We present theory and experiment for the high-speed modulation response of a quantum-well (QW) laser in the presence of an external microwave modulated optical pump in the gain region. The model includes the effects of pump-induced stimulated recombination and cross-gain saturation. Expressions for the small-signal modulation response of the test laser under gain modulation are derived. We also present experimental results using a multiple-QW InGaAlAs Fabry-Perot (FP) laser at 1.552 /spl mu/m as the test laser and an external pump by a 1.542 /spl mu/m DFB laser. Comparison between electrical modulation and optical cross-gain modulation (XGM) of the test laser is also presented, which shows improvement of the modulation bandwidth by optical XGM. Our data show a reduction of carrier lifetime with increasing optical pumping, a shift of the test-laser threshold current, a change in the K factor, and a variation of the relaxation frequency with different pump powers. The experimental results agree very well with the theoretical results.