Lateral mode frequency locking in fiber pump lasers: observations disputing nonlinear coupling mechanisms

Summary form only given. Beam steering in the far-field emission of high-power semiconductor lasers (accompanied by L-I curve kinks) limits their ability to effectively pump Er-doped fibers. This beam steering has been attributed to frequency-locking between the first order and fundamental lateral modes. In order to eliminate beam steering, it is necessary to understand the locking mechanism between the lateral modes. Third-order nonlinearities (resulting from intensity-dependent carrier modulation) and other intensity-dependent nonlinearities induce coupling between the modes, which theoretically can contribute to frequency locking. Coupling can also occur from geometrical asymmetries, gain or finite Q (resonator quality)-induced modal nonorthogonality, optical scattering from defects, and other linear processes. To characterize the effect of intermodal coupling outside the frequency locking band, we analyze a generalized form of Adler´s equation for the relative dynamic phase evolution between the two modes. Observations of the laser output showed harmonic patterns near beam-steering regions of operation (only). Since all harmonics were observed (and have the expected qualitative descending order), the locking mechanism responsible for the observed beam steering is thus most likely linear.