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 The Franz–Keldysh effect is a change in optical absorption by a semiconductor when an electric field is applied. The effect is named after the German physicist Walter Franz and Russian physicist Leonid Keldysh. Karl W. Böer observed first t ...

, 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 (i ...

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 ...

or wells. 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 An electro-optic modulator (EOM) is an optical device in which a signal-controlled element exhibiting an electro-optic effect is used to modulate a beam of light. The modulation may be imposed on the phase, frequency, amplitude, or polarization ...

(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. 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. Compared with direct modulation of the

, a higher bandwidth and reduced chirp 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 occupy ...

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 ( excitons) 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 exciton. 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