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An optical modulator is an optical device which is used to modulate a beam of light with a 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 methods 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.〔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)〕 However, in silicon photonics, 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.〔Liu, A. et al. Nature 427, 615–618 (2004)〕 Their work showed a high-speed optical active device on silicon—a critical 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 cells, 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. Micrometre-scale silicon electro-optic modulator〔Nature 435, 325–327 (19 May 2005)〕 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. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Optical modulators using semiconductor nano-structures」の詳細全文を読む スポンサード リンク
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