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Optical modulator
Wednesday, 13 December 2006

An optical modulator is a device that modulates or varies the amplitude of an optical signal in a controlled manner. An optical modulator generates desired intensity, color and the like in the passing light by changing optical parameters such as the transmission factor, refractive index, reflection factor, degree of deflection and coherency of light in the optical system according to the modulating signal. Optical modulators are one of the key devices for realizing such high-performance optical networks. In recent years, enormous volumes of data in communications have been transmitted through high-performance information and communication instruments so that it becomes essential to popularize widespread optical communication networks using optical fibers. In the optical communication networks, high-speed semiconductor lasers or the like are used as key devices thereof, and semiconductor optical modulators are also used for modulating input light beams generated by the semiconductor lasers. Digital communications employing laser beams as optical carriers generally having carrier frequencies in the terahertz range that permit extremely high modulation bandwidths. Much of the optical communications network in place utilizes optical fibers. Optical fiber transmission has played a key role in increasing the bandwidth of telecommunications networks. Optical fiber offers much higher bandwidths than copper cable and is less susceptible to various types of electromagnetic interferences and other undesirable effects. As a result, optical fiber is the preferred medium for transmission of data at high data rates and over long distances. One property of optical fibers that is of concern is dispersion. Dispersion in optical fiber occurs as a result of variation in the refractive index of the optical fiber with wavelength. Modulation of an optical signal results in optical harmonics of the modulation frequency about the carrier frequency. The optical carrier signal is modulated by an RF information signal or many information signals on respective sub-carriers. One common method of modulation uses an optical modulator consisting of complementary electrically responsive optical phase shifters in each of two branches of an interferometer such as a Mach-Zehnder interferometer. The optical modulator is interposed between the optical carrier source (a laser) and the communication channel (a fiber optic cable). Optical modulators have many different uses in optical communication systems. For example, high-speed optical modulators are used to encode information into an optical signal generated by an optical source such as an optical laser, where the information is represented by changes in the amplitude of the optical signal. A low-speed optical modulator, also referred to as an optical attenuator, may be used in conjunction with an optical amplifier in the amplifier stage of an optical communication system. The optical attenuator controls the overall gain of the amplifier stage to account for gradual changes in the received optical signal, for example, as the optical source ages.

In optical fiber communication systems, data is transmitted as light energy over optical fibers. The data is modulated on an optical light beam with an optical modulator. Optical modulators modulate the amplitude or the phase of the optical light beam. A semiconductor optical modulator used for signal modulation typically functions to regulate an intensity of incident light. In this manner, digital signals, which are subjected to intensity modulation (IM), are simply differentiated as ones having intensity higher than a predetermined reference level and ones having intensity lower than the predetermined reference level. Optical modulators are typically based on direct or external modulation. Direct detection is the simplest technique to use for an electro-optic conversion circuit employed in an optical communications system. Direct optical modulators modulate the optical wave as it is generated at the source. In direct modulation, a current that activates a semiconductor laser is turned on and off directly by the "0"s and "1"s of a data signal to control the emission and extinction of the laser beam. When a laser per se is turned on and off directly, however, the light signal experiences a fluctuation in wavelength (chirping) owing to the properties of the semiconductor. The higher the transmission speed (bit rate) of the data, the greater the influence of chirping. When chirping is caused by direct modulation, propagation velocity fluctuates, waveforms are distorted during propagation through optical fiber and it becomes difficult to perform long-distance transmission and transmission at high speed. With direct modulation, the optical source is turned on and off at intervals. The direct modulation method can be realized with a simple system configuration, while the indirect method using an external modulator provides high-quality modulation. For communications involving ultrahigh speeds and long distances, modulation is usually carried out using an external modulator. External optical modulators modulate the optical wave after it has been generated by an optical source. External modulation is used for high transmission speeds of 2.5 to 10 Gbps. In external modulation, a laser diode emits light continuously and the emitted light is turned on and off by the "1"s and "0"s of data using an external modulator. With external modulation, the optical source is operated continuously and its output light is modulated using an optical external modulator. Optical external modulators are superior to direct modulation in many ways. For example, optical external modulators are suitable for many high-speed applications and do not typically affect the wavelengths carrying the data signal as much as direct modulation. Optical external modulators are often based on electro-optic, magneto-optic, acousto-optic, and/or electric field absorption type effects, thus providing additional design flexibility. Accordingly, there are available on markets an electro-optical modulator, a magneto-optical modulator, an acousto-optic modulator, an electric field absorption type modulator and the like. The electro-optical modulator uses the electro-optical effect, the magneto-optical modulator uses the magneto-optical effect, the acousto-optic modulator uses the acousto-optic effect, and the electric field absorption type modulator uses the Franz-Keldysh effect and the quantum-confined Stark effect.

