Actively Mode-Locked Fiber Lasers

We review recent investigation into the active mode-locking of rare earth doped silica, monomode fiber lasers. Dopants include Nd3+, Er3+ and Yb3+:Er3+ and mode-locking has been accomplished either by amplitude modulation or by phase modulation. Both bulk optic and integrated optic modulators have been used. Pulses of 4 psec duration have been observed in an Er3+-doped fiber laser with an integrated amplitude modulator. 20 psec pulses have been produced from a Nd3-doped fiber laser with a bulk phase modulator. In both these systems, the introduction of anomalous group velocity dispersion compensates for self-phase modulation, resulting in bandwidth-limited operation. The paper concentrates mainly on three experiments involving the mode-locking of fiber lasers by active phase modulation. A bulk LiNb03 phase modulator produces pulses of 20 psec and 37 psec respectively in Nd3-doped and Yb3:Er3-doped fiber lasers. The third system incorporates intra-fiber phase modulation. This allows considerable simplification of the laser cavity, resulting in a diode-pumpable source of 80 psec mode-locked pulses with a threshold pump power of 520 pW and a slope efficiency of 48%.