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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 e.g. 1–2 μm 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 energy.
SOAs are often used in telecom systems in the form of fiber-pigtailed components, operating at signal wavelengths near 1.3 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 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 in pump power or signal power within nanoseconds (instead of milliseconds). Changes in 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 type of SOAs 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 obtaining a reasonably high gain, since the single-pass gain through a few quantum wells is fairly low, but it also reduces substantially the gain bandwidth. Comparing with standard edge-emitting SOAs, VCSOAs can be made significantly smaller and cheaper, and can be operated with lower drive currents. (Typically, the order of 10 mA should be sufficient for 20 dB of gain.) Also, they can be fabricated in arrays.
Application in Channel Translation
Amplification is actually not the only application of SOAs: 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 in the refractive index via the carrier density in a SOA. 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”, Opt. Quantum Electron. 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] | W. Loh et al., “Noise figure of Watt-class ultralow-confinement semiconductor optical amplifiers”, IEEE J. Quantum Electron. 47 (1), 66 (2011) |
| [5] | M. J. Conelly, Semiconductor Optical Amplifiers, Springer, 1st edn., 2002 |
| [6] | B. E. A. Saleh and M. C. Teich, Fundamentals of Photonics, John Wiley & Sons, Inc., New York (1991) |
| [7] | P. Urquhart (ed.), Advances in Optical Amplifiers (open-access online edition available), InTech, Rijeka, Croatia (2011) |
See also: amplifiers, erbium-doped fiber amplifiers, semiconductor lasers, laser diodes
