We investigate a new phenomenon, where a reciprocal fber ring laser switches from bidirectional to unidirectional operation above a certain pump power threshold. Signifcant simplifcations regarding earlier experiments are presented, which for the frst time allow the identifcation of individual nonlinear efects. We highlight the unique role of stimulated Raman scattering in triggering unidirectional operation, and that additional conditions apply. The threshold is reduced from 30 to 3.8W, which eases potential applications.