Optical modulator

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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. Depending on the parameter of a light beam which is manipulated, modulators may be categorized into amplitude modulators, phase modulators, polarization modulators etc. Often the easiest way to obtain modulation of intensity of a light beam, is to modulate the current driving the light source, e.g. a laser diode. This sort of modulation is called direct modulation, as opposed to the external modulation performed by a light modulator. For this reason light modulators are, e.g. in fiber optic communications, called external light modulators.

With laser diodes where narrow linewidth is required, direct modulation is avoided due to a high bandwidth "chirping" effect when applying and removing the current to the laser.

Classification of optical modulators

According to the properties of material that are used to modulate the light beam, modulators are divided into two groups: absorptive modulators and refractive modulators. In absorptive modulators absorption coefficient of the material is changed, in refractive modulators changed is refractive index of the material.

Absorption coefficient of the modulator's material can be manipulated by Franz-Keldysh effect, Quantum-confined Stark effect, excitonic absorption, or changes of free carrier concentration. Usually, if several such effects appear together, the modulator is called electro-absorptive modulator.

Refractive modulators most often make use of electro-optic effect, other modulators are made with acousto-optic effect or magneto-optic effect or take advantage of polarization changes in liquid crystals. The refractive modulators are named by the respective effect: i.e. electrooptic modulators, acousto-optic modulators etc. Immediate effect of refractive modulator operation is change of the phase of a light beam. This can be converted into amplitude modulation by interferometers or directional couplers.

Separate case of modulators are spatial light modulators (SLMs). The role of SLM is modification two dimensional distribution of amplitude and/or phase of an optical wave.

See:

• Electro-optic modulator, exploiting the electro-optic effect
• Acousto-optic modulator
• Magneto-optic modulators, using magnetooptic effects such as Faraday and Cotton-Mouton effects, one can modulate the amplitude and frequency of light up to tens of GHz.

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