定义：
放大光束功率的装置。

光纤放大器是接受入射信号光，然后产生更高功率的输出信号的装置。通常输入和输出信号都是激光光束，是在自由空间或者光纤中传输的高斯光束。放大过程发生在增益介质中，需要外加光源泵浦（用来提供能量）。大多数的放大器是有光或者电泵浦的。 

目录

• 激光放大器与基于光学非线性放大器的比较
• 多通道装置，正反馈放大器和放大器链
• 增益饱和
• 有害效应
• 超快放大器
• 光学放大器的重要参数
• 应用

激光放大器与基于光学非线性放大器的比较 
大部分放大器都是激光放大器，放大过程基于受激辐射。增益介质包含激发态的原子、粒子或者分子，受到信号光激发后辐射更多具有相同模式的光。有的增益介质为掺有激光活性离子的绝缘体，有的是半导体，这些介质可以被电或者光泵浦。用于激光放大器的掺杂绝缘体为激光晶体和体形式的玻璃，或者一些波导结构，例如光纤（光纤放大器）。激光活性离子是稀土离子或者过渡金属离子（不常见）。掺铒光纤放大器是一种非常重要的激光放大器，它一般用于光纤通讯领域中。 
在光纤放大过程中，除了受激辐射外，还存在其它的物理机制，这些物理过程基于各种不同的非线性效应。光学参量放大器的原理是利用介质的二阶非线性效应，也有光纤参量装置是基于光纤中的三阶非线性过程。其它类型的非线性放大器，如拉曼放大器和布里渊放大器利用介质的延迟非线性响应。 

多通道装置，正反馈放大器和放大器链 
光学体激光放大器通常只能提供中等程度的增益，一般只有几个dB。这可以应用于超短脉冲激光器，它们大多采用宽带增益介质，具有比较小的发射截面。有效增益可以通过在同一增益介质中设置多个通道的辐射（多通放大器）来提高，或者也可以采用一个放大器组成一个链的形式（放大器链）来提高增益。 

图1.多通飞秒放大器的示意图。 
可以采用多个镜子（多个通道的方向之间存在夹角）的结合来实现多个通道工作，或者也可以采用正反馈放大器（通常用于超短脉冲）。 
多级放大器（放大器链）更适宜于得到非常高的放大系数。例如，一个正反馈放大器可以将脉冲能量放大到几个mJ，然后多通放大器将脉冲能量继续放大到几百个mJ。在各级放大级之间，需要对脉冲进行空间滤波或者光谱滤波，从而得到更高的光束质量和更短的脉冲持续时间。 

增益饱和 
如果输入光光强或者频率过高，增益介质的增益因子则会发生饱和现象，也就是说会变小（增益饱和）。这很自然，因为放大器不可能放大具有任意能量的输入光。然而，当激光放大器（尤其采用固态增益介质的）在增益介质中存贮了一些能量后，可以在非常短的时间内将这些能量提取出来。因此，输出光功率可以在很短时间间隔内比泵浦光功率高许多数量级。 

有害效应 
在高增益情况下，寄生的若反射光也可以产生寄生激光，也就是说即使没有输入信号，也可以产生震荡，也可以说是不由输入信号产生的额外输出信号。这种效应限制了可以得到的增益。即使不存在任何的寄生反射，自发辐射放大信号也从放大器中提取了相当的能量。 
放大器通常也会在输出光束中加入一些附加噪声。这不仅在激光放大器中存在，其中自发辐射效应是附加噪声的部分来源，而且在非线性放大器中也存在。 

超快放大器 
有些放大器可以用来放大超短脉冲。有时需要放大的是高重复速率的脉冲列，这是得到了很高的平均输出功率但是脉冲能量强度是中等的。有时，低重复率的脉冲列经过更高的增益后得到了很高输出的脉冲能量，对应的峰值功率非常高。在超快放大器的词条中，介绍了这些装置的很多其它的细节。 

光学放大器的重要参数 

• 峰值增益，也被称作放大因子，由dB值表示 
• 饱和功率，与增益效率相关 
• 饱和输出功率（对应某一泵浦功率而言） 
• 功率效率和泵浦功率 
• 饱和能量 
• 储存能量时间（上能态寿命） 
• 增益带宽（增益谱的平滑度） 
• 噪声系数和更详细的噪声指标 
• 背向散射灵敏度 

