定义：
基于掺铒光纤的光纤放大器。

掺铒光纤放大器是目前在长距离光纤通信领域最重要的光纤放大器；因为它们可以放大1500nm附近的光，而通信光纤在这一波长区域内损耗最小。 

目录

• 装置和工作原理
• 增益谱
• 通信系统中的掺铒放大器
• 技术细节
• 掺铒光纤放大器的其它应用

装置和工作原理 
图1是一种典型的掺铒光纤放大器的装置图。光纤是标准的单模光纤，纤芯掺铒。在如图所示的这种情况下，光纤是由两个激光二极管发出的光（双向泵浦）来泵浦的，尽管单方向的泵浦（同方向和反方向泵浦）也非常常见。泵浦光通常是采用980nm，有时也用1450nm的光，将铒离子激发到4I13/2态（用980nm光泵浦是通过4I11/2 ），从该态通过受激辐射过程返回基态4I15/2 实现将1500nm的光放大。（也可看词条掺铒增益介质中的图1.） 

图1：一种简单的掺铒光纤放大器的示意图。两个激光二极管（LDs）为掺铒光纤提供泵浦能量。泵浦光通过二向色性光纤耦合器耦合进入光纤。马尾式光隔离器用来阻止背向反射的光使其不会影响装置的性能。 
这个装置还包含两个马尾式（光纤耦合）的光学隔离器。输入端的隔离器阻止放大的自发辐射产生的光以防其干扰前端的器件，而输出端的隔离器则抑制（也可能完全消除）被反射的输出光重新进入放大器而产生的激光效应。如果没有隔离器，光纤放大器将会对背向反射非常敏感。 
除了光学隔离器，商用光纤放大器还可以包含其它的器件。例如，光纤耦合器和光探测器用来操控光功率值，泵浦激光二极管需要采用控制电路和增益平坦滤波器。对于封装非常紧凑的放大器，一些无源光学器件可以集成到光电集成回路上（平板光波回路）。 
在需要非常高增益的情况下，例如放大高能量的超短脉冲，则通常采用放大器链来实现，包含多级放大器和放大器之间的光学器件（如隔离器，滤波器，调制器）。 

增益谱 
增益谱的形状依赖于吸收和发射截面，也就是依赖于基质玻璃。增益的幅值和增益谱的形状都极大的受铒离子被激发的程度的影响，因为这是准三能级跃迁。图2是一种普通玻璃的值，这种玻璃是在二氧化硅内加入掺杂来防止铒离子聚集在一起。其它种类的玻璃会得到非常不同的增益谱。 

图2：铒离子在磷酸盐玻璃的激发程度从0到100%时的增益谱和吸收谱，各个谱之间激发程度差值为20%。 
强三能级过程（在>50%激发情况下透明）发生在1535nm处。在这一光谱区域，未被泵浦的光纤具有非常大的损耗，但是由于发射界面非常大，所以强激发仍然可以得到高增益。在更长波长处（如1580nm），需要低激发能级来得到增益，但是最大增益更小。 
峰值增益通常发生在波长为1530-1560nm区域，在1530nm峰值处具有最高的激发能级，然而稍低的激发能级使峰值增益发生在稍长的波长处。激发能级依赖于发射和吸收截面，还依赖于泵浦光和信号光强度（ASE光除外）。整个光纤的平均激发能级，与净增益谱相关，是依赖于泵浦光和信号光功率的，还和光纤长度和掺铒浓度有关。这些参数（还包括玻璃成分选择）都用来优化EDFAs在特定波长区域的性能，例如通讯C或者L带。 
为了在一个大的波长范围得到平坦的增益谱（增益均衡），例如在波分复用中要求的，可以通过优化玻璃基底（例如碲化物光纤或者氟化物光纤，或者不同的放大器区域采用不同玻璃）或者采用合适的光学滤波器，例如长周期光纤布拉格光栅。 

