The purpose of this study is the active control of flow separation on an ONERA D airfoil, using continuous blowing microjets. Particle Image Velocimetry (PIV) is used to measure velocity fields at midspan around the airfoil in both controlled and uncontrolled cases. Post-processing based on Proper Orthogonal Decomposition (POD) is developed to exhibit the main energetic features of the flow. Therefore, the detailed study of the POD modes for separated and stalled cases leads to a better understanding of the control mechanisms. Moreover, the POD basis computed for uncontrolled and controlled cases, opens the way to optimisation of control strategies.