'Polarisation collapse' of the radiation emitted by a neodymium glass laser with a passive LiF : $F_2^-$ crystal switch

The 'polarisation collapse' of the radiation of a neodymium glass laser with a passive LiF : $F_2^-$ crystal switch was discovered. This effect involves lifting, as the number of the output pulses increases, the degeneracy of the multi-stable polarisation states of the radiation during the initial stage of the laser operation. The polarisation collapse was found to be related to the bleaching of the $F_2^-$ colour centres as a result of two-photon absorption by an excited state of these centres.