Absorptive, dispersive and modal bistabilities are considered in semiconductor injection lasers. Previous work is briefly reviewed and the discussion is concerned with experimental work with transverse modal bistability in twin stripe injection lasers. A simple circuit analogy indicates how modal bistability can arise and how it is possible to have stronger light output on the side where the current drive is weakest. Experimental results are reviewed for an optical flip-flop with the use of twin-stripe lasers with three such lasers coupled together in an optical logic circuit. Various mode patterns that have been observed by using different geometries of twin-stripe lasers are discussed. Use is foreseen for active bistable devices in front-end optical signal processing.
