An electro-absorption modulator (EAM) is a

semiconductor A semiconductor is a material which has an electrical conductivity value falling between that of a conductor, such as copper, and an insulator, such as glass. Its resistivity falls as its temperature rises; metals behave in the opposite way. ...

device which can be used for modulating the intensity of a laser beam via an electric voltage. Its principle of operation is based on the Franz–Keldysh effect, i.e., a change in the absorption spectrum caused by an applied electric field, which changes the

bandgap In solid-state physics, a band gap, also called an energy gap, is an energy range in a solid where no electronic states can exist. In graphs of the electronic band structure of solids, the band gap generally refers to the energy difference ( ...

energy (thus the photon energy of an absorption edge) but usually does not involve the excitation of carriers by the electric field. For modulators in telecommunications, small size and modulation voltages are desired. The EAM is candidate for use in external modulation links in telecommunications. These modulators can be realized using either bulk semiconductor materials or materials with multiple

quantum dots Quantum dots (QDs) are semiconductor particles a few nanometres in size, having optical and electronic properties that differ from those of larger particles as a result of quantum mechanics. They are a central topic in nanotechnology. When the ...

wells Wells most commonly refers to: * Wells, Somerset, a cathedral city in Somerset, England * Well, an excavation or structure created in the ground * Wells (name) Wells may also refer to: Places Canada * Wells, British Columbia England * Wel ...

. Most EAMs are made in the form of a

waveguide A waveguide is a structure that guides waves, such as electromagnetic waves or sound, with minimal loss of energy by restricting the transmission of energy to one direction. Without the physical constraint of a waveguide, wave intensities de ...

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 (QCSE) in a quantum well structure. Compared with an Electro-optic modulator (EOM), an EAM can operate with much lower voltages (a few volts instead of ten volts or more). They can be operated at very high speed; a modulation

bandwidth Bandwidth commonly refers to: * Bandwidth (signal processing) or ''analog bandwidth'', ''frequency bandwidth'', or ''radio bandwidth'', a measure of the width of a frequency range * Bandwidth (computing), the rate of data transfer, bit rate or thr ...

of tens of gigahertz can be achieved, which makes these devices useful for

optical fiber communication Fiber-optic communication is a method of transmitting information from one place to another by sending pulses of infrared light through an optical fiber. The light is a form of carrier wave that is modulated to carry information. Fiber is pref ...

. A convenient feature is that an EAM can be integrated with distributed feedback

laser diode The laser diode chip removed and placed on the eye of a needle for scale A laser diode (LD, also injection laser diode or ILD, or diode laser) is a semiconductor device similar to a light-emitting diode in which a diode pumped directly with e ...

on a single chip to form a data transmitter in the form of a

photonic integrated circuit A photonic integrated circuit (PIC) or integrated optical circuit is a microchip containing two or more photonic components which form a functioning circuit. This technology detects, generates, transports, and processes light. Photonic integrated c ...

. Compared with direct modulation of the

, a higher bandwidth and reduced

chirp A chirp is a signal in which the frequency increases (''up-chirp'') or decreases (''down-chirp'') with time. In some sources, the term ''chirp'' is used interchangeably with sweep signal. It is commonly applied to sonar, radar, and laser system ...

can be obtained. Semiconductor

quantum well A quantum well is a potential well with only discrete energy values. The classic model used to demonstrate a quantum well is to confine particles, which were initially free to move in three dimensions, to two dimensions, by forcing them to occup ...

EAM is widely used to modulate near-infrared (NIR) radiation at frequencies below 0.1 THz. Here, the NIR absorption of undoped quantum well was modulated by strong electric field with frequencies between 1.5 and 3.9 THz. The THz field coupled two excited states (

exciton An exciton is a bound state of an electron and an electron hole which are attracted to each other by the electrostatic Coulomb force. It is an electrically neutral quasiparticle that exists in insulators, semiconductors and some liquids. Th ...

s) of the quantum wells, as manifested by a new THz frequency-and power- dependent NIR absorption line. The THz field generated a coherent quantum superposition of an absorbing and a nonabsorbing

. This quantum coherence may yield new applications for

modulators in optical communications. Recently, advances in crystal growth have triggered the study of self organized

. Since the EAM requires small size and low modulation voltages, possibility of obtaining

with enhanced electro-absorption coefficients makes them attractive for such application.

References

* S. G. Carter, ''Quantum Coherence in an Optical Modulator'', Science 310 (2005) 651 * I. B. Akca, ''Electro-optic and electro-absorption characterization of InAs quantum dot waveguides'', Opt. Exp. 16 (2008) 3439 * X. Xu, ''Coherent Optical Spectroscopy of a Strongle Driven Quantum Dot'', Science 317 (2007) 929 {{DEFAULTSORT:Electro-Absorption Modulator Optical devices Nonlinear optics

See also

References