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.