Abstract

Passively mode-locked erbium-doped fiber lasers (EDFLs) have been attractive sources of ultrashort pulses at the communication wavelength of 1.55 μm.^1,2 Major disadvantages of the passively mode-locked EDFLs are the limited repetition rate to the cavity's fundamental mode frequency and the relatively poor synchronization property of the mode-locked pulses due to self-initiation of the lasing conditions caused by noise fluctuation during polarization controller movements or by evolution of a particular polarization mode. The repetition rate and pulse synchronization properties can be improved in actively mode-locked EDFLs with amplitude or phase modulation, but typically these lasers deliver longer laser pulses than the passively mode-locked lasers do unless a pulse compression method is applied.^3,4 A hybrid-type passively and actively mode-locked erbium-doped fiber ring laser has been attempted to utilize the meritorious properties of both schemes previously, but has shown a nonuniformly distributed laser pulse output because it emitted short pulse trains only within the actively mode-locked pulse envelopes.⁵ In this research we have demonstrated a hybrid-type mode-locked fiber laser in a figure-of-eight geometry with a nonlinear amplifier loop mirror (NALM) and a directional-coupler type electro-optic modulator to generate a uniformly distributed laser pulse stream with a relatively short pulse width.

© 1996 Optical Society of America