Experiments are described in which the intensity and spectral distribution of amplified spontaneous radiation is measured on optically pumped AlxGa(1-x)As-AlyGa(1-y)As double-heterojunction structures as a function of the length of the region where the amplification takes place. From these data information is obtained on the gain versus wavelength dependence and gain saturation as a function of the excess carrier densities created. Furthermore, the optical absorption of a short unpumped region was also determined. Results are presented for structures with a GaAs active region (y approx 0) and for a structure withy approx 0.10. The results are compared with those of calculations based on various recombination models. Furthermore, a comparison is made with the values of the normalized threshold current density obtained on laser diodes fabricated from the same wafers.