Optical modulators using semiconductor nano-structures: Difference between revisions

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== Optical Modulator ==

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 modulators (majority), phase modulators, polarization modulators etc.

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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.

In an AOM a laser beam is caused to interact with a high frequency ultrasonic sound wave inside an optically polished block of crystal or glass (the interaction medium). By carefully orientating the laser with respect to the sound waves the beam can be made to reflect off the acoustic wave-rontsfronts (Bragg diffraction). Therefore, when the sound field is present the beam is deflected and when it is absent the beam passes through undeviated. By switching the sound field on and off very rapidly the deflected beam appears and disappears in response (digital modulation). By varying the amplitude of the acoustic waves the intensity of the deflected beam can similarly be modulated (analogue modulation).

'''Megneto-optic modulator'''

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.

== Optical modulators using semiconductor nano-structures ==

1) Electro-optic modulator of 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

Micrometre-scale silicon electro-optic modulator[1]

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.

1) '''Electro-optic modulator of nano-structures'''

Micrometre-scale silicon electro-optic modulator[1]<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.

2) '''Acousto-optic modulator of nano-structures'''

Acoustic solitons in semiconductor nanostructures[2]<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.

3) '''Magneto-optic modulator of nano-structures'''

Wideband magneto-optic modulation in a bismuth-substituted yttrium iron garnet waveguide[3]<ref>Optics Communications Volume 220, Issues 4-6</ref>

A schematic diagram of the MO modulator is shown in Fig. 1. The MO active layer is a 4.5 μm (Y0.6Bi0.4LuPr)3(FeGa)5O12 film that has been grown on a 450-μm thick (1 1 1)-oriented gadolinium gallium garnet substrate by means of liquid-phase epitaxy. The MO film has an in-plane magnetization with a saturation value (μ0Ms) of 9 mT and a specific Faraday rotation of 5400°/cm at 800 nm. A linearly polarized optical beam from an 800 nm laser diode is focused and edge-coupled to the thin film waveguide. At this wavelength the optical absorption of the MO film is 400 cm−1 and, therefore, the length of the device is designed to be 60 μm. On the surface of the Bi-YIG film, a 50-Ω terminated microstrip transmission line is patterned and used to carry the high-speed electrical signals, I(t). The current transient creates a time-varying magnetic field that has a component, bz(t), along the direction of optical propagation. This component (underneath the microstrip line) acts to tip the magnetization, M, along the propagation direction of the optical beam. A static in-plane magnetic field, by, is applied perpendicular to the light propagation direction, thus ensuring the return of M to its initial orientation after the passage of the current transient. Depending on the component of the magnetization along the z-direction, Mz, the optical beam experiences a rotation of its polarization due to the Faraday effect. The polarization modulation is converted into an intensity modulation via a polarization analyzer, which is detected by a high-speed photodiode.

4) '''Other semiconductor nanostructures of optical modulator'''

MODULATION OF THz RADIATION BY SEMICONDUCTOR NANOSTRUCTURES[4]<ref>MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 35, No. 5, December 5 2002</ref>

As a result of increased demand for bandwidth, wireless short-range communication systems are expected to extend into the THz frequency range. Therefore the fundamental interactions between THz radiation and semiconductors are receiving increasing attention. This new quantum structure is based on the well-established technology for producing high electron mobility transistors where an electron gas is confined at a GaAs/AlxGa1 xAs interface. The electron density at the hetero-interface can be controlled by the application of an external gate voltage, which in turn will alter the transmission/reflection characteristics of the device to an incident THz beam.

3. Applications and Commercial products

1) Electro-optic modulator

3.== Applications and Commercial products ==

1) '''Electro-optic modulator'''

* from THORLABS

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Applications ; Chirp Control for High-Speed Communications (SONET OC-768 Interfaces, SDH STM-256 Interfaces), Coherent communications, C & L Band Operation, Optical Sensing, All-optical frequency shifting.

2) '''Acousto-optic modulator of nano-structures'''

* from ELECTRO-OPTICAL PRODUCTS CORPORATION

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== References ==

{{reflist|2}}

1. Nature 435, 325-327 (19 May 2005)

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2. Journal of Physics: Conference Series 92 (PHONONS 2007)

3. Optics Communications Volume 220, Issues 4-6

4. MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 35, No. 5, December 5 2002