定义：
在光纤数据链中的光纤放大器，在很长的传输光纤上发生的放大过程。

对于应用于长途数据传输的长光纤链路，需要一个或者多个光纤放大器来保证接收器处有足够大的信号功率并且在保证误码率的前提下保持足够的信噪比。许多情况下，这些放大器是分立的，由几米稀土掺杂光纤来实现，它被光纤耦合的二极管激光器泵浦，有时会作为发射器的一部分或者仅仅在接收器前面使用，或者在传输光纤中间的某个地方使用。也可以采用传输光纤本身的分布放大器，泵浦光通常是在接收器或者发射器端口注入，或者两个端口同时注入。这一分布放大器可能会得到类似的总体增益，但是单位长度的增益低很多。也就是说这可以在存在传输损耗的情况下保持一个合理的信号功率水平，而不是将功率提高几十分贝。

目录

• 1 优点和缺点
• 2 分布激光放大器
• 3 分布拉曼放大器

优点和缺点
采用分布放大器的一个优点就是较低的链路上积累的放大器噪声。这主要是因为信号功率始终被维持着而没有讲到很低的程度，而在分立放大器中这种情况是会发生的。于是可以在没有附加放大器噪声的前提下，峰值信号功率可以减小。这实际上减小了潜在的不利的光纤非线性效应。
分布放大器的一个非常大的缺点就是需要更高的泵浦功率。这适用于拉曼放大器和稀土掺杂放大器，在下面会有讨论。
不同类型放大器的优点取决于传输系统和其特性。例如，对于仅仅基于孤子的系统，重点考虑的因子是波长范围和信号带宽。

分布激光放大器
分布放大器可以通过两种不同的形式来实现。第一种方法是采用包含一些稀土掺杂离子的传输光纤，例如铒离子，但是相比于一般的放大器光纤掺杂浓度需要低很多。尽管通信通常采用二氧化硅光纤，它对稀土离子的可溶性很低，低的掺杂可以避免淬灭效应。但是，由于传输光纤还具有一些其它的限制，很难优化光纤使其具有大的增益带宽。尤其是，任何的掺杂都会提高传输损耗，然而在短的分立放大器中这就不是非常严重的问题。
由于分布放大器的泵浦光也需要传输很长的距离，因此会经历传输损耗，如果泵浦波长比信号波长小很多，损耗甚至比信号光还多。因此长的分布掺铒放大器需要采用1.45微米的泵浦光，而不是常用的980nm光。而这个又会对放大器增益的光谱形状产生了更多的限制。即使采用长的泵浦波长，相比于分立光纤放大器，由于存在泵浦损耗，所以对泵浦功率需求更高。

分布拉曼放大器
另外一种分布放大器是拉曼放大器，它不需要稀土掺杂，而是李利用受激拉曼散射实现放大过程。同样的，传输光纤很难优化到适合拉曼放大过程，因为传输损耗需要很低，而且泵浦光也会经历传输损耗。因此，需要非常大的泵浦功率。
采用一个泵浦光源泵浦的增益谱取决于光纤纤芯的化学成分。经过调整的更宽的增益谱可以通过结合不同泵浦波长来实现。

Definition: fiber amplifiers in fiber-optic data links, where the amplification occurs within a large length of transmission fiber

More general term: optical amplifiers

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.