Digital holography is a well established technique for the study of dynamic volume of particle fields. In this work, an original digital in-line holography system is developed and used to determine 3D displacements of particles tracer in an air-flow. This system allows us to investigate a sample volume of few mm 3 and reconstruct a moving particle with velocities ranging up to 1m.s-1. The sample volume is reconstructed by computing the wavelet transformation (WT) for different value of the scale parameter. This parameter is related to the axial distance between a reconstruction plane and the CCD sensor. A fiber coupled laser diode is used as the recording light sources. The curvature radius of the reference wave emerging from this fiber forms geometrically magnified diffraction patterns of particles and thus acts as an optical magnification. We show that this enables to enhance the spatial resolution of the system. Here, a transverse magnification ratio of 4 allows us to reconstruct images of droplets with a diameter smaller than 5µm. Note that this configuration needs an accurate knowledge of curvature radius of the recording wave. By using an original calibration method, presented here, we show that the droplets can be localized from the reconstructed holograms with an accuracy of 1µm for lateral position and 20µm in depth. The potential of this system is shown in the case of a laminar air flow seeded with oil micro-droplets.