Optical Modulator - Classification of Optical Modulators

Classification of Optical Modulators

According to the properties of the 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 refractive index of the material is changed.

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

Refractive modulators most often make use of an electro-optic effect. Some modulators utilize an 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. The effect of a refractive modulator of any of the types mentioned above is to change the phase of a light beam. The phase modulation can be converted into amplitude modulation using an interferometer or directional coupler.

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.
• Mechano-optical modulators, in which a mechanical element such as a cantilever or clamped-clamped beam perturbs the optical evanascent field of a guided wave.