Mode switching in ring lasers with delayed optical feedback

We have demonstrated that a weak external optical feedback with delay can lead to the switching of the counterpropagating modes in a ring laser. The period of this switching is almost equal to the doubling delay time. This time should be longer than any system characteristic time. The equations describing the ring laser with the delayed optical feedback have been obtained and the analytical formula for the necessary external feedback strength was derived. We have shown that the effect can take place in semiconductor lasers on quantum dots.