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Definition: amplifiers based on Raman gain
A Raman amplifier is an optical amplifier based on Raman gain, which results from the effect of stimulated Raman scattering. The Raman-active medium is usually an optical fiber, although it can also be a bulk crystal, or a cell with a gas or liquid medium. An input signal can be amplified while co- or counterpropagating with a pump beam, the wavelength of which is typically a few tens of nanometers shorter. For silica fibers, maximum gain is obtained for a frequency offset of ∼10 to 15 THz between pump and signal, depending on the core composition.
For application in telecom systems, fiber Raman amplifiers compete with erbium-doped fiber amplifiers. Compared with those, their typical features are:
- Raman amplifiers have the potential for generating gain in very different wavelength regions, provided that a suitable pump source is available.
- The gain spectrum can be tailored by using different pump wavelengths simultaneously.
- A Raman amplifier requires high pump power (possibly raising laser safety issues) and high pump brightness, but it can also generate high output powers.
- A longer length of fiber is required. However, the transmission fiber in a telecom system may be used, so that no additional fiber is required.
- Raman fiber amplifiers can have a lower noise figure.
- They also have a fast reaction to changes of the pump power, particularly for co-propagating pump, and very different saturation characteristics.
- If the pump wavelength is polarized, the Raman gain is polarization-dependent. This effect is often unwanted, but can be suppressed e.g. by using two polarization-coupled pump diodes or a pump depolarizer.
Fibers used for Raman amplifiers are not doped with rare-earth ions. In principle, any ordinary single-mode fiber could be used, and in practice the transmission fibers themselves are often suitable (→ distributed amplifiers). However, there are special fibers with increased Raman gain, resulting from certain dopants (e.g. germania) for enhanced Raman cross sections, or simply from a small effective mode area. Such fibers are used for lumped Raman amplifiers, where a shorter piece of fiber is dedicated to amplification only.
Bibliography
| [1] | R. H. Stolen and E. P. Ippen, "Raman gain in glass optical waveguides", Appl. Phys. Lett. 22, 276 (1973) |
| [2] | A. J. Stentz, "Applications of Raman lasers and amplifiers in fiber communication systems", Proc. SPIE 3263, 91 (1998) |
| [3] | Y. Emori et al., "100 nm bandwidth flat-gain Raman amplifiers pumped and gain-equalized by 12-wavelength-channel WDM laser diode unit", Electron. Lett. 35, 1355 (1999) |
| [4] | S. A. E. Lewis et al., "Gain and saturation characteristics of dual-wavelength-pumped silica-fiber Raman amplifiers", Electron. Lett. 35, 1178 (1999) |
| [5] | D. Bayart et al., "Broadband optical fiber amplification over 17.7 THz range", Electron. Lett. 36, 1569 (2000) |
| [6] | V. E. Perlin and H. G. Winful, "Optimal design of flat-gain wide-band fiber Raman amplifiers", J. Lightwave Technol. 20 (2), 250 (2002) |
| [7] | O. Boyraz and B. Jalali, "Demonstration of 11 dB fiber-to-fiber gain in a silicon Raman amplifier", IEICE Elect. Expr. 1, 429 (2004) |
| [8] | B. Jalali, "Raman-based silicon photonics", IEEE J. Sel. Top. Quantum Electron. 12 (3), 412 (2006) |
| [9] | G. P. Agrawal, "Nonlinear Fiber Optics", Academic Press, 4th edition, 2006 |
| [10] | ITU standard G.665 (01/05), "Generic characteristics of Raman amplifiers and Raman amplified subsystems" |
See also: Raman scattering, Raman lasers, Raman gain, amplifiers, fiber amplifiers, fibers, nonlinearities, noise figure
Categories: amplifiers, communications, fibers and other waveguides, nonlinear optics
