Violation of the symmetry of waveguide laser modes caused by the lens effect

It is shown that the formation of a negative gas lens in a waveguide gas laser of medium pressure can break the symmetry and the number (classification) of waveguide modes. The violation of symmetry can be caused by imperfect waveguide construction or can occur spontaneously. The spontaneous violation of symmetry is caused by the optical nonlinearity of an active medium (the saturation effect) and leads to the appearance of new modes, which are absent in the linear waveguide with the same optical characteristics. The violation of symmetry of waveguide modes can cause, in particular, a decrease in coherence of laser radiation.