The easiest way to obtain modulation is modulation of intensity of a light by the current driving the light source (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 called external light modulators.
According to manipulation of the properties of material modulators are divided into two groups, absorptive modulators (absorption coefficient) and [[refractive modulators]] (refractive index of the material). Absorption coefficient 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]] (amplitude & phase modulation), other modulators are made with acousto-optic effect, [[magneto-optic effect]] such as Faraday and [[Cotton-Mouton effects]]. The other case of modulators is spatial light modulator (SLM) which is modified two dimensional distribution of amplitude & phase of an optical wave.
