Optical modulator
Encyclopedia
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.
of the material is changed.
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 crystal
s. 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:
Modulation
In electronics and telecommunications, modulation is the process of varying one or more properties of a high-frequency periodic waveform, called the carrier signal, with a modulating signal which typically contains information to be transmitted...
a beam of light
Light beam
A light beam or beam of light is a narrow projection of light energy radiating from a source into a beam. Sunlight is a natural example of a light beam when filtered through various mediums...
. 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
Laser diode
The laser diode is a laser where the active medium is a semiconductor similar to that found in a light-emitting diode. The most common type of laser diode is formed from a p-n junction and powered by injected electric current...
. 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 refractive indexRefractive index
In optics the refractive index or index of refraction of a substance or medium is a measure of the speed of light in that medium. It is expressed as a ratio of the speed of light in vacuum relative to that in the considered medium....
of the material is changed.
Absorption coefficient of the modulator's material can be manipulated by Franz-Keldysh effect
Franz-Keldysh effect
The Franz–Keldysh effect is a change in optical absorption by a semiconductor when an electric field is applied. The effect is named after the German physicist Walter Franz and Russian physicist Leonid Keldysh ....
, Quantum-confined Stark effect
Quantum-confined Stark effect
The quantum-confined Stark effect describes the effect of an external electric field upon the light absorption spectrum or emission spectrum of a quantum well . In the absence of an external electric field, electrons and holes within the quantum well may only occupy states within a discrete set...
, 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
Electro-optic effect
An electro-optic effect is a change in the optical properties of a material in response to an electric field that varies slowly compared with the frequency of light...
, other modulators are made with acousto-optic effect or magneto-optic effect
Magneto-optic effect
A magneto-optic effect is any one of a number of phenomena in which an electromagnetic wave propagates through a medium that has been altered by the presence of a quasistatic magnetic field...
or take advantage of polarization changes in liquid crystal
Liquid crystal
Liquid crystals are a state of matter that have properties between those of a conventional liquid and those of a solid crystal. For instance, an LC may flow like a liquid, but its molecules may be oriented in a crystal-like way. There are many different types of LC phases, which can be...
s. 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 modulatorElectro-optic modulatorElectro-optic modulator is an optical device in which a signal-controlled element displaying electro-optic effect is used to modulate a beam of light. The modulation may be imposed on the phase, frequency, amplitude, or polarization of the modulated beam...
, exploiting the electro-optic effectElectro-optic effectAn electro-optic effect is a change in the optical properties of a material in response to an electric field that varies slowly compared with the frequency of light... - 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.