A passively mode-locked external-cavity semiconductor laser emitting 60-fs pulses

Optical pulses with durations of a few tens of femtoseconds are used to drive applications ranging from attoscience to metrology 1 , but femtosecond lasers remain bulky and expensive items based on vibronic crystal or ytterbium-doped silica fibre gain media. Although, in principle, bandgap-engineered quantum-confined semiconductor materials have significant advantages in terms of their gain bandwidth and spectral versatility, semiconductor lasers have not, to date, been capable of generating comparably short pulses. Here, we describe an optically pumped InGaAs/GaAs quantum-well laser that is passively mode-locked using an intracavity semiconductor saturable absorber mirror 2 that emits 60-fs pulses at 1,037 nm with an optical spectrum bandwidth of 20 nm (full-width at half-maximum). In this laser, fast optical Stark self-absorption modulation and strong gain saturation combine to shape pulses on the timescale of carrier–carrier scattering. The cavity contains a stable group of circulating pulses, regularly spaced at an interval fixed by the time over which the saturated gain recovers. A compact source of 60-fs pulses is demonstrated, based on a semiconductor laser that is mode-locked using a saturable absorber mirror in an external cavity.