In summary, passive optical receivers are the main components that convert optical signals into electrical signals, while acousto-optic modulators are devices that dynamically modulate optical signals through sound waves. 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 essential functions of that absorber are the following: It needs to initiate the mode locking.