Optical amplifier
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An optical amplifier is a device that amplifies an optical signal directly, without the need to first convert it to an electrical 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. Optical amplifiers are important in optical communication and laser physics.
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[edit] Laser amplifiers
Almost any laser active gain medium can be pumped to produce gain for light at the wavelength of a laser made with the same material as its gain medium. Such amplifiers are commonly used to produce high power laser systems. Special types such as regenerative amplifiers and chirped-pulse amplifiers are used to amplify ultrashort pulses.
[edit] Doped fibre amplifiers
Doped fibre amplifiers (DFAs) are optical amplifiers which use a doped optical fibre as a gain medium to amplify an optical signal. They are related to fibre lasers. The signal to be amplified and a pump wavelength are multiplexed into the doped fibre, and the signal is amplified through interaction with the doping ions.
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 of an optical amplifier is the range of optical wavelengths for which the amplifier yields a usable gain. The amplification window is defined by the dopant ions used, the glass structure of the optical fibre and the pump wavelengths used. The gain of an optical amplifier also depends on the local beam intensity at any point inside the amplifier.
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.
[edit] Noise
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. Amplifier noise in an ideal amplifier would cause at least 3 dB, and practical amplifiers can have amplifier noise as large as 6-8 dB. Noise figure in an ideal DFA is 3 dB, while practical amplifiers can have noise figure as large as 6-8 dB.
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.
[edit] 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 decreases, 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.
[edit] 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.
[edit] 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.
[edit] Erbium-doped fibre amplifiers
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 two different amplifiers, each optimized for one of the bands.
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 commonly-used 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 [1] by a group including David Payne, R. Mears, and L. Reekie, from the University of Southampton and a group from AT&T Bell Laboratories, E. Desurvire, P. Becker, and J. Simpson. [2]
[edit] 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.
[edit] Semiconductor optical amplifier (SOA)
Semiconductor optical amplifiers are amplifiers which use a semiconductor to provide the gain medium. These 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 which can reduce endface reflection to less than 0.001%. Since this creates a loss of power from the cavity which is greater than the gain it prevents the amplifier from acting as a laser.
Semiconductor optical amplifiers are typically made from group III-V compound semiconductors such as GaAs/AlGaAs, InP/InGaAs, InP/InGaAsP and InP/InAlGaAs, though any direct band gap semiconductors such as II-VI could conceivably be used. Such amplifiers are often used in telecommunication systems in the form of fibre-pigtailed components, operating at signal wavelengths between 0.85 μm and 1.6 μm and generating gains of up to 30 dB.
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 originates from the short nanosecond or less upper state lifetime, so that the gain reacts rapidly to changes of pump or signal power and the changes of gain also cause phase changes which can distort the signals. This nonlinearity presents the most severe problem for optical communication applications. However it provides the possibility for gain in different wavelength regions form the EDFA. "Linear optical amplifiers" using gain-clamping techniques have been developed.
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 elements for optical signal processing, wavelength conversion, clock recovery, signal demultiplexing, and pattern recognition.
A recent addition to the SOA family is the vertical-cavity SOA (VCSOA). These devices are similar in structure to, and share many features with, vertical-cavity surface-emitting lasers (VCSELs). The major difference when comparing VCSOAs and VCSELs is the reduced mirror reflectivities used in the amplifier cavity. With VCSOAs, reduced feedback is necessary to prevent the device from reaching lasing threshold. Due to the extremely short cavity length, and correspondingly thin gain medium, these devices exhibit very low single-pass gain (typically on the order of a few percent) and also a very large free spectral range (FSR). The small single-pass gain requires relatively high mirror reflectivities to boost the total signal gain. In addition to boosting the total signal gain, the use of the resonant cavity structure results in a very narrow gain bandwidth; coupled with the large FSR of the optical cavity, this effectively limits operation of the VCSOA to single-channel amplification. Thus, VCSOAs can be seen as amplifying filters.
Given their vertical-cavity geometry, VCSOAs are resonant cavity optical amplifiers that operate with the input/output signal entering/exiting normal to the wafer surface. In addition to their small size, the surface normal operation of VCSOAs leads to a number of advantages, including low power consumption, low noise figure, polarization insensitive gain, and the ability to fabricate high fill factor two-dimensional arrays on a single semiconductor chip. These devices are still in the early stages of research, though promising preamplifier results have been demonstrated. Further extensions to VCSOA technology are the demonstration of wavelength tunable devices. These MEMS-tunable vertical-cavity SOAs utilize a microelectromechanical systems (MEMS) based tuning mechanism for wide and continuous tuning of the peak gain wavelength of the amplifier.
[edit] Raman amplifier
In a 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. There are two types of Raman amplifier: distributed and lumped. 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 fibre 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 500 mW 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.
[edit] Optical parametric amplifier
An optical parametric amplifier allows the amplification of a weak Signal-Impulse in a noncentrosymmetric nonlinear medium (e.g. BBO). In contrast to the previously mentioned amplifiers, which are mostly used in telecommunication environmets, this type finds its main application in expanding the frequency tunability of ultrafast solid-state lasers (e.g. Ti:Sapphire). By using a noncollinear interaction geometry Optical Parametric Amplifiers are capable of extreme broad amplification bandwidths.
[edit] References
- ^ R.J. Mears, L. Reekie, I.M. Jauncey and D.N. Payne: “Low-noise Erbium-doped fibre amplifier at 1.54pm”, Electron. Lett., 1987, 23, pp.1026-1028
- ^ E. Desurvire, J. Simpson, and P.C. Becker, High-gain erbium-doped traveling-wave fiber amplifier," Optics Letters, vol. 12, No. 11, 1987, pp. 888-890
