定义：
衡量放大器中附加噪声的参数。
 
光放大器的噪声系数F（例如，光纤放大器或者半导体光放大器）是衡量放大器附加到信号上的噪声大小。更准确的说，是用来表征放大的输出光束的功率谱密度比入射信号功率谱密度与放大因子G的乘积大了多少的因子，假设入射光束噪声在散粒噪声水平： 

假设入射光在散粒噪声极限非常重要：当然如果入射信号光存在强的附加噪声，噪声水平相对提高的要少一些。 

噪声系数也采用分贝值来衡量，也就是对F取以10为底的对数值。理论上无噪声的放大器的噪声系数为1，对应的是0dB。 

放大器的附加噪声在高的输入噪声水平时重要性会比较低。例如，如果采用两级放大器，第二级的输入噪声要远高于散粒噪声（假设第一级增益很大）。因此，第二级的附加噪声一般不是太相关（除非特别强），它对放大器链的噪声系数的贡献不是很大。 

光放大器的量子噪声 
量子光学的一个很重要的影响就是相位不敏感的高增益光纤放大器的噪声系数至少为3dB。四能级放大器可以达到这个极限，非简并的光学参量放大器或者拉曼放大器都是准三能级放大器，噪声系数更大。在放大器输入处的附加损耗会增大噪声系数。

只有相位敏感的放大器，如简并参量放大器的噪声系数可以在3dB以下。 

当放大器被应用于光纤通信领域是噪声系数需要考虑。掺铒光纤放大器，光纤拉曼放大器和半导体光放大器的噪声系数都不是理想的，依赖于设计细节，并且在输入端的额外信号损耗也会使其增大。在准三能级放大器中，背向泵浦得到的噪声系数更大，也就是泵浦光与信号光的传播方向相反时。 

电子放大器的噪声系数 
电子放大器的噪声系数定义是不同的：它是以热噪声水平作为基准，而不是以散粒噪声水平。这重要是因为在电子器件中，热噪声是限制因素，因为光子由于频率很低所以能量远低于热能。 

Definition: a measure of the excess noise added in an amplifier

German: Rauschzahl

Categories: optical amplifiers, fluctuations and noise

Formula symbol: F

Units: dB

For an ideal optical amplifier, one may expect that any amplitude or phase noise is simply the noise at the amplifier input multiplied with the amplification factor – with no excess noise (amplifier noise) added by the amplifier. Unfortunately, however, that is not even physically possible due to unavoidable quantum noise influences. The noise figure F of an optical amplifier (e.g. a fiber amplifier or semiconductor optical amplifier) is a measure of how much excess noise the amplifier adds to the signal. More precisely, it is a factor which indicates how much higher the noise power spectral density of the amplified output is compared with the input noise power spectral density times the amplification factor G, assuming that the input noise is at the shot noise level:

The noise figure is often specified in decibels (dB), i.e. as ten times the logarithm (to base 10) of F. A (hypothetical) noiseless amplifier would have a noise figure of 1, corresponding to 0 dB.

The significance of excess noise of an amplifier is lower for higher input noise levels. For example, if two fiber amplifier stages are used in series, the second stage is fed with noise well above the shot noise limit (assuming that the first stage has significant gain). Therefore, the excess noise of the second stage is not very relevant (unless it is very strong), and it does not contribute significantly to the noise figure of the amplifier chain.

Quantum Noise of Optical Amplifiers

An important result of quantum optics [1] is that a phase-insensitive high-gain optical amplifier has a noise figure of at least 3 dB. This limit can be reached for a four-level laser amplifier, a non-degenerate optical parametric amplifier, or a Raman amplifier, whereas quasi-three-level amplifiers have higher noise figures. The noise figure can be increased by excess losses at the amplifier input. Noise figures below 3 dB are only possible for phase-sensitive amplifiers, based on, e.g., degenerate parametric amplification.

The noise figure is often relevant for amplifiers used in optical fiber communications. Erbium-doped fiber amplifiers, fiber Raman amplifiers and semiconductor optical amplifiers have non-ideal noise figures, which depend on design details, and can be increased further e.g. due to extra signal losses at the input end. In quasi-three-level amplifiers, the noise figure is larger for backward pumping, i.e., when the pump light propagates in the direction opposite to that of the signal light, because that leads to a lower excitation of the laser-active ions at the signal input end.

Noise Figure of Electronic Amplifiers

Note that the noise figure of electronic amplifiers is defined differently: the thermal noise level rather than the shot noise level is taken as a basis. This is essentially because thermal noise is the limiting factor for electronics, where the quantum energy is far below the thermal energy due to the much lower frequencies.

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