Definition: a measure of the strength of optical amplification
More specific terms: small-signal gain, laser gain, Raman gain
Formula symbol: g, G
Units: %, dB or dimensionless number
In photonics, the term gain is usually used to quantify the amplification of optical amplifiers or of a laser gain medium. Different meanings occur in the literature:
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.
Apart from its magnitude, important properties of gain are its spectral bandwidth and its saturation characteristics.
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.
Bibliography
[1]
R. Paschotta, tutorial on "Fiber Amplifiers", part 2 on gain and pump absorption
