Optical Modulators

Definition: devices allowing one to manipulate properties of light beams, such as the optical power or phase

An optical modulator is a device which can be used for manipulating a property of light – often of an optical beam, e.g. a laser beam. Depending on which property of light is controlled, modulators are called intensity modulators, phase modulators, polarization modulators, spatial light modulators, etc. A wide range of optical modulators are used in very different application areas, such as in optical fiber communications, displays, for active Q switching or mode locking of lasers, and in optical metrology.

Types of Optical Modulators

There are very different kinds of optical modulators:

• Acousto-optic modulators are based on the acousto-optic effect. They are used for switching or continuously adjusting the amplitude of a laser beam, for shifting its optical frequency, or its spatial direction.
• Electro-optic modulators exploit the electro-optic effect in a Pockels cell. They can be used for modifying the polarization, phase or power of a beam, or for pulse picking in the context of ultrashort pulse amplifiers.
• Electroabsorption modulators are intensity modulators, used e.g. for data transmitters in optical fiber communications.
• Interferometric modulators, e.g. Mach–Zehnder modulators, are often realized in photonic integrated circuits for optical data transmission.
• Fiber-optic modulators can exploit various physical principles. They can be true fiber devices, or contain fiber pig-tailed bulk components.
• Liquid crystal modulators are suitable for, e.g., optical displays and pulse shapers. They can serve as spatial light modulators, i.e. with a spatially varying transmission, e.g. for displays.
• Chopper wheels can periodically switch the optical power of a light beam, as required for certain optical measurements (e.g. those using a lock-in amplifier).
• Micromechanical modulators (which are microelectromechanical systems = MEMS), e.g. silicon-based light valves and two-dimensional mirror arrays, are particularly useful for projection displays.

Bulk-optical modulators, e.g. of the electro-optic type, can be used with large beam areas, and handle correspondingly large optical powers. On the other hand, there are fiber-coupled modulators, often realized as a waveguide modulator with fiber pigtails, which can easily be integrated into fiber-optic systems.

See also: acousto-optic modulators, electro-optic modulators, electroabsorption modulators, phase modulators

Categories: communications, photonic devices