In this book, we have shown first results on how to improve the coherence of an as-grown broad-area laser diode based on quantum dots. The achieved bandwidth of 5-10 GHz at power levels of 10-140 mW is sufficient for many applications such as investigating the nonlinear optics of QD though at higher current the spatial coherence is not optimal. We calculate the nonlinear optical response of a sample of self-assembled QD to a cw driving field via numerical simulations. It is found that a saturation model based on inhomogeneous broadening fits the numerical results but that the saturation power depends on detuning in contrast to a strongly inhomogeneously broadened system. This is interpreted to be due to that fact that QD are in the Voigt-parameter regime between homogeneous and inhomogeneous broadening. Following the numerical simulations we report studies on the interaction strength between laser and self assembled quantum dots, as a function of power in the form of the absorption and gain coefficient. Our analysis is concentrated on continuous wave (cw) beam interacting with quantum dot structures. Several setups have been arranged and relevant results are presented.