Structures confining light along one or two directions (microcavities) are of prime importance for the construction of opto-electronic devices based on luminescent semiconductor materials. Our work is devoted to the making of microcavities centered at 2.4 eV (515 nm) formed by two TiO2/SiO2 distributed Bragg reflectors, surrounding an emitting layer. The emitting layer is an organic semiconductor (trihydroxyquinoline-aluminum, Alq(3)) chosen for its strong luminescent efficiency. For the fabrication of such complex structure, the layer optical path length is an determining parameter to obtain a high quality microcavity. First, a particular attention has been paid to the characterization of the optical properties of the active layer and the mirror layers. Optical indices have been obtained by means of spectroellipsometry. Alq(3) and TiO2 dispersion laws were successfully modeled with a model developed by Forohoui and Bloomer. Secondly, we applied in situ ellipsometry to control the mirror layer deposition. Finally, we show that this method allows microcavity fabrication with good accuracy.