Optical amplifier
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In optics, an optical amplifier is a device that amplifies an optical signal directly, without the need to first convert it to an electrical signal, then amplify it electrically, and finally reconvert it to an optical signal. An optical amplifier may be thought of as a laser without an optical cavity, or one in which feedback from the cavity is suppressed. Stimulated emission in the amplifier's gain medium causes amplification of incoming light.
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Doped fibre amplifiers
Image:Simple edfa.jpg Doped Fiber Amplifiers (DFAs) are optical amplifiers which use a doped optical fiber as a gain medium to amplify an optical signal. The signal to be amplified and a pump wavelength are multiplexed into the doped fibre and through interaction with the doping ions the signal is amplified.
Amplification is achieved by stimulated emission of photons from dopant ions in the doped fibre. The pump wavelength excites electrons into a higher energy from where they can decay via stimulated emission of a photon back to a lower energy level. The energy levels involved generally form a three- or four-level system and hence include a non-radiative transition either from the highest energy level and/or back to the bottom energy level.
The amplification window is defined by the dopant ions used, the glass structure of the optical fibre and the pump wavelengths used.
Although the electronic transitions of a single ion are very well defined, electron level broadening occurs when the ions are incorporated into the glass of the optical fibre and thus the wavelengths that can be amplified are also broadened. This broadening is both homogeneous and heterogeneous which leads to a gain spectrum that is not uniform against wavelength. The broad gain-bandwidth of fibre amplifiers make them particularly useful in wavelength-division multiplexed communications systems as a single amplifier can be utilised to amplify all signals being carried on a fibre.
Noise in Doped Fibre Amplifiers
The principal source of noise in DFAs is Amplified Spontaneous Emission (ASE), which has a spectrum approximately the same as the gain spectrum of the amplifier.
As well as decaying via stimulated emission, electrons in the upper energy level can also decay by spontaneous emission, which occurs at random, depending upon the glass structure and inversion level. Photons are emitted spontaneously in all directions, but a proportion of those will be emitted in a direction that falls within the Numerical aperture of the fibre and are thus captured and guided by the fibre. Those photons captured may then interact with other dopant ions, and are thus amplified by stimulated emission. The initial spontaneous emission is therefore amplified in the same manner as the signals, hence the term Amplified Spontaneous Emission. ASE is emitted by the amplifier in both the forward and reverse directions, but only the forward ASE is a direct concern to system performance since that noise will co-propagate with the signal to the receiver where it degrades system performance. Counter-propagating ASE can, however, lead to degradation of the amplifier's performance since the ASE can deplete the inversion level and thereby reduce the gain of the amplifier.
Gain saturation
Gain is achieved in a DFA due to population inversion of the dopant ions. The inversion level of a DFA is set, primarily, by the power of the pump wavelength and the power at the amplified wavelengths. As the signal power increases, or the pump power reduces, the inversion level will reduce and thereby the gain of the amplifier will be reduced. This effect is known as gain saturation - as the signal level increases, the amplifier saturates and cannot produce any more output power, and therefore the gain reduces. Saturation is also commonly known as gain compression.
To achieve optimum noise performance DFAs are operated under a small amount of gain compression, since that reduces the rate of spontaneous emission, thereby reducing ASE.
Inhomogeneous effects
Due to the inhomogeneous portion of the linewidth broadening of the dopant ions, the gain spectrum has an inhomogeneous component and gain saturation occurs, to a small extent, in an inhomogeneous manner. This effect is known as Spectral hole burning due to the fact that a high power signal at one wavelength can 'burn' a hole in the gain of wavelengths close to that signal by saturation of the inhomogeneously broadened ions. Spectral holes vary in width depending on the characteristics of the optical fibre in question, but are typically less than 1nm at the short wavelength end of the C-band, and a few nm at the long wavelength end of the C-band.
Polarisation effects
Although the DFA is principally a polarisation independent amplifier, a small proportion of the dopant ions interact preferentially with certain polarisations and a small dependence on polarisation may occur (typically <0.5dB). The change in gain is principally dependent on the alignment of the polarisation of the pump and signal wavelengths - ie. whether the two wavelengths are interacting with the same sub-set of dopant ions or not.
