the invention is the response of the saturable absorber with a fast saturable absorber contributing substantially to the formation of short pulses it is known that in general the exciton absorption is saturated more effectively by the excitons themselves than by the free carriers the excitons decomposed to at room temperature the exciton lifetime in an iii v semiconductor such as gaalas is typically about 300 fsec thus a very fast saturable absorber can be realized by designing a quantum well laser according to the invention to operate at a frequency u03bd o corresponding to e ex the energy of the exciton transition of the quantum well or wells thus it is advantageous to make the quantum well or wells and the waveguide means such that e ex is approximately equal to i e within u00b1 10 of h u03bd o where h is planck 39 s constant pulse generating means according to the invention can be incorporated into a wide variety of apparatus in place of conventional pulse generating laser means exemplary of such apparatus is a time division multiplexed optical fiber communication system of the type that is schematically depicted in fig4 in this figure electrical signals on each of channels 1 n are converted to streams of optical pulses in electrical to optical e o conversion means 401 40n the resulting n streams of optical pulses travel on.