通信系统中的掺铒放大器 
EDFAs在光纤通信系统中具有多种用途，最重要的几种应用包括： 

• 数据传输器的功率在进入长光纤或者损耗很大的器件（如光纤分束器）之前可以先通过EDFA得到高功率。光纤分束器在有线电视系统中应用非常广泛，在这个系统中一个传输器将信号传入很多光纤中。 
• 当信号很弱时，光纤放大器也可应用于数据接收器之前。除了引入放大器噪声外，这可以提高信噪比和数据传输速率，因为放大器噪声比接收器的输入噪声要小。雪崩二极管更加常用，它内部本身具有信号放大功能。 
• 在无源光纤传输中，同轴的EDFA被放置于长跨度光纤中。在长光纤链中采用多个放大器的优势在于大的传输损耗被补偿，同时（a）不会降低光功率，而光功率过低会极大降低信噪比；（b）也不会使其它位置传输多余光功率，从而避免了有害的非线性效应。许多这种同轴的EDFAs即使在环境恶劣情况下，如在海平面，也可以正常工作，当然了维护比较难。 
• 尽管数据传输器一般不基于掺铒装置，但是EDFAs通常是测试传输硬件的一部分。EDFAs也可以用于光学信号处理。 

这些功能在通信C和L波段都可以实现。其它的光纤放大器，例如采用镨的，也可以用于其它波段，但是在增益和增益效率方面都无法和掺铒的器件相比拟。 
EDFAs的一个显著优点就是它非常大的增益带宽，通常具有几十个纳米，因此足够放大所有信号通道并且还能保持最高的数据速率而不引入任何增益变窄的效应。一个EDFA就可以再增益区域内同时用于放大多个不同波长的信号通道，这项技术被称为波分复用。在这个光纤放大器技术出现之前，没有任何实际方法来放大长光纤跨度中的所有通道。因此引入光纤放大器极大的简化了系统，并且提高了可靠性。 
在1500nm区域唯一可以与掺铒光纤放大器比拟的就是拉曼放大器，它得益于高功率泵浦激光器的发展。在传输光纤中就可以实现拉曼放大。尽管如此，EDFAs仍然占据主导地位。 

技术细节 
EDFAs中常用的泵浦光波长为980nm。980nm光可以将铒离子从基态 泵浦到激发态 ，然后很快转移到能级 。由于转移过程很快，泵浦光的受激辐射不受影响，可以达到非常高的激发能级。因此尽管由于量子数亏损泵浦效率并不是理想情况，但是它仍然可以得到最高的增益效率（10dB/mW量级）和最低的噪声系数。 
由于激光截面不是很大，EDFA的饱和功率相比于半导体光放大器来说相对比较高。因此在高比特率数据传输中，单一的字节具有很低的能量不会产生任何的增益饱和。只有当字节具有成千上百万的时候，增益会自调整到平均信号功率。 
在高增益放大器中，放大的自发辐射通常是制约增益的一个因素。由于铒离子准三能级的特性，前向和后向的ASE功率是不同的，而且峰值ASE所处的波长与峰值增益对应的波长是不同的。 
EDFA的噪声系数比理想的高增益放大器理论上的3dB要稍大，这主要是由于准三能级的特性。可以通过合理设计放大器来降低噪声，要考虑到输入信号处铒被激发的程度，它受泵浦光方向的影响很大。 
通过光纤模拟软件可以从许多不同角度来分析掺铒光纤放大器。得到的定量记过可以作为优化装置性能和需要哪些器件的基础。 

掺铒光纤放大器的其它应用 
采用掺镱光纤（也被称为Er:Yb:光纤）可以在较短波长处取得高增益。这种光纤除了掺杂铒离子，还包含一部分的镱离子（通常镱的浓度要远大于铒）。镱离子也可以由980nm泵浦光（也可以由1064nm泵浦光）激发，然后将它们的能量转移到铒离子中。通过合理配置光纤纤芯中掺杂成分的浓度，能量转移可以达到很高的效率。但是，纯掺铒光纤在通讯领域应用更广泛，因为额外掺镱在这个领域应用并没有显著优势，并且由于化学成分改变可能会降低增益带宽。 
掺铒的双包层光纤可用来产生几十瓦甚至更高的输出功率。在这种情况下，泵浦吸收效率比较弱，掺镱的纤芯有很大的用途。 
也可以将1500nm波长区域的超短脉冲放大到比较高的能量，这需要采用EDFAs作为一个放大器链。利用了这些放大器的高饱和能量，尤其是采用掺铒的具有很大模式面积光纤。

