more of the technologies described above will illuminate the future. Now that we have a background in the directi