Synthesis of the spectrum in ring lasers with coupling of counterpropagating waves in a spectral filter and the filtering by real spectral lines

The formation of radiation with a complex spectrum in a ring titanium laser, with counterpropagating waves coupled in an extracavity spectral filter, is studied. It is shown experimentally that when absorption lines in a flame are used for filtering, weak lenses formed in the flame cause distortions of the spectrum, which can be described only within the framework of a modified method for calculating coupling coefficients. The mechanism of this effect is shown to be associated with a narrow peak of matching fields of counterpropagating waves in unstable cavities in the vicinity of the zero power of a lens in the coupling channel. An optimum regime of the synthesis of the spectrum without distortions but with a high efficiency is found.