If a strong light beam is amplified in a laser gain medium, the gain can be saturated. That saturation is called homogeneous if the shape of the gain spectrum is not modified, i.e., if the relative change of gain (in decibels) is the same at all optical wavelengths.
Homogeneous saturation usually occurs when the finite gain bandwidth is caused by homogeneous broadening. On the other hand, inhomogeneous saturation typically results when different laser atoms or ions exhibit different characteristics, for example due to different lattice sites occupied in a solid-state host medium. 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.
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