Abstract

The achievement of a stable optical pulse source in the 1 5 µm wavelength region with a high repetition rate is extremely important for its use as a transmitter in soliton-based optical transmission systems In recent works [1] we proposed and demonstrated a novel non-resonant technique for the generation of high-bit-rate transform-limited pulse trains, based on asynchronous frequency modulation (FM) of an Er-Yb laser with external spectral filtering by a Bragg fiber grating This technique enables to relax the strict constraint of cavity length control needed with conventional active mode-locking methods In this work we present a comparative analysis of pulse train stability, based on BER measurements, of a 2 5-GHz frequency-modulated Er-Yb laser operated in FM mode-locking (synchronous modulation) and in the FM regime (asynchronous modulation), and demonstrate the significant lower sensitivity to cavity length detuning of the latter configuration as compared to standard mode-locking The laser transmitter is similar to that reported in [1] and consists of a compact three-mirror 18-cm long folded cavity with an intracavity LiNbO₃ phase modulator operated at 2 49 GHz in the third-harmonic configuration Under exact synchronism between modulation frequency and cavity free-spectral range, standard FM mode-locking is attained, corresponding to the generation of ≅ 10 ps-duration chirped Gaussian pulse trains. As the cavity is detuned by a few MHz, the laser switches into the frequency modulation regime; in this case ≅ 40 ps-duration transform-limited pulse trains can be generated by external spectral filtering of the FM laser spectrum by a fiber grating [1].

© 2000 IEEE