Abstract

Actively mode-locked semiconductor lasers need high quality antireflection coatings to avoid multiple pulse formation and mode-locking in clusters. The addition of an intrawaveguide saturable absorber eliminates these problems by absorbing the reflected pulses from the antireflection coating. Saturable absorption also causes pulse shortening and introduces self-phase modulation opposite in sign to that of saturable gain. Two section lasers with four GaAs quantum well active regions were compared using active and hybrid mode-locking techniques. The saturable absorber is formed by reverse biasing an 80-μm end segment and high reflection coating the facet. The opposite facet has a 10^-3 antireflection coating for a 200-ps roundtrip time external cavity. Active mode-locking produced 9.5-ps pulses with a 400-GHz spectral width at 1-mW average power. Mode-locking was in clusters as evidenced by 8-dB nulls in the spectrum due to laser diode Fabry-Perot modes. The addition of the saturable absorber reduced the pulse width to 2.5 ps with an 800-GHz spectral width. The spectral ripple was reduced to 0.5 dB and the time–bandwidth product improved from 3.8 to 2.0. The impulse response of the saturable absorber photocurrent was measured as 45 ps showing that a p-i-n reversed biased absorber can recover quickly without the need for proton bombardment or other means to shorten electronic lifetime.

© 1991 Optical Society of America