I.INTRODUCTION
An optical modulator is a device which can be used for manipulating a property of light – often of an optical beam, e.g. a laser beam. Depending on which property of light is controlled, modulators are called intensity modulators, phase modulators, polarization modulators, spatial light modulators, etc. A wide range of optical modulators are used in very different application areas, such as in optical fiber communications, displays, for active Q switching or mode locking of lasers, and in optical metrology.[1]
According to the properties of the material that are used to modulate the light beam, modulators are divided into two groups: absorptive modulators and refractive modulators. In absorptive modulators the absorption coefficient of the material is changed, in refractive modulators the refractive index of the material is changed. [2]
The absorption coefficient of the material in the modulator can be manipulated by the Franz-Keldysh effect, the Quantum-confined Stark effect, excitonic absorption, changes of Fermi level, or changes of free carrier concentration. Usually, if several such effects appear together, the modulator is called an electro-absorptive modulator.
Refractive modulators most often make use of an electro-optic effect. Some modulators utilize an acousto-optic effect or magneto-optic effect or take advantage of polarization changes in liquid crystals. The refractive modulators are named by the respective effect: i.e. electrooptic modulators, acousto-optic modulators etc. The effect of a refractive modulator of any of the types mentioned above is to change the phase of a light beam. The phase modulation can be converted into amplitude modulation using an interferometer or directional coupler. [2]
Separate case of modulators are spatial light modulators (SLMs). The role of SLM is modification two dimensional distribution of amplitude and/or phase of an optical wave. [2]
II.TYPES OF OPTICAL MODULATORS
An acousto-optic modulator (AOM) is a device which can be used for controlling the power, frequency or spatial direction of a laser beam with an electrical drive signal. It is based on the acousto-optic effect, i.e. the modification of the refractive index by the oscillating mechanical pressure of a sound wave.
The key element of an AOM is a transparent crystal (or piece of glass) through which the light propagates. A piezoelectric transducer attached to the crystal is used to excite a sound wave with a frequency of the order of 100 MHz. Light can then experience Bragg diffraction at the traveling periodic refractive index grating generated by the sound wave; therefore, AOMs are sometimes called Bragg cells. The optical frequency of the scattered beam is increased or decreased by the frequency of the sound wave (depending on the propagation direction of the acoustic wave relative to the beam) and propagates in a slightly different direction. (The change in direction is smaller than shown in Figure 1, because the wavenumber of the sound wave is very small compared with that of the light beam.) The frequency and direction of the scattered beam can be controlled via the frequency of the sound wave, whereas the acoustic power is the control for the optical powers. For sufficiently high acoustic power, more than 50% of the optical power can be diffracted – in extreme cases, even more than 95%.[2]
Figure 1: Schematic setup of a non-resonant acousto-optic modulator. A transducer generates a sound wave, at which a light beam is partially diffracted. The diffraction angle is exaggerated. [3]
The acoustic wave may be absorbed at the other end of the crystal. Such a traveling-wave geometry makes it possible to achieve a broad modulation bandwidth of many megahertz. Other devices are resonant for the sound wave, exploiting the strong reflection of the acoustic wave at the other end of the crystal. The resonant enhancement can greatly increase the modulation strength (or decrease the required acoustic power), but reduces the modulation bandwidth.
Common materials for acousto-optic devices are tellurium dioxide (TeO2), crystalline quartz, and fused silica. There are manifold criteria for the choice of the material, including the elasto-optic coefficients, the transparency range, the optical damage threshold, and required size. One may also use different kinds of acoustic waves. Most common is the use of longitudinal (compression) waves. These lead to the highest diffraction efficiencies, which however depend on the polarization of the optical beam. Polarization-independent operation is obtained when using acoustic shear waves (with the acoustic movement in the direction of the laser beam), which however make the diffraction less efficient.
There are also integrated-optical devices containing one or more acousto-optic modulators on a chip. This is possible, e.g., with integrated optics on lithium niobate (LiNbO3), as this material is piezoelectric, so that a surface-acoustic wave can be generated via metallic electrodes on the chip surface. Such devices can be used in many ways, e.g. as tunable optical filters or optical switches. [3]
An electro-optic modulator (EOM) (or electrooptic 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. Different types of Pockels cells are shown in Figure 1 and are described more in detail in the article on Pockels cells. The principle of operation is based on the linear electro-optic effect (also called the Pockels effect), i.e., the modification of the refractive index of a nonlinear crystal by an electric field in proportion to the field strength.
