Optical modulators using semiconductor nano-structures: Difference between revisions

Revision as of 14:02, 3 June 2009

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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.

=== Electro-optic modulator ===

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 [[PockelPockels 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]]. The ideal electro-optic material possesses all of the following properties:

• large change in [[refractive index]] per volt

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• low dielectric loss tangent (no dielectric heating due to a high-frequency electric field)

• no distortions in modulators output from [[piezoelectric resonances]]

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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-fronts ([[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).

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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]] integration on silicon.

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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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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]].