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(Acronym: SOA)
Definition: optical amplifiers based on semiconductor gain media
A semiconductor optical amplifier is an optical amplifier based on a semiconductor gain medium. It is essentially like a laser diode where the end mirrors have been replaced with anti-reflection coatings. The signal light is usually sent through a semiconductor single-mode waveguide with transverse dimensions of a few micrometers and a length of the order of 0.5-2 mm. The waveguide mode has significant overlap with the active (amplifying) region, which is pumped with an electric current. The injection current creates a certain carrier density in the conduction band, allowing for optical transitions from the conduction band to the valence band. The gain maximum occurs for photon energies slightly above the bandgap (somewhat increasing with increasing injection current).
Semiconductor optical amplifiers are often used in telecom systems in the form of fiber-pigtailed components, operating at signal wavelengths near 1.3 μm or 1.5 μm, and offering a gain of up to ∼30 dB.
Comparison with Erbium-doped Fiber Amplifiers
The technology of semiconductor amplifiers competes with that of erbium-doped fiber amplifiers (EDFAs). The main differences compared with EDFAs are:
- The setup is much more compact, containing only a small semiconductor chip with electrical and fiber connections.
- The achievable output powers are significantly smaller.
- The gain bandwidth is smaller, but devices operating in different wavelength regions can be made.
- The upper state lifetime and thus the stored energy are much smaller, so that the gain reacts to changes of pump power or signal power within nanoseconds (instead of milliseconds).
- Changes of gain also cause phase changes (→ linewidth enhancement factor).
- SOAs exhibit much stronger nonlinear distortions in the form of self-phase modulation and four-wave mixing. These are often unwanted, but can also be used e.g. for optical signal processing (see below).
- The noise figure is typically higher.
- The amplification is normally polarization-sensitive.
Vertical-cavity SOAs
A special breed of semiconductor optical amplifiers is that of the vertical-cavity SOA (VCSOA). This is very similar to a vertical cavity surface-emitting laser, but the top mirror reflectivity is reduced so that the laser threshold is not reached. The still significant top reflectivity is required for generating a reasonably high gain, since the single-pass gain through a few quantum wells is quite low, but it also strongly reduces the gain bandwidth.
Application for Channel Translation
Amplification is actually not the only application of semiconductor optical amplifiers: there are also applications in optical fiber communications based on nonlinearities such as gain saturation, or a kind of cross-phase modulation, which is associated with changes of the refractive index via the carrier density in a semiconductor optical amplifier. Such effects can be used for channel translation (wavelength conversion) in wavelength division multiplexing systems, for modulation format conversion, clock recovery, signal regeneration, and pattern recognition.
Bibliography
| [1] | Y. Yamamoto and T. Mukai, "Fundamentals of optical amplifiers", Optical Quantum Electr. 21, S1 (1989) |
| [2] | N. A. Olsson, "Lightwave systems with optical amplifiers", J. Lightwave Technol. LT-7, 1071 (1989) |
| [3] | C. Schubert, R. Ludwig, and H.-G. Weber, "High-speed optical signal processing using semiconductor optical amplifiers", J. Opt. Fiber Commun. Rep. 2, 171–208 (2004) |
| [4] | M. J. Conelly, "Semiconductor Optical Amplifiers", Springer, 1st edition, 2002 |
| [5] | B. E. A. Saleh and M. C. Teich, "Fundamentals of Photonics", John Wiley & Sons, Inc., New York (1991) |
See also: amplifiers, erbium-doped fiber amplifiers, semiconductor lasers, laser diodes
Categories: amplifiers, communications
