Common optical active components in optical communications include: semiconductor light sources, semiconductor photodetectors, fiber lasers, optical amplifiers, optical modulators, etc. An optical modulator is a device which is used to modulate a beam of light. The beam may be carried over free space, or propagated through an optical waveguide (optical fibre). Depending on the parameter of a light beam which is manipulated, modulators may be categorized into amplitude modulators. Optical modulators are devices that modify the properties of light, such as its amplitude, phase, frequency, or polarization, in response to an external signal. The inverse process that recovers the encoded information is demodulation.