electroabsorption modulators (original) (raw)
Acronym: EAM
Definition: optical modulators based on the Franz–Keldysh effect
Categories: 
photonic devices, 
optoelectronics
- optical modulators
- acousto-optic modulators
- electro-optic modulators
- Pockels cells
- electroabsorption modulators
- liquid crystal modulators
- intensity modulators
- phase modulators
Related: electro-optic modulatorsoptical modulatorstelecom transmittersintensity modulatorsmodulator drivers
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Contents
What are Electroabsorption Modulators?
An electroabsorption modulator (or electro-absorption modulator) is a semiconductor-based optical modulator. It can be used for controlling (modulating) the intensity (more precisely: the optical power) of a laser beam via an electric voltage (→ intensity modulators). Its principle of operation is based on the Franz–Keldysh effect [1, 2], i.e., a change in the absorption spectrum caused by an applied electric field, which changes the band gap energy (thus the photon energy corresponding to an absorption edge) but usually does not involve a substantial excitation of carriers by the electric field.
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.
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.
Performance; Comparison with Electro-optic Devices
Drive Voltage
Compared with bulk-optical electro-optic modulators, electroabsorption modulators can operate with far lower voltages: a few volts only instead of hundreds or thousands of volts. However, a more fair comparison is with integrated electric-optic modulators, e.g. based on LiNbO3 and used in similar applications, which require voltages on the order of 10 V.
Modulation Bandwidth
Electroabsorption modulators 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.
See also the article on modulator drivers.
Frequently Asked Questions
What is an electroabsorption modulator?
An electroabsorption modulator (EAM) is a semiconductor device used to control the optical power of a laser beam with an electric voltage.
What is the operating principle of an electroabsorption modulator?
It works based on the Franz–Keldysh effect, where an applied electric field changes the semiconductor's band gap energy, thus altering its absorption spectrum for light.
What are the main advantages of electroabsorption modulators?
They can operate with low voltages (a few volts) and at very high bandwidths of tens of gigahertz. They can also be integrated on a single chip with a laser diode.
Why use an EAM instead of direct modulation of a laser?
Using an external electroabsorption modulator allows for a higher modulation bandwidth and can achieve reduced chirp compared to directly modulating the drive current of a laser diode.
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Bibliography
| [1] | 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) |
|---|---|
| [2] | W. Franz, “Einfluß eines elektrischen Feldes auf eine optische Absorptionskante”, Z. Naturforsch., Teil A 13, 484 (1958) |
| [3] | E. Lach et al., “Application of electroabsorption modulators for high-speed transmission systems”, J. Opt. Fiber Commun. Rep. 2, 140–170 (2005) |
| [4] | J. Liu et al., “Waveguide-integrated, ultralow-energy GeSi electro-absorption modulators”, Nature Photon. 2, 433 (2008); doi:10.1038/nphoton.2008.99 |
(Suggest additional literature!)
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