Frequently used nonlinear crystal materials for EOMs are potassium di-deuterium phosphate (KD*P = DKDP), potassium titanyl phosphate (KTP), beta-barium borate (BBO) (the latter for higher average powers and/or higher switching frequencies), also lithium niobate (LiNbO3), lithium tantalate (LiTaO3) and ammonium dihydrogen phosphate (NH4H2PO4, ADP). In addition to these inorganic electro-optic materials, there are also special polymers for modulators. [4]
Figure 2: Pockels cells of various types. [4]
The voltage required for inducing a phase change of π is called the half-wave voltage (Vπ). For a Pockels cell, it is usually hundreds or even thousands of volts, so that a high-voltage amplifier is required. Suitable electronic circuits can switch such large voltages within a few nanoseconds, allowing the use of EOMs as fast optical switches. In other cases, a modulation with smaller voltages is sufficient, e.g. when only a small amplitude or phase modulation is required. [4]
3. Electroabsorption modulators
An electroabsorption modulator (or electro-absorption modulator) is a semiconductor device which can be used for controlling (modulating) the intensity of a laser beam via an electric voltage (→ optical modulators). Its principle of operation is based on the Franz–Keldysh effec[5,6], i.e., a change in the absorption spectrum caused by an applied electric field, which changes the bandgap energy (thus the photon energy of an absorption edge) but usually does not involve the excitation of carriers by the electric field. [7]
Most electroabsorption modulators are made in the form of a waveguide with electrodes for applying an electric field in a direction perpendicular to the modulated light beam. For achieving a high extinction ratio, one usually exploits the quantum-confined Stark effect in a quantum well structure.
Compared with electro-optic modulators, electroabsorption modulators can operate with much lower voltages (a few volts instead of hundreds of thousands of volts). They can be operated at very high speed; a modulation bandwidth of tens of gigahertz can be achieved, which makes these devices useful for optical fiber communications. A convenient feature is that an electroabsorption modulator can be integrated with a distributed feedback laser diode on a single chip to form a data transmitter in the form of a photonic integrated circuit. Compared with direct modulation of the laser diode, a higher bandwidth and reduced chirp can be obtained.
4.Interferometric Modulator of Mach-Zehnder
An interferometer is an optical device which utilizes the effect of interference. Typically, it starts with some input beam, splits it into two separate beams with some kind of beam splitter (a partially transmissive mirror), possibly exposes some of these beams to some external influences (e.g. some length changes or refractive index changes in a transparent medium), and recombines the beams on another beam splitter. The power or the spatial shape of the resulting beam can then be used e.g. for a measurement.
The Mach–Zehnder interferometer was developed by the physicists Ludwig Mach and Ludwig Zehnder. As shown in Figure 3, it uses two separate beam splitters (BS) to split and recombine the beams, and has two outputs, which can e.g. be sent to photodetectors. The optical path lengths in the two arms may be nearly identical (as in the figure), or may be different (e.g. with an extra delay line). The distribution of optical powers at the two outputs depends on the precise difference in optical arm lengths and on the wavelength (or optical frequency). [8]
Figure 3: Mach–Zehnder interferometer.[8]
If the interferometer is well aligned, the path length difference can be adjusted (e.g. by slightly moving one of the mirrors) so that for a particular optical frequency the total power goes into one of the outputs. For misaligned beams (e.g. with one mirror being slightly tilted), there will be some fringe patterns in both outputs, and variations of the path length difference affect mainly the shapes of these interference patterns, whereas the distribution of total powers on the outputs may not change very much. [8]
III.CONCLUSION
In a nutshell, there are four conclusions that i get according to the references that i have read about optical modulators. First, Acousto-optic modulators are based on the acousto-optic effect. They are used for switching or continuously adjusting the amplitude of a laser beam, for shifting its optical frequency, or its spatial direction.; second, Electro-optic modulators exploit the electro-optic effect in a Pockels cell. They can be used for modifying the polarization, phase or power of a beam, or for pulse picking in the context of ultrashort pulse amplifiers.; third, Electroabsorption modulators are intensity modulators, used e.g. for data transmitters in optical fiber communications.; fourth, Interferometric modulators, e.g. Mach–Zehnder modulators, are often realized in photonic integrated circuits for optical data transmission.
REFERENCES
[1]Optical modulator, accessed on 9 october 2016, https://www.rp-photonics.com/optical_modulators.html
[2]optical modulator, accessed on 10 october 2016, https://en.wikipedia.org/wiki/Optical_modulator
[3]Acousto-optic modulator, accessed on 10 october 2016, https://www.rp-photonics.com/acousto_optic_modulators.html
[4]Electro-optic modulator, accessed on 10 october 2016, https://www.rp-photonics.com/electro_optic_modulators.html
[5] L. V. Keldysh, “Behaviour of non-metallic crystals in strong electric fields”, J. Exp. Theor. Phys. (USSR) 33, 994 (1957); translation: Sov. Phys. JETP 6, 763 (1958)
[6]W. Franz, “Einfluß eines elektrischen Feldes auf eine optische Absorptionskante”, Z. Naturforsch., Teil A 13, 484 (1958).
[7]Electroabsorption modulators, accessed on 10 october 2016, https://www.rp-photonics.com/electroabsorption_modulators.html
[8]Interferometers, accessed on 11 october 2016, https://www.rp-photonics.com/interferometers.html