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.
See also: gain, laser threshold, threshold pump power
and other articles in the categories optical amplifiers, physical foundations
If you like this article, share it with your friends and colleagues, e.g. via social media: