rts, the mir- ror at the intersection of output 1 and output 2 rises and redirects the light signal coming from input 1, while the other mirrors on the matrix remain flat. MEMS devices are attractive because of their miniaturization and potential mass production. The main drawback of an MEMS device is the issue of movable parts and reliability. Bubble Technology With bubble technology, an input signal enters the device with the goal of exiting at output 1. When an input signal and out- put signal intersect in the device, a vapor bubble is formed upon command. This point of intersection houses an index-matching liquid that enables the formation of the bubble. In the presence of a bubble, the input signal is reflected toward output 2. (Without the formation of a bubble, the light sig- Optics Primer nal passes straight through the device toward output 1.) The bubble is of interest because of its connection with a proven technology and lack of mov- ing parts. However, scaling the system can become a problem. Each time the service provider adds a block, more insertion loss is generated and accu- mulated across the system. Liquid Crystal Technology Liquid crystal displays (LCDs) consist of active and passive elements. The active element is the liquid crystal cell and the passive element is the polarization beam splitter. The switching action is dependent on whether or not an electrical field is present. When there is no electrical field in the device, the input signal reflects off the liq- uid crystal pixel and rotates 90 degrees. When an electrical field is present, the light signal travels straight through the active cell without changing direction. Two advantages of LCD technology are the lack of movable parts, making it more reliable than MEMS, and the manufacturing process (inter- estingly, the same used for flat panel displays), which uses a technology that is mature so that materials, fabrication, infrastructure, and processes are already in place and scaled for large quantities. The drawbacks of liquid crystal 