The amplified spontaneous emission of a semiconductor-optical amplifier saturated by two input waves is investigated theoretically and experimentally for both co- and counterpropagating injection. For the first time, an analytical expression for counterpropagating input waves is presented and compared with the copropagating case. It is shown, that copropagating injection can significantly reduce the noise level. The theoretical results are also compared with the experimental data and found to be in good agreement.