不同种类的放大器差别很大，例如在饱和特性方面。例如，稀土掺杂增益介质可以储存相当多的能量，然而光学参量放大器则只有在泵浦光存在的情况下才能提供放大。半导体放大器储存的能量比光纤放大器少很多，这在光纤通信领域是非常重要的应用。 

应用 
光学放大器的典型应用包括： 

• 放大器可以将激光器功率提高到很到的水平（主振荡功率放大器，MOPA） 
• 可以得到非常高的峰值功率，尤其是对于超短脉冲，当储存的能量在很短的时间内提取出来时。 
• 可以在光探测器之前放大弱信号，因此可以减小探测噪声，放大器噪声很大时例外。 
• 光纤通信中长距离光纤链路传输中，在长距离光纤之间需要将光功率提高到某一值，以防信息淹没在噪声中。

Definition: devices for amplifying the power of light beams

More specific terms: amplifier chains, distributed amplifiers, multipass amplifiers, fiber amplifiers, ultrafast amplifiers, regenerative amplifiers, semiconductor optical amplifiers, power amplifiers, high-power amplifiers, tapered amplifiers, optical parametric amplifiers, Raman amplifiers

Opposite terms: optical attenuators

An optical amplifier is a device which receives some input signal and generates an output signal with higher optical power. Typically, inputs and outputs are laser beams, either propagating as Gaussian beams in free space or in a fiber. The amplification occurs in a so-called gain medium, which has to be “pumped” (i.e., provided with energy) from an external source. Most optical amplifiers are either optically or electrically pumped.

Laser Amplifiers

Most optical amplifiers are laser amplifiers, where the amplification is based on stimulated emission. Here, the gain medium contains some atoms, ions or molecules in an excited state, which can be stimulated by the signal light to emit more light into the same radiation modes. Such gain media are either insulators doped with some laser-active ions, or semiconductors (see below). Doped insulators for laser amplification are laser crystals and glasses used in bulk form, or some types of waveguides, such as optical fibers (→ fiber amplifiers, see Figure 1). The laser-active ions are usually either rare earth ions or (less frequently) transition-metal ions. A particularly important type of laser amplifier is the erbium-doped fiber amplifier, which is used mostly for optical fiber communications.

Semiconductor optical amplifiers can be electrically or optically pumped. In most cases, electrical pumping is used, which is highly convenient and allows for very compact amplifiers. While ordinary semiconductor optical amplifiers are quite limited in output power, substantially higher pulse (up to several watts) can be obtained from tapered amplifiers.

Figure 1: Schematic setup of a simple erbium-doped fiber amplifier as an example for an optical amplifier. Two laser diodes (LDs) provide the pump power for the erbium-doped fiber, allowing it to amplify light with wavelength around 1550 nm. Two pig-tailed Faraday isolators strongly reduce the sensitivity of the device to back-reflections.

Amplifiers Based on Optical Nonlinearities

In addition to stimulated emission, there also exist other physical mechanisms for optical amplification, which are based on various types of optical nonlinearities. Optical parametric amplifiers are usually based on a medium with χ⁽²⁾ nonlinearity, but there are also parametric fiber devices using the χ⁽³⁾ nonlinearity of a fiber. Other types of nonlinear amplifiers are Raman amplifiers and Brillouin amplifiers, exploiting the delayed nonlinear response of a medium.

An important difference between laser amplifiers and amplifiers based on nonlinearities is that laser amplifiers can store some amount of energy, whereas nonlinear amplifiers provide gain only as long as the pump light is present.

Ultrafast Amplifiers

Amplifiers of different kinds may also be used for amplifying ultrashort pulses. In some cases, a high repetition rate pulse train is amplified, leading to a high average power while the pulse energy remains moderate. In other cases, a much higher gain is applied to pulses at lower repetition rates, leading to high pulse energies and correspondingly huge peak powers. An essential point is the ability of a laser amplifier to store some amount of pump energy until the amplified pulse extracts led energy. For more details, see the article on ultrafast amplifiers.

Multipass Arrangements, Regenerative Amplifiers, and Amplifier Chains

A bulk-optical laser amplifier often provides only a moderate amount of gain, typically only few decibels. This applies particularly to ultrashort pulse amplifiers, since they must be based on broadband gain media, which tend to have lower emission cross sections. The effective gain may then be increased either by arranging for multiple passes of the radiation through the same amplifier medium (multipass amplifier), or by using several amplifiers in a sequence (→ amplifier chains).

