- The gain can simply be an amplification factor, i.e., the ratio of output power and input power.
- Particularly for small gains, the gain is often specified as a percentage. For example, 3% correspond to a power amplification factor of 1.03.
- Particularly large gains are often specified in decibels (dB), i.e., as 10 times the logarithm (to base 10) of the amplification factor. For example, a fiber amplifier may have a small-signal gain of 40 dB, corresponding to an amplification factor of 104 = 10 000.
- One also often specifies a gain per unit length, or more precisely the natural logarithm of the amplification factor per unit length, or alternatively the decibels per unit length.
The gain achieved e.g. in a fiber amplifier or the gain medium of a laser depends on the population densities in different electronic levels, which themselves depend on the optical intensities. Rate equation modeling may be used for calculating the gain and investigating its dependence on various influences.
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See also: laser gain media, gain bandwidth, gain clamping, optical amplifiers, fiber amplifiers, gain saturation, gain narrowing, gain switching, gain efficiency, homogeneous saturation, inhomogeneous saturation
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