Optical Modulators

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, 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.

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.
• Plasmonic modulators are a special type of electro-optic modulators which exploit the formation of plasmons (a special type of electromagnetic excitation) at metal surfaces, which lead to surface plasmon polaritons (SPPs). They can be extremely fast intensity or phase modulators while having a low energy consumption.
• Electroabsorption modulators are semiconductor-based intensity modulators, used e.g. for data transmitters in optical fiber communications.
• Interferometric modulators, e.g. Mach–Zehnder modulators, are mostly exploiting the electro-optic effect in conjunction with interference. They are often fabricated in photonic integrated circuits for optical data transmission.
• Liquid crystal modulators are suitable for, e.g., optical displays and ultrafast pulse shapers. They are also available as spatial light modulators, i.e. with a spatially varying transmission, e.g. for displays.
• Micromechanical modulators (which are microelectromechanical systems = MEMS), e.g. silicon-based light valves and two-dimensional mirror arrays, are particularly useful for projection 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), and may thus be considered as optical modulators in a wider sense. Of course, that kind of device cannot provide arbitrary modulation controlled with an electrical input signal.

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.

Different types of optical modulators are characterized by a set of common performance parameters:

• Modulation bandwidth: The range of modulation frequencies which can be handled. This can range from a few kilohertz (e.g. for liquid crystal modulators) to over 100 GHz (for fast electro-optic modulators).
• Modulation depth and contrast: The maximum achievable change in the optical property (e.g. intensity). For intensity modulators, the extinction ratio is the ratio of the maximum to the minimum output power.
• Insertion loss: The optical power loss caused by the device even in its state of maximum transmission.
• Drive voltage: For electro-optic modulators, the half-wave voltage ($V_\pi$) is a key parameter, indicating the voltage required to induce a phase change of ($\pi$).
• Power handling: The maximum optical power (average or peak) the device can withstand without damage or performance degradation.
• Chirp: Some modulators induce an unwanted phase modulation when modulating the intensity; this is characterized by the chirp parameter.