Erbium-doped fiber amplifiers (EDFA)
The Erbium-Doped Fibre Amplifier (EDFA) is the most deployed fibre amplifier as its amplification window coincides with the third transmission window of silica-based optical fibre.
Two bands have developed in the third transmission window - the Conventional, or C-band, from approximately 1525nm - 1565nm, and the Long, or L-band, from approximately 1570nm to 1610nm. Both of these bands can be amplified by EDFAs, but it is normal to use a different amplifier, each optimised for the respective band, for each band.
The principal difference between C- and L-band amplifiers is that a longer length of doped fibre is used in L-band amplifiers. The longer length of fibre allows a lower inversion level to be used, thereby giving at longer wavelengths (due to the band-structure of Erbium in silica) while still providing a useful quantity of gain.
EDFAs have two pumping bands - 980nm and 1480nm. The 980nm band has a higher absorption cross-section and is generally used where low-noise performance is required. The absorption band is relatively narrow and so wavelength stabilised laser sources are typically needed. The 1480nm band has a lower, but broader, absorption cross-section and is generally used for higher power amplifiers. A combination of 980nm and 1480nm pumping is generally utilised in amplifiers.
The EDFA was invented by a group including David Payne from the University of Southampton.
Doped fibre amplifiers for other wavelength ranges
Thulium doped fibre amplifiers have been used in the S-band (1450-1490nm) and Praseodymium doped amplifiers in the 1300nm region. However, those regions have not seen any significant commercial use and so those amplifiers have not been the subject of as much development as the EDFA.
Semiconductor optical amplifier (SOA)
Semiconductor optical amplifiers have a similar structure to Fabry-Perot laser diodes but with anti-reflection design elements at the endfaces. Recent designs include anti-reflective coatings and tilted waveguide and window regions to eliminate endface reflection almost perfectly. This effectively prevents the amplifier from acting as a laser.
The semiconductor optical amplifier is of small size and electrically pumped. It can be potentially less expensive than the EDFA and can be integrated with semiconductor lasers, modulators, etc. However, the performance is still not comparable with the EDFA. The SOA has higher noise, lower gain, moderate polarization dependence and high nonlinearity with fast transient time. This nonlinearity presents the most severe problem for optical communication applications. In 2001 a "linear optical amplifier" using a gain-clamping technique was developed. As of 2006 this device is no longer commercially available.
High optical nonlinearity makes semiconductor amplifiers attractive for all optical signal processing like all-optical switching and wavelength conversion. There has been much research on semiconductor optical amplifiers as optical computing components.
Raman amplifier
The signal is intensified by Raman amplification. Unlike the EDFA and SOA the amplification effect is achieved by a nonlinear interaction between the signal and a pump laser within an optical fibre. A distributed Raman amplifier is one in which the transmission fibre is utilised as the gain medium by multiplexing a pump wavelength with signal wavelength, while a lumped Raman amplifier utilises a dedicated, shorter length of fibre to provide amplification. In the case of a lumped Raman amplifier highly nonlinear fibre with a small core is utilised to increase the interaction between signal and pump wavelengths and thereby reduce the length of fibre required.
The pump light may be coupled into the transmission fiber in the same direction as the signal (co-directional pumping), in the opposite direction (contra-directional pumping) or both. Contra-directional pumping is more common as the transfer of noise from the pump to the signal is reduced.
The pump power required for raman amplification is higher than that required by the EDFA, with in excess of 500mW being required to achieve useful levels of gain in a distributed amplifier. Lumped amplifiers, where the pump light can be safely contained to avoid safety implications of high optical powers, may use over 1W of optical power.
The principal advantage of Raman amplification is its ability to provide distributed amplification within the transmission fibre, thereby increasing the length of spans between amplifier and regeneration sites. The amplification bandwidth of Raman amplifiers is defined by the pump wavelengths utilised and so amplification can be provided over wider, and different, regions than may be possible with other amplifier types which rely on dopants and device design to define the amplification 'window'.
Note: The text of an earlier version of this article was taken from the public domain Federal Standard 1037C.