Electro-absorption modulator
Encyclopedia
Electro-absorption modulator (EAM) is a semiconductor
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 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 or wells
.
Most EAM 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 (QCSE) in a quantum well structure.
Compared with Electro-optic modulator
(EOM), 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
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
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.
Semiconductor quantum well
EAM is widely used to modulate near-infrared (NIR) radiation at frequencies below 0.1THz. 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
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 exciton
. This quantum coherence may yield new applications for quantum well
modulators in optical communications.
Recently, advances in crystal growth have triggered the study of self organized quantum dots. Since the EAM requires small size and low modulation voltages, possibility of obtaining quantum dots with enhanced electro-absorption coefficients makes them attractive for such application.
Semiconductor
A semiconductor is a material with electrical conductivity due to electron flow intermediate in magnitude between that of a conductor and an insulator. This means a conductivity roughly in the range of 103 to 10−8 siemens per centimeter...
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
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 ....
, 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.
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 or wells
Quantum well
A quantum well is a potential well with only discrete energy values.One technology to create quantization is to confine particles, which were originally free to move in three dimensions, to two dimensions, forcing them to occupy a planar region...
.
Most EAM are made in the form of a waveguide
Waveguide
A waveguide is a structure which guides waves, such as electromagnetic waves or sound waves. There are different types of waveguides for each type of wave...
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 Electro-optic modulator
Electro-optic modulator
Electro-optic modulator is an optical device in which a signal-controlled element displaying electro-optic effect is used to modulate a beam of light. The modulation may be imposed on the phase, frequency, amplitude, or polarization of the modulated beam...
(EOM), 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 (computing)
In computer networking and computer science, bandwidth, network bandwidth, data bandwidth, or digital bandwidth is a measure of available or consumed data communication resources expressed in bits/second or multiples of it .Note that in textbooks on wireless communications, modem data transmission,...
of tens of gigahertz can be achieved, which makes these devices useful for optical fiber communication
Fiber-optic communication
Fiber-optic communication is a method of transmitting information from one place to another by sending pulses of light through an optical fiber. The light forms an electromagnetic carrier wave that is modulated to carry information...
. A convenient feature is that an EAM can be integrated with distributed feedback
Distributed feedback laser
A distributed feedback laser is a type of laser diode, quantum cascade laser or optical fibre laser where the active region of the device is periodically structured as a diffraction grating...
laser diode
Laser diode
The laser diode is a laser where the active medium is a semiconductor similar to that found in a light-emitting diode. The most common type of laser diode is formed from a p-n junction and powered by injected electric current...
on a single chip to form a data transmitter in the form of a photonic integrated circuit
Photonic integrated circuit
A photonic integrated circuit or integrated optical circuit is a device that integrates multiple photonic functions and as such is analogous to an electronic integrated circuit...
. Compared with direct modulation of the laser diode
Laser diode
The laser diode is a laser where the active medium is a semiconductor similar to that found in a light-emitting diode. The most common type of laser diode is formed from a p-n junction and powered by injected electric current...
, a higher bandwidth and reduced chirp
Chirp
A chirp is a signal in which the frequency increases or decreases with time. In some sources, the term chirp is used interchangeably with sweep signal. It is commonly used in sonar and radar, but has other applications, such as in spread spectrum communications...
can be obtained.
Semiconductor quantum well
Quantum well
A quantum well is a potential well with only discrete energy values.One technology to create quantization is to confine particles, which were originally free to move in three dimensions, to two dimensions, forcing them to occupy a planar region...
EAM is widely used to modulate near-infrared (NIR) radiation at frequencies below 0.1THz. 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
Exciton
An exciton is a bound state of an electron and 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...
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 exciton
Exciton
An exciton is a bound state of an electron and 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...
. This quantum coherence may yield new applications for quantum well
Quantum well
A quantum well is a potential well with only discrete energy values.One technology to create quantization is to confine particles, which were originally free to move in three dimensions, to two dimensions, forcing them to occupy a planar region...
modulators in optical communications.
Recently, advances in crystal growth have triggered the study of self organized quantum dots. Since the EAM requires small size and low modulation voltages, possibility of obtaining quantum dots with enhanced electro-absorption coefficients makes them attractive for such application.