© 2017 by Taylor & Francis Group, LLC. Lasers, being oscillators at optical frequencies, usually produce laser radiation, which is characterized by an extremely high degree of monochromaticity, coherence, directionality, and brightness. These unique properties of laser light have inspired the widespread use of laser sources in numerous applications. Monochromaticity, or narrow spectrum output, which is usually much smaller than the gain profile of the amplification material, is considered to be one of the inherent laser features. Even media with a very broad, homogeneously broadened gain profile (e.g., laser dyes, Ti3+:sapphire, alexandrite, forsterite, color center), being placed in a nondispersive cavity, provide a significant line narrowing of the output laser radiation. The physical mechanism of the line narrowing is a consequence of the spectral dependence of 358the gain profile of the amplification media: frequency modes with highest gain build up in the laser cavity faster than others, lower the total inversion, and eventually suppress other frequency modes with lower gain. 1 - 4.