定义：
小信号强度的光学增益，并且不会发生放大饱和现象。

增益介质（激光器或者放大器中的介质）的小信号增益是输入信号非常弱时不会引起任何的增益饱和现象的情况。在连续波工作时，这表示增益介质的任何地方的信号强度都低于饱和强度。 

在四能级激光增益介质中，ASE和寄生激光效应都可以忽略时，用分贝值来表示的稳态的小信号增益通常正比于吸收的泵浦功率和储存的能量。当小信号增益等于谐振腔损耗时，会达到激光器阈值或者光纤参量放大器的阈值。在调Q激光器中，高的小信号增益可以得到短的脉冲长度。在高增益放大器中（例如，光纤放大器），小信号增益通常受放大的自发辐射或者寄生激光效应的限制。

Definition: optical gain for small signal intensities, where no amplifier saturation occurs

More general term: gain

Formula symbol: g_ss

Units: %, dB

The small-signal gain of a gain medium (e.g. a laser medium in a laser or amplifier) is the gain obtained for an input signal which is so weak that it does not cause any gain saturation. In continuous-wave operation, this means that everywhere in the gain medium the signal intensity is well below the saturation intensity.

In a four-level laser gain medium with negligible ASE and no parasitic lasing, the small-signal gain in the steady state, as measured in decibels, is usually proportional to the absorbed pump power and to the stored energy.

The threshold of a laser or an optical parametric oscillator is reached when the small-signal gain equals the total resonator losses. In a Q-switched laser, a high small-signal gain helps to achieve a short pulse duration. In a high-gain amplifier (e.g. a fiber amplifier), the small-signal gain achievable is often limited by amplified spontaneous emission (ASE) or by parasitic lasing.

Measurement of Small-signal Gain

For measuring the small-signal gain of a fiber amplifier (based on a rare-earth-doped fiber), for example, one needs to inject a test signal at the relevant wavelength, which is weak enough not to significantly modify the amplifier gain. Then one needs to measure both the launched signal power and the resulting amplified signal power.

Such measurements would be substantially more difficult e.g. in a bulk laser, where the injected input signal would be influenced by the resonator mirrors. In such cases, one may take an indirect approach – for example, calculate the required small-signal gain for reaching the laser threshold (based on known reflectivities of the resonator mirrors) and measuring at what pump power that is achieved. The gain at other pump power levels may then be estimated – in simple cases based on the assumption that it is linearly dependent on the pump power.