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== Optical modulators using semiconductor nano-structures ==▼
An [[optical modulator]] is a optical device which is used to modulate a beam of light with perturbation device. it is a kind of transmitter to convert information to optical binary signal through [[optical fiber]] ([[optical waveguide]]) or transmission medium of optical frequency in fiber optic communication. There are several method to manipulate this device depending on the parameter of a light beam like [[amplitude modulator]] (majority), [[phase modulator]], polarization modulator etc.▼
▲An [[optical modulator]] is a optical device which is used to modulate a beam of light with perturbation device. it is a kind of transmitter to convert information to optical binary signal through [[optical fiber]] ([[optical waveguide]]) or transmission medium of optical frequency in fiber optic communication. There are several method to manipulate this device depending on the parameter of a light beam like [[amplitude modulator]] (majority), [[phase modulator]], polarization modulator etc.
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.
[[Optical modulators]] can be implemented using Semiconductor Nano-structures to increase the performance like high operation, high stability, high speed response, and highly compact system. Highly compact [[electro-optical modulators]] have been demonstrated in compound semiconductors.<ref>Sadagopan, T., Choi, S. J., Dapkus, P. D. & Bond, A. E. Digest of the LEOS Summer Topical Meetings MC2–-3 (IEEE, Piscataway, New Jersey (2004)</ref> However, in silicon, electro-optical modulation has been demonstrated only in large structures, and is therefore inappropriate for effective on-chip▼
integration. Electro-optical control of light on silicon is challenging owing to its weak electro-optical properties. The large dimensions of previously demonstrated structures were necessary to achieve a significant modulation of the transmission in spite of the small change of refractive index of silicon. Liu et al. have recently demonstrated a high-speed silicon [[optical modulator]] based on a metal–oxide–semiconductor (MOS) configuration<ref>Liu, A. et al. Nature 427, 615–618 (2004)</ref>. Their work showed a high-speed optical active device on silicon—acritical milestone towards [[optoelectronic]] integration on silicon.▼
=== Electro-optic modulator of nano-structures ===
An electro-optic modulator is a device which can be used for controlling the power, phase or polarization of a laser beam with an electrical control signal. It typically contains one or two [[Pockels cell]]s, and possibly additional optical elements such as polarizers. The principle of operation is based on the linear [[electro-optic effect]] (the [[Pockels effect]], the modification of the [[refractive index]] of a nonlinear crystal by an electric field in proportion to the field strength).
The crystal which is covered by electrode may be considered to be a voltage-variable wave-plate. When a voltage is applied, the retardation of laser polarization of the light would be changed while a beam passes through an ADP crystal. This variation in polarization results in intensity modulation downstream from the output polarizer. The output polarizer converts the phase shift into an [[amplitude modulation]].
[[File:optical_mod1.jpg]]▼
Micrometre-scale silicon electro-optic modulator<ref>Nature 435, 325-327 (19 May 2005)</ref>▼
This device was fabricated a shape of the p-i-n ring resonator on a [[silicon-on-insulator]] substrate with a 3-mm-thick buried oxide layer. Both the waveguide coupling to the ring and that forming the ring have awidth of 450 nm and a height of 250 nm. The diameter of the ring is 12 mm, and the spacing between the ring and the straight waveguide is 200 nm.▼
=== Acousto-optic modulator of nano-structures ===▼
▲[[File:optical_mod1.jpg]]
Acousto-optic modulators are used to vary and control laser beam intensity. A Bragg configuration gives a single first order output beam, whose intensity is directly linked to the power of RF control signal. The rise time of the modulator is simply deduced by the necessary time for the acoustic wave to travel through the laser beam. For highest speeds the laser beam will be focused down, forming a beam waist as it passes through the modulator.
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[[File:optical_mod3.jpg]]
A dc magnetic field Hdc is applied perpendicular to the light propagation direction to produce a single ___domain, transverse directed 4~Ms. The rf modulation field Hrf, applied by means of a coil along the light propagation direction, wobbles 4~Ms through an angle of @ and produces a time varying magnetization component in the longitudinal direction. This component then produces an ac variation in the plane of polarization via the longitudinal Faraday effect. Conversion to [[amplitude modulation]] is accomplished by the indicated analyzer.▼
[[File:optical_mod2.jpg]]▼
▲== Optical modulators using semiconductor nano-structures ==
▲[[Optical modulators]] can be implemented using Semiconductor Nano-structures to increase the performance like high operation, high stability, high speed response, and highly compact system. Highly compact electro-optical modulators have been demonstrated in compound semiconductors.<ref>Sadagopan, T., Choi, S. J., Dapkus, P. D. & Bond, A. E. Digest of the LEOS Summer Topical Meetings MC2–-3 (IEEE, Piscataway, New Jersey (2004)</ref> However, in silicon, electro-optical modulation has been demonstrated only in large structures, and is therefore inappropriate for effective on-chip
▲integration. Electro-optical control of light on silicon is challenging owing to its weak electro-optical properties. The large dimensions of previously demonstrated structures were necessary to achieve a significant modulation of the transmission in spite of the small change of refractive index of silicon. Liu et al. have recently demonstrated a high-speed silicon optical modulator based on a metal–oxide–semiconductor (MOS) configuration<ref>Liu, A. et al. Nature 427, 615–618 (2004)</ref>. Their work showed a high-speed optical active device on silicon—acritical milestone towards [[optoelectronic]] integration on silicon.
▲Micrometre-scale silicon electro-optic modulator<ref>Nature 435, 325-327 (19 May 2005)</ref>
▲This device was fabricated a shape of the p-i-n ring resonator on a [[silicon-on-insulator]] substrate with a 3-mm-thick buried oxide layer. Both the waveguide coupling to the ring and that forming the ring have awidth of 450 nm and a height of 250 nm. The diameter of the ring is 12 mm, and the spacing between the ring and the straight waveguide is 200 nm.
▲=== Acousto-optic modulator of nano-structures ===
Acoustic [[solitons]] in semiconductor nanostructures<ref>Journal of Physics: Conference Series 92 (PHONONS 2007)</ref>
Acoustic [[solitons]] strongly influence the electron states in a semiconductor nanostructure. The amplitude of [[soliton]] pulses is so high that the electron states in a [[quantum well]] make temporal excursions in energy up to 10 meV. The subpicosecond duration of the [[solitons]] is less than the coherence time of the optical transition between the electron states and a frequency modulation of emitted light during the coherence time (chirping effect) is observed. This system is for an ultrafast control of electron states in semiconductor nanostructures.
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=== Magneto-optic modulator of nano-structures ===
▲A dc magnetic field Hdc is applied perpendicular to the light propagation direction to produce a single ___domain, transverse directed 4~Ms. The rf modulation field Hrf, applied by means of a coil along the light propagation direction, wobbles 4~Ms through an angle of @ and produces a time varying magnetization component in the longitudinal direction. This component then produces an ac variation in the plane of polarization via the longitudinal Faraday effect. Conversion to [[amplitude modulation]] is accomplished by the indicated analyzer.
▲[[File:optical_mod2.jpg]]
Wideband magneto-optic modulation in a bismuth-substituted yttrium iron garnet waveguide<ref>Optics Communications Volume 220, Issues 4-6</ref>
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