Optical-confinement-factor Γ dependencies of the K factor, differential gain dg/dN, and nonlinear gain coefficient e, for 1.55 μm InGaAs/InGaAsP multiple-quantum-well (MQW) and compressively strained MQW lasers, were investigated experimentally. For both MQW and strained-MQW lasers, when Γ is increased, the K factor is reduced, dg/dN is increased but e is almost constant. These results indicate that the Γ dependence of the K factor mainly results from a change in dg/dN, and does not result from a change in ∊. For the strained MQW lasers, the K factor, dg/dN and ∊ are, respectively, half as large, twice as large and the same as those for the MQW lasers, when both types of lasers have the same r (= 0.05). This suggests that the strained MQW lasers with a large Γ are expected to have a small K factor and thus be preferable in achieving large modulation bandwidths.