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From Wikipedia

Optical modulator

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.


Modulator, optical

From Yahoo Answers


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From Youtube

Phase Modulation :Phase ModulationFrequency and phase modulation have become widely spread in the VHF range.One of the advantages of phase modulation is that it's easy to use.For example, VHF direct transformation radio stations which use phase modulation are among the simplest ones, but still remain as effective as the radio stations that use single-band modulation.In addition, phase modulation has other advantages.Thus, the power of a transmitter does not change during the modulation. It remains constant and equals the peak value.The transmitter's power amplifier operates at a constant signal amplitude, so there are no special requirements to its linearity.The transmitter does not require high-power audio signal, so its scheme and design turn out to be simpler than a single-band transmitter.During phase modulation the phase of high-frequency oscillations varies in accordance with the sound signal.The maximum phase deviation, expressed in radians, is called a modulation index.When you change the phase of the signal, the frequency also changes, as the frequency is actually the speed of the phase change over time.In general, the phase and frequency modulations are very similar.They differ only in the spectral composition of the modulating signal applied to the modulator.During the pure sound modulation the phase and the frequency modulations are indistinguishable at all.Phase-modulated signal at the modulator's output has a constant modulation index at any modulation frequency.The ...