Abstract

Novel intelligent fiber telecommunication networks¹ are in need of simple high-power subpicosecond pulse sources. Particularly in the context of clock recovery and synchronization, Nd fiber oscillators are an attractive candidate. The short length of the gain medium (<20 cm) potentially permits derivation of a high-repetition-rate high-power (>200 pJ) clock signal at the fundamental cavity frequency, whereas in Er oscillators the long length of the gain medium imposes severe restrictions on both the pulse energy and the repetition rate of the pulse trains.^2,3 However, a major limitation of passively mode-locked Nd fiber lasers to date⁴ has been that an active modulator is required for initiating the pulses, which greatly increases the cost of a potential device. In addition, the absence of the modulator decreases material dispersion considerably and thus increases the compactness and stability of the system. As a further benefit, the output power increases because of reduced intracavity losses. Here we demonstrate for the first time to our knowledge passive mode-locked operation of a Nd fiber oscillator, where a simple translation of a cavity mirror is sufficient to initiate pulse formation. The method yields pulses as short as 53 fs with an energy content of 1 nJ. In an additional derivation of the cavity design we also demonstrate direct 34-fs pulse generation in the oscillator; these are to our knowledge the shortest pulses yet obtained with a waveguide laser.

© 1993 Optical Society of America