Acronym: EDFA

Definition: fiber amplifiers based on erbium-doped optical fibers

More general term: fiber amplifiers

Erbium-doped fiber amplifiers are the by far most important fiber amplifiers in the context of long-range optical fiber communications; they can efficiently amplify light in the 1.5-μm wavelength region, where silica-based telecom fibers have their loss minimum. The core element of a fiber amplifier is a piece of rare-earth-doped fiber, which can provide laser amplification via stimulated emission when it is optically pumped with other light injected into the fiber. See the article on fiber amplifiers for information on more general aspects.

Setup and Operation Principle

A typical setup of a simple erbium-doped fiber amplifier (EDFA) is shown in Figure 1. Its core component is the erbium-doped optical fiber, which is typically a single-mode fiber (or sometimes a few-mode fiber).

In the shown case, the active fiber is “pumped” with light from two laser diodes (bidirectional pumping), although unidirectional pumping in the forward or backward direction (co-directional and counter-directional pumping) is also very common. The pump light, which most often has a wavelength around 980 nm or sometimes around 1450 nm, excites the erbium ions (Er³⁺) into the ⁴I_13/2 state (in the case of 980-nm pumping via ⁴I_11/2), from where they can amplify light in the 1.5-μm wavelength region via stimulated emission back to the ground-state manifold ⁴I_15/2. (See also Figure 1 in the article on erbium-doped laser gain media.)

The setup shown also contains two “pig-tailed” (fiber-coupled) Faraday isolators. The isolator at the input prevents light originating from amplified spontaneous emission from disturbing any previous stages, whereas that at the output suppresses lasing (or possibly even destruction) if output light is reflected back to the amplifier. When used without optical isolators, fiber amplifiers can be sensitive to back-reflections.

Figure 1: Schematic setup of a simple erbium-doped fiber amplifier. Two laser diodes (LDs) provide the pump power for the erbium-doped fiber. The pump light is injected via dichroic fiber couplers. Pig-tailed optical isolators reduce the sensitivity of the device to back-reflections.

Various other components can also be contained in a commercial fiber amplifier. For example, there can be fiber couplers and photodetectors for monitoring optical power levels, pump laser diodes with control electronics and gain flattening filters for a wider gain bandwidth (e.g. C + L band). For particularly compact packages, various passive optical components can be combined into a photonic integrated circuit (planar lightwave circuit).

For low-power EDFAs, one often uses erbium-doped fibers having an effective mode area which is substantially smaller than that of passive telecom single-mode fibers. That could in principle lead to high splice losses. However, the high alumina content of such fibers leads to substantial diffusion during the splicing process, effectively generating a kind of mode field adapter, so that the splice losses can actually be quite small – far smaller than with mechanical splicing.

Very high signal gains, as required e.g. for the amplification of ultrashort pulses to high energies, are usually realized with amplifier chains, consisting of several amplifier stages with additional optical elements (e.g. isolators, filters, or modulators) in between.

Gain Spectrum

The shape of the erbium gain spectrum depends on the absorption and emission cross sections, which substantially depend on the used host glass of the fiber core. Also, the spectral shape of the gain and not only its magnitude is substantially influenced by the average degree of excitation of the erbium ions, because these have a quasi-three-level transition. Figure 2 shows data for a common type of glass, which is some variant of silica with additional dopants e.g. to avoid clustering of erbium ions. Other glass compositions can lead to substantially different gain spectra.

Figure 2: Gain and absorption (negative gain) of erbium (Er³⁺) ions in a phosphate glass for excitation levels from 0 to 100% in steps of 20%.

Strong three-level behavior (with transparency reached only for > 50% excitation) occurs at 1535 nm. In that spectral region, the unpumped fiber exhibits substantial absorption losses, but the high emission cross section allows for a high gain for strong excitation. At longer wavelengths (e.g. 1580 nm), a lower excitation level is required for obtaining gain, but the maximum gain is smaller.

