Optical active products are devices and equipment that actively manipulate, process, or generate optical signals for various applications in telecommunications, data communications, and other fields where optical communication is required. Compared to conventional metallic cables, optical fiber provides an advantage of low loss (~ 0. 2dB/km) and wide bandwidth (several hundred MHz to THz) to enable long-distance, high-capacity communication. ▶. Active components require some type of external energy either to perform their functions or to be used over a wider operating range than a passive device, thereby offering greater application flexibility. This chapter teaches how stimulated emission produces laser beams in semiconductor materials.