For longer fiber-optic links as used for long-haul data transmission, one or several fiber amplifiers are usually needed for obtaining a sufficiently high signal power at the receiver and maintaining a high enough signal-to-noise ratio for the required bit error rate. In many cases, such amplifiers are discrete amplifiers, realized with a few meters of some rare-earth-doped fiber, which is pumped with a fiber-coupled diode laser and used as part of the transmitter, or just before the receiver, or somewhere between parts of the transmission fiber. However, it is also possible to employ so-called distributed amplification in a long length of the transmission fiber itself, where the pump power is typically injected at the receiver or transmitter end, or from both directions. Such a distributed amplifier may have a similar overall gain, but a much lower gain per unit length. It is meant to approximately maintain a reasonable signal power level in the presence of propagation losses, rather than increasing the power level by tens of decibels.
General Advantages and Disadvantages
An advantage of using distributed amplifiers is that this approach normally leads to a lower accumulation of amplifier noise within the link. This is essentially because the signal power level is prevented from dropping to a very low level, as it can occur between discrete (lumped) amplifiers. The maximum signal power level can then actually be reduced without obtaining excessive amplifier noise. This also reduces the potentially detrimental effect of fiber nonlinearities.
An important disadvantage is that distributed amplifiers generally require higher pump powers. This applies to both Raman amplifiers and rare-earth-doped amplifiers, which are discussed below.
The detailed advantages of different types of amplifiers depend on the type of transmission system and its characteristics. For example, there are specific aspects which are relevant only for soliton-based systems, and the wavelength region and signal bandwidth are also important factors to be considered.
Distributed Laser Amplifiers
A distributed amplifier can be realized in essentially two different forms. The first one is to use a transmission fiber which contains some rare-earth dopant such as erbium (Er3+), but with a much lower doping concentration than a regular amplifier fiber. Although the material of silica fibers, as normally used for transmission, exhibits a low solubility for rare earth ions, a low concentration can be incorporated without quenching effects. However, it is difficult to optimize the fiber also for a large gain bandwidth, as the transmission fiber is subject to further constraints. In particular, any dopants need to be avoided which substantially raise the propagation losses, whereas in a short discrete amplifier these are typically not a serious issue.
Note also that the pump light for a distributed amplifier needs to be delivered over a substantial length, and is therefore also subject to propagation losses – even more than the signal light, if the pump wavelength is significantly shorter than the signal wavelength. A long distributed erbium amplifier should thus be pumped around 1.45 μm rather than the otherwise often used wavelength of 980 nm. This introduces further restrictions on the spectral shape of the amplifier gain. Even with a long pump wavelength, the pump losses lead to the requirement of a higher pump input power, compared with that of a discrete fiber amplifier.
Distributed Raman Amplifiers
Another type of distributed amplifier is the Raman amplifier, where no rare earth dopant is required, and stimulated Raman scattering is used for amplification. Again, the transmission fiber can hardly be optimized for Raman amplification, as the propagation losses need to be low, and the pump light is also subject to propagation losses. Therefore, substantial pump powers are needed.
The gain spectrum achieved with a single pump source is essentially determined by the chemical composition of the fiber core. Broader gain spectra, possibly with a tailored shape, can be achieved by using some combination of different pump wavelengths.
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