  We theoretically investigate the possibility of generating pulses in an
excitable (asymmetric) semiconductor ring laser (SRL) using optical trigger
pulses. We show that the phase difference between the injected field and the
electric field inside the SRL determines the direction of the perturbation in
phase space. Due to the folded shape of the excitability threshold, this has an
important influence on the ability to cross it. A mechanism for exciting
multiple consecutive pulses using a single trigger pulse (i.e. multi pulse
excitability) is revealed. We furthermore investigate the possibility of using
asymmetric SRLs in a coupled configuration, which is a first step toward an
all-optical neural network using SRLs as building blocks.
