On average, data traffic in the internet grows by 40% each year. This growth, and, in particular, the rapidly increasing interest in videos on demand, in multiplayer online games, and in selling music and software over the World Wide Web, naturally places huge demands on the speed of data transfer. When sending data using conventional electric data transfer, only one electric signal can be transferred per cable. Using optical data transmission, it is possible to send several optical signals simultaneously in one glass fiber without them interfering with one another. This technology, known as wavelength division multiplexing (WDM), is of considerable importance and has one major advantage, namely that even without new infrastructure and using as few as two colors, the transmission capacity of the glass fiber cable can be doubled. The more wavelengths used, therefore, means more data can be transferred. This technology allows an enormous amount of data to be transferred through just one glass fiber 10 times thinner than a human hair. Particularly in large cities where the laying of new glass fiber cables involves considerable cost and difficulty, WDM technology has revolutionized data transfer.