If a strong light beam is amplified in a gain medium, the gain can be saturated. This saturation is called homogeneous if the shape of the gain spectrum is not modified. Homogeneous saturation usually occurs when the finite gain bandwidth is caused by homogeneous broadening. However, quasi-homogeneous saturation is also sometimes observed despite inhomogeneous broadening in situations where many different Stark level transitions overlap. For example, some rare-earth-doped glasses (e.g. neodymium-doped phosphate glasses) exhibit quasi-homogeneous saturation, leading to laser operation with relatively low optical bandwidth.
See also: homogeneous broadening, inhomogeneous saturation
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