intensity modulation (fiber optics) (IM)
- Intensity modulation( IM) is a light modulation used in optical transmission systems in which the intensity of a light or laser beam changes in rhythm with the modulation signal. The intensity change of the light beam is analogous to the modulation signal, hence the name Analog Intensity Modulation( AIM). In intensity modulation, the analog modulation signal follows the instantaneous value of the light intensity.
- For intensity modulation of laser light, as applied in laser projectors, there are the electro-optical and the acousto-optical modulation( AOM) methods. In the electro-optical modulator( EOM), the laser beam is changed in its plane of polarization. For this purpose, the polarization of crystals is changed by applying an electric field. The laser light is passed through a crystal that rotates the plane of polarization of the laser light, causing intensity changes. The difference in brightness between the bright and dark beams is 1,000:1, providing high contrast during projection. Acousto-optic modulation is based on the diffraction of the laser beam in an Acoustic Optical Modulator (AOM), a device that affects light in its wavelength and direction of propagation. In this process, sound waves create a refraction grating, Bragg grating, in a crystal that changes with sound frequency. The laser beam, which is passed through the crystal and aligned perpendicular to the sound waves, has its wavelength changed at the refraction grating and is diffracted in rhythm with the sound waves. The diffraction and the resulting interference can be influenced by slightly varying the angle of incidence of the laser beam and the sound waves.