Abstract

Semiconductors have recently been shown to be useful as saturable absorbers. Both bulk and quantum well materials have been used to generate picosecond and femtosecond pulses in color center[1], erbium-doped fiber[2], dye[3] and semiconductor[4] laser systems. These applications typically use damage sites or traps to decrease the absorption recovery time. In semiconductor diode lasers, monolithic structures incorporating waveguide saturable absorber sections for passive modelocking have produced pulses of 0.6 picoseconds, at repetition rates up to 350 GHz. However, the exact mechanism for such high frequency modelocking is not well understood. In order to produce a stable train of short pulses at these millimeter-wave repetition rates, the absorption recovery must be fast enough to recover on each round trip in the cavity, e.g., 10 ps for a 100 GHz repetition rate. The saturable absorber sections are typically p-i-n structures operated under reverse bias to remove photogenerated carriers and achieve this fast recovery, as in a p-i-n photodiode. Knowledge of the dynamics of the saturable absorbers is necessary for design optimization, as well as insight into the potential of these devices for producing short optical and electrical pulses. Absorption recovery is also thought to be responsible for elimination of multiple pulses in external cavity lasers[5]. In this paper we will present a study of the bleaching and recovery dynamics of these waveguide saturable absorbers for different bias conditions, and discuss the implications of the results for modelocked laser systems.

© 1992 The Author(s)