Optical modulators based on the external modulation principles include a Mach-Zehnder interferometric optical modulator (MZ, or MZI). A Mach-Zehnder interferometer optical modulator utilizes a mechanism such that when light propagated through a waveguide is branched in two directions and a modulation signal current is flowed through the center of each branch, there occur magnetic fields of opposite phases with respect to grounds provided on opposite sides in a sandwiching relation to the waveguides, so that the phases of light signals propagated through the respective routes become opposite to each other and the phase lead and lag are offset each other when both lights are later combined together. A Mach-Zehnder interferometric modulator is a dual waveguide device. In operation, an electromagnetic signal, such as a RF or microwave signal, interacts with an optical signal in one of the waveguides over a predetermined distance that is known as the interaction distance. The RF signal propagates in a coplanar waveguide (CPW) mode. A Mach-Zehnder optical modulator is normally constructed such that the phase difference between the two optical waveguides that propagate the split beams is 0 when voltage is not being applied. A typical Mach-Zehnder modulator includes an interferometer having an input waveguide, two arms that branch from the input waveguide, and an output waveguide at the junction of the two arms. The Mach-Zehnder optical modulator includes an optical waveguide formed on an electro-optic substrate, which for exemplary purposes is lithium niobat. The optical waveguide includes a first Y-branch, a first interferometer arm, a second interferometer arm, and a second Y-branch. An optical signal is directed into and propagates in the input waveguide, and is split between the two arms so that approximately one-half of the input optical signal propagates in each of the interferometer arms. A drive voltage is applied to one arm of the interferometer which changes the effective refractive index of that arm and introduces a phase-shift in an optical signal propagating in that arm. In a Mach-Zehnder optical modulator, input light is split into two beams which each undergoes phase modulation and then are combined. In this way, modulation of light intensity is effected by mutual interference. The Mach-Zehnder interferometric optical modulator is widely used as an external modulator particularly for ultra-high-rate optical communication systems because it can provide modulation characteristics which are stable against disturbance and have a good S/N ratio by canceling out in-phase noise components with the push-pull application of a drive voltage. MZ modulators are important because they can be integrated with other optical devices, such as semiconductor lasers, optical amplifiers, or other electronic circuits. Integrated electro-optical devices, such as Mach-Zehnder interferometric optical modulators, are fabricated on substrates of electro-optic material. Among all the known substrate materials, lithium niobate is probably the most widely used because of the enhanced electro-optic properties thereof and the possibility of making low loss optical waveguides.

An electroabsorption modulator (EAM) is used as an optical modulator for use with the external modulation system. The electroabsorption optical modulator (EA modulator) is an optical modulator that utilizes the electroabsorption effect that the optical absorption coefficient of a substance varies depending on the electric field applied to it. The electric absorption type modulator utilizes a mechanism such that if a modulation signal voltage is applied to light propagated through a waveguide, the resulting electric field causes an electric absorption coefficient in a medium to change, thereby intercepting the light. Electroabsorption modulators can be roughly divided into two types: that is, an FAM using a single thick light-absorption layer, and an EAM employing a multiple quantum well (MQW) structure formed by means of stacking thin quantum well layers, each quantum well layer being capable of forming excitons at room temperature. The absorption edge is shifted toward the longer wavelength direction by applying an electric field to the modulator, so that the absorption coefficient is changed, thus modulating a light intensity. An MQW structure comprises a stack of thin layers of narrow bandgap semiconductor material alternating with layers of wide bandgap semiconductor material so that each layer of narrow bandgap material is sandwiched between two layers of wide bandgap material. The alternating structure forms a series of quantum wells located in the narrow bandgap layers that are capable of confining conduction band electrons and valence band holes. The former type of EAM effects extinction by utilization of variation in an absorption spectrum due to the Franz-Keldysh effect, and the latter type of EAM effects extinction by utilization of variation in absorption spectrum due to the Stark effect. The electroabsorption optical intensity modulator can remarkably reduce the waveform chirping phenomenon, as compared with the direct modulation system by the semiconductor laser diode; however, the waveform chirping amount cannot be zero. Among RF optical modulators, an electroabsorption optical modulator having a multiple quantum well is a device having a high-frequency operating speed, low-power consumption and a capability to be integrated with other devices. For these reasons, the electro-absorption optical modulator attracts attention in the optical transmission technology for the ROF link. In an electroabsorption type optical modulator, the amount of carriers comprised of pairs of electrons and holes generated by light absorption increases in accordance with incident light intensity. The electron and hole pairs form an internal electric field so as to cancel an externally applied electric field. The screening effect on the externally applied electric field increases with the intensity level of the incident light, and there is a correlation between the intensity level of the incident light and the change in the absorption coefficient.

Optical modulator is used as the means of converting the electrical signal array into the light signal array. In the field of optical communication, the distributed feed back semiconductor laser (DFB laser) that have stable wavelength with narrow spectrum width are commonly used as light source and generates continuous wave (CW) and the laser beam is modulated responding transmission data by the optical modulator placed in external side of the semiconductor laser, and the modulated light is sent by the transmission line such as optical fiber. In detail, various optical modulators are utilized depending on the speed of signal array used, the transmission distance and the light wavelength. Typical high-speed electro-optical external modulators use a traveling-wave electrode structure to apply the RF signal. Such modulators have a RF transmission line in the vicinity of the optical waveguide. In an optical modulator using a semiconductor material, an absorption coefficient or refraction index can be significantly changed by the Franz-Keldysh effect or quantum confined Stark effect. Each optical modulator shares the same type of material with each light-emitting device so that it can be integrated into a small high-efficiency modulator for external light. Such an optical modulator has achieved a high speed operation in a certain modulation frequency band to a degree as achieved by a dielectric optical modulator. An electromechanical optical modulator is basically a Fabry-Perot cavity comprising the air gap between an optical membrane and a substrate. Modulation of reflected light is based on voltage-controlled movement of the membrane in relation to the substrate. They are particularly useful as optical equalizers, switches for wavelength Add/Drop modules and optical cross-connect mirrors. Optical modulators can generally be categorized into two types depending on their principle of operation. Resonant modulators operate by changing the resonant wavelength to effect switching between the resonant and non-resonant state at a particular wavelength. This change is achieved by altering the optical phase change of the signal as it passes through an "active" medium. Non-resonant modulators operate by modulating the phase and/or the intensity of the optical signal in the "active" medium within the modulator. This switching can be achieved by a wide range of physical phenomena e.g. electro-optic, electro-absorption, electro-mechanical or thermal effects.


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