Figure 2: Setup of a multipass femtosecond amplifier.

Multipass operation (Figure 2) can be achieved with combinations of mirrors (for several passes with slightly different angular directions), or (mostly for ultrashort pulses) with regenerative amplifiers.

For very large amplification factors, multi-stage amplifiers (amplifier chains) are often better suited. For example, one may have a low-power but high-gain preamplifier followed by a power amplifier or booster amplifier, which delivers a modest gain but a high output power level. Between the amplifier stages, the optical signal may be spatially or spectrally filtered in various ways, helping e.g. to achieve a high beam quality and/or a shorter pulse duration. Reasons for choosing a multi-stage amplifier design may be exactly the opportunity for filtering between the stages, or the use of adapted parts for the different power levels – for example, a high-gain amplifier fiber with small effective mode area for a preamplifier and a double-clad large mode area fiber for the power amplifier.

Multimode and Single-mode Amplifiers

Some types of optical amplifiers are intrinsically multimode devices. For example, an amplifier based on bulk laser crystal can amplify multimode beams and can also handle beams with different angular positions and directions in certain ranges, which makes it possible to construct a multipass amplifier (see above).

Other optical amplifiers are single-mode devices. For example, many fiber amplifiers and semiconductor optical amplifiers are based on a single-mode fiber or waveguide. In such cases, only a single mode can be amplified, and that has to be mode-matched to the amplifier.

When a single-mode laser amplifier provides a high gain within some bandwidth, it inevitably generates a substantial level of amplified spontaneous emission (ASE). In a multimode amplifier, this effect is correspondingly stronger. Even if ASE is negligible (for lower gain), a multimode amplifier typically exhibits more power losses by spontaneous emission (and thus has a lower gain efficiency) because a larger amount of laser-active material has to be kept in the excited state.

Gain Saturation

For high values of the input light intensity or fluence, the amplification factor of a gain medium saturates, i.e., is reduced (→ gain saturation). This is a natural consequence of the fact that an amplifier cannot add arbitrary levels of energy or power to an input signal. However, as laser amplifiers (particularly those based on solid-state gain media) store some amount of energy in the gain medium, this energy can be extracted within a very short time. Therefore, during some short time interval the output power can exceed the pump power by many orders of magnitude.

Detrimental Effects of High Gain

For high gain, weak parasitic reflections can cause parasitic lasing, i.e., oscillation without an input signal, or additional output components not caused by the input signal. This effect then limits the achievable gain. Even without any parasitic reflections, amplified spontaneous emission may extract a significant power from an amplifier.

Amplifier Noise

Generally, amplifiers do not only amplify any intensity or phase noise of the input, but also add some excess noise. This applies not only to laser amplifiers, where excess noise can partly be explained as the effect of spontaneous emission, but also to nonlinear amplifiers. The noise figure e.g. of a fiber amplifier is a measure for how much excess noise occurs. Quantum optics dictates a minimum amount of excess intensity and phase noise for phase-insensitive amplifiers.

Important Parameters of an Optical Amplifier

Important parameters of an optical amplifier include:

• the maximum gain, specified as an amplification factor or in decibels (dB)
• the saturation power, which is related to the gain efficiency
• the saturated output power (for a given pump power)
• the power efficiency and pump power requirements
• the saturation energy
• the time of energy storage (→ upper-state lifetime)
• the gain bandwidth (and possibly smoothness of gain spectrum)
• the noise figure and possibly more detailed noise specifications
• the sensitivity to back-reflections
• the number of modes it can amplify (see above, multimode and single-mode amplifiers)

Different kinds of amplifiers differ very much e.g. in terms of saturation properties. For example, rare-earth-doped laser gain media can store substantial amounts of energy, whereas optical parametric amplifiers provide amplification only as long as the pump beam is present. As another example, semiconductor optical amplifiers store much less energy than fiber amplifiers, and this has important implications for optical fiber communications.

Applications of Optical Amplifiers

Typical applications of optical amplifiers are:

• An amplifier can boost the (average) power of a laser output to higher levels (→ master oscillator power amplifier = MOPA).
• It can generate extremely high peak powers, particularly in ultrashort pulses, if the stored energy is extracted within a short time.
• It can amplify weak signals before photodetection, and thus reduce the detection noise, unless the added amplifier noise is large.
• In long fiber-optic links for optical fiber communications, the optical power level has to be raised between long sections of fiber before the information is lost in the noise.

Bibliography