We present an investigation of the optical gain and its saturation of the electron-hole plasma confined in GaAs/(Ga,Al)As quantum wells. For such investigations the variable stripe length method is a simple but quite powerful tool. The spacial longitudinal variation of luminous and electron-hole pair densities in narrow stripes of various lengths has been investigated theoretically and experimentally. For this also the spectra of spontaneous luminescence (emitted in a direction perpendicular to the stripe) have been recorded at different places on the stripe. The experimental results are compared with the predictions of a theoretical model of the optical quantum well amplifier. We find that saturation is caused by carrier depopulation through stimulated electron-hole recombination, and by the loss of light travelling in the direction of the stripe due to scattering and/or reabsorption. However, we do not find evidence of intra band saturation, ie. of a non thermal electron or hole distribution.