Thanks to its unique properties, diamond is intensively investigated for the development of optical and elec-tronic devices. These applications, such as pseudo-vertical Schottky diodes or Bragg mirrors, rely on the synthesis of boron-doped (p+) and non-intentionally doped (nid) stacked epilayer with well-controlled thick-nesses, doping level and sharp interfaces. Such structures require a time-consuming optimization of the growth processes throughout the use of destructive techniques such as Secondary Ion Mass Spectroscopy (SIMS), Transmission Electron Microscopy (TEM) or transport measurements. From this perspective, the use of an in situ characterization tool is a considerable asset. In this paper, we demonstrate that spectroscopic ellipsometry, implemented to be used in situ during the Plasma Enhanced Chemical Vapor Deposition (MPCVD) process, is a powerful and non-destructive technique to characterize diamond based devices. Moreover, it can also be sensitive enough to access the gas flow dynamics in the reactor. To this aim, two doped and nid multilayer stack have been investigated. The doping profile of a doped layer, extracted from the optical spectra is compared to the one obtained by SIMS and the growth rate of nid epilayers grown under various flow rate, is derived based on this technique.