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.
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.
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