The maximum gain typically occurs in the wavelength region around 1530–1565 nm (the telecom C band), with the 1530-nm peak being most pronounced for high excitation levels, whereas low excitation levels lead to gain maxima at longer wavelengths. The local excitation level depends on the emission and absorption cross sections and on the pump and signal intensity (apart from that of ASE light). The average excitation level over the whole fiber length, as is relevant for the net gain spectrum, depends on the pump and signal powers, but also on the fiber length and the erbium concentration. Such parameters (together with the choice of glass composition) are used to optimize an EDFA for sufficiently high and spectrally flat gain in a particular wavelength region, such as the telecom C or L band. L band amplifiers generally work with a low degree of erbium excitation and are designed for a longer cut-off wavelength.

A good flatness of the gain in a wide wavelength region (→ gain equalization), as required e.g. for wavelength division multiplexing (see below), can be obtained by using optimized glass hosts (e.g. telluride or fluoride fibers, or some combination of amplifier sections with different glasses) or by combination with appropriate optical filters, such as long-period fiber Bragg gratings. The latter approach can be realized with a tailored long-period fiber Bragg grating, but at the expense of reduced output power and efficiency.

Erbium-doped Amplifiers in Telecom Systems

After the first demonstration of an erbium-doped fiber amplifier in 1987 [1], it took only five years until commercial amplifier products became available. This is because the benefits for optical fiber communications were easily recognized as being substantial. In many years of further research and development, no viable alternative has been found which could widely replace erbium-doped fiber amplifiers.

EDFAs can serve various functions in systems for telecom systems; the most important applications are the following:

• The power of a data transmitter may be boosted with a high-power EDFA before entering a long fiber span, or a device with large losses, such as a fiber-optic splitter. Such splitters are widely used e.g. in cable-TV systems, where a single transmitter is used to deliver signals into many fibers.
• A fiber amplifier may also be used in front of a data receiver, if the arriving signal is weak. Despite the introduction of amplifier noise, this can improve the signal-to-noise ratio and thus the possible data transmission rate, since the amplifier noise may be weaker than the input noise of the receiver. It is more common, however, to use avalanche photodiodes, which have some built-in signal amplification.
• In-line EDFAs are used between long spans of passive transmission fiber. Using multiple amplifiers in a long fiber-optic link has the advantage that large transmission losses can be compensated without (a) letting the optical power drop to too low levels, which would spoil the signal-to-noise ratio, and (b) without transmitting excessive optical powers at other locations, which would cause detrimental nonlinear effects due to the unavoidable fiber nonlinearities. Many of these in-line EDFAs are operated even under difficult conditions, e.g. on the ocean floor, where maintenance would be hardly possible.
• Although data transmitters are normally not based on erbium-doped devices, an EDFA can be part of equipment for testing transmission hardware. It can also be used in the context of optical signal processing.

These functions can be realized in the telecom C and L bands. Other types of fiber amplifiers, e.g. based on praseodymium, have been considered for other bands, but none can compete with erbium-based devices in terms of gain and gain efficiency.

Particularly high capacity WDM transmission systems profit a lot from the broadband EDFA gain.

A particular attraction of EDFAs is their large gain bandwidth, which is typically tens of nanometers and thus actually more than enough to amplify data channels with the highest data rates without introducing any effects of gain narrowing. A single EDFA may be used for simultaneously amplifying many data channels at different wavelengths within the gain region; this technique is called wavelength division multiplexing. Before such fiber amplifiers were available, there was no practical method for amplifying all channels e.g. between long fiber spans of a fiber-optic link: one had to separate all data channels, detect and amplify them electronically, optically resubmit and again combine them. The introduction of fiber amplifiers thus brought an enormous reduction in the complexity, along with a corresponding increase in reliability. Very long lifetimes are possible by using redundant down-rated pump diodes.

Some EDFAs are specially designed for space division multiplexing.

