Single-Mode Operation of Fiber Lasers Based on Time Domain Suppression of Non-Lasing Modes

We experimentally and theoretically demonstrate a new method to obtain a single-mode operation in long cavity lasers. The method is based on suppressing non-lasing modes in the time domain by attenuating their intensity beating with the lasing mode. This suppression is implemented in our experiments by adding into the cavity an intensity modulator, which is controlled by the photo-detected beating signal. We have demonstrated the method in a long Erbium-doped fiber ring laser with a ring cavity length of about 13 m. Single-mode operation without mode-hopping was obtained for a long time duration of about 100 minutes. Although Erbium-doped fiber amplifiers are homogeneously broadened at room temperature, a single-mode operation was achieved in several cavity modes, which do not have the highest small-signal gain. A theoretical model, based on a small-signal stability analysis, is given. The model indicates that the control delay should be accurately chosen.