(Ga,Al)As CW diode lasers with a linear light output versus pumping current characteristic have been coupled to an external optical cavity. This coupling is shown to induce nonlinearities in the characteristic. The light output of external-cavity-coupled CW diode lasers is self-pulsing at a resonance frequency of the external cavity and/or at a low frequency in the range of 3-30 MHz. It is found that irregularities in the light output-current characteristic are strictly related to changes of the type of self-pulsation. On the basis of separate measurements of the stimulated and spontaneous emission, it appears that the coupling to the external cavity affects the ratio between stimulated and spontaneous emission but not the total radiative recombination rate. It is supposed that the irregularities are due to variations of the stimulated-emission rate that are related to the external-cavity-induced instability in the time domain.