Abstract

Mode-locked semiconductor diode lasers are compact and simple sources of short optical pulses that are well suited for a number of applications. However their technology is not as mature as that of mode-locked solid-state lasers and their potential has not been fully exploited yet. In this paper we present our experimental results showing that picosecond pulses have been reliably generated with a semiconductor diode laser operated with a ring cavity. An angle etched travelling-wave amplifier (InGaAs) designed to be operated at 860 nm was used as the active medium. The mode-locked operation was purely passive and it produced an average power of 3 mW per beam. We observed from 1 to 10 pulses per round trip in the cavity along both directions (repetition rates of 300 MHz to 3 GHz). The optical spectra associated to the countepropagating pulses had a width of up to 4 nm; their peak wavelengths would differ from 1 nm (as shown in Fig. 1) to as much as 6 nm; in some cases the spectra did not overlap and the laser was indeed a dual-wavelength picosecond source. The pulse duration could be as short as one picosecond, but was more typically around 1.6 ps (as shown in Fig. 2). Mode-locked operation was achieved through a proper cavity alignment. The active medium was uniformly biased; however the asymmetry of its spontaneous emission spectrum indicates the presence of a defect at one facet. That defect very likely acts as a saturable absorber, but other effects do contribute to the mode-locked operation: cross-phase modulation between counterpropagating pulses, spectral filtering due to the chromatic dispersion of the intracavity optics, self-bending at the angled facets. A model integrating all these effects will be described.

© 2000 IEEE