Most erbium-doped fiber amplifiers are based on single-mode fiber. However, other types of fiber amplifiers have recently been developed, which can be used in the context of space division multiplexing. Here, the fiber is either a multi-core fiber or a few-mode fiber. Such amplifiers need to be optimized such that the gain in the different cores, or in the second case for the different fiber modes, is quite similar.

The only competitors to erbium-doped fiber amplifiers in the 1.5-μm region are Raman amplifiers, which profit from the development of higher power pump lasers. Raman amplification can also be done in the transmission fiber. Nevertheless, EDFAs remain very dominant.

Further Technical Details

The most common pump wavelength for EDFAs is around 980 nm. Light at this wavelength pumps erbium ions from their ground-state manifold ⁴I_15/2 to the ⁴I_11/2 manifold, from where there is a quick non-radiative transfer to the upper laser level ⁴I_13/2. Due to that quick transfer, there is essentially no deexcitation via stimulated emission by pump light, and very high excitation levels can be achieved. Therefore, this approach makes it possible to achieve the highest gain efficiency (order of 10 dB/mW) and the lowest noise figure, although the power efficiency is not ideal due to the significant quantum defect.

A higher power efficiency can be achieved by in-band pumping around 1450 nm. However, stimulated emission by pump light then limits the achievable excitation level, hence also the gain per unit length, and the maximum gain occurs at longer wavelengths. The noise figure will also be higher.

Due to the not very high transition cross sections, the saturation power of an EDFA is fairly high compared with that of a semiconductor optical amplifier. Therefore, single symbols in high bit rate data transmissions have a much too low energy to cause any significant gain saturation. Only over thousands or millions of symbols, the gain adjusts itself to the average signal power level.

In high-gain amplifiers, amplified spontaneous emission (ASE) is often the limiting factor for the achievable gain. Due to the quasi-three-level nature of the erbium ions, ASE powers can be different between forward and backward direction, and the maximum ASE can occur at a wavelength which differs from that of maximum gain.

The noise figure of an optimized EDFA is slightly larger than the theoretical limit of 3 dB for a high-gain amplifier; this is mainly due to the quasi-three-level nature. Relatively low-noise performance can be achieved with suitable amplifier design, taking into account particularly the erbium excitation level near the signal input end, which can be strongly influenced by the pump direction.

Various aspects of erbium-doped amplifiers (magnitude and spectral shape of the gain, power conversion efficiency, gain saturation effect, ASE, sensitivity to back-reflections etc.) can be analyzed with suitable fiber simulation software. The resulting quantitative understanding can be the basis for optimization of devices in terms of performance and required components. Generally, the operation details are too complex for optimization based on trial and error.

Variants and Other Applications of Erbium-doped Fiber Amplifiers

A high gain in a shorter length can be achieved with ytterbium-sensitized fibers, i.e., fibers doped with erbium and ytterbium. They are also called Er:Yb:glass fibers or ytterbium-codoped fibers. Their concentration of Yb³⁺ ions is an order of magnitude higher than that of Er³⁺. The ytterbium ions may be excited e.g. with 975-nm pump light (or even at longer wavelengths such as 1064 nm) and transfer their energy to erbium ions. For a proper choice of the material composition of the fiber core (often with substantial amounts of phosphorus), the energy transfer can be fairly efficient. However, the use of pure erbium-doped fibers is more common in the telecom area, because ytterbium sensitization has no essential advantages here and possibly leads to a reduced gain bandwidth due to the modified chemical composition.

Erbium-doped double-clad fibers can be used for generating very high output powers of tens of watts or even more. Ytterbium codoping is in that case usually used, because the pump absorption efficiency would otherwise be poor. Even for moderately high signal power levels around 1 to 2 W, as needed e.g. for cable TV (CATV) amplifiers, double-clad Er/Yb amplifiers are used, since suitable pump diodes for core pumping would not be available.

It is also possible to amplify ultrashort pulses in the 1.5-μm region to relatively high energies, using EDFAs in the form of amplifier chains. One exploits the relatively high saturation energy of such amplifiers, particularly when using erbium-doped large mode area fibers.

Bibliography