Study of mode locking in (GaAl) as injection lasers

This paper describes the important experimental parameters affecting the mode locking of a variety of (GaAl)As injection lasers operating in an external optical cavity. We find that short detector limited pulses (less than 60 ps) and 100 percent modulation depth can only be obtained using lasers which exhibit either an anomalous narrow-band noise or self-pulsations. Little or no mode locking is observed in lasers having the normal broad noise resonance. The observed amplitude, pulsewidth, and frequency of the mode-locked pulses are correlated to the degree of self-pulsation and the external cavity length. The experimental results obtained are in qualitative agreement with a model which uses the rate equations modified by either electron traps or saturable absorbers and a delayed feedback term. Our results appear to imply that mode locking in injection lasers is related to laser defects and is very similar to passive mode locking in dye lasers.