Self pulsing versus self locking in a cw pumped neodymium doped double clad fiber laser

We investigated the emission behavior of cw diode pumped double clad fiber lasers with and without a semiconductor saturable absorber mirror (SESAM) present. Without the SESAM, we verified that cw-pumped multi longitudinal mode lasers start oscillating in a pulsed fashion. Maximizing the emission rate (maximum emission principle [C.L. Tang, H. Statz, Maximum emission principle and phase locking in multimode lasers, J. Appl. Phys. 38 (7) (1967) 2963]), the lasing system favors a pulsed emission (self-pulsing) over a cw one. Simultaneous oscillation of a large number of modes in homogeneously broadened lasers is not exclusively due to spatial hole burning but due to spontaneous mode locking spanning a certain frequency range regardless of the number of modes contained in this range. Third order nonlinear polarization terms (combination tones) help to equalize non equidistant (dispersion shifted) mode intervals. The combination tones act as seed signals onto which the dispersion shifted mode frequencies lock establishing moreover, at the same time, definite phase relationships between the modes. With the SESAM self pulsing goes over into a self locked mode of emission. The conditions to achieve this type of emission behavior are inferred from the self pulsing state.