A theoretical model was developed to simulate the apparent substrate temperature oscillation during the growth of AlAs/AlxGa1-xAs, x = 0 and 0.25, distributed Bragg reflectors (DBR) for 980- and 780-nm vertical cavity surface emitting lasers, respectively. The simulated data were then used for in situ monitoring and feedback control of layer thickness by a simple pyrometric interferometry technique to obtain a highly reproducible DBR. These measurements can be performed with continuous substrate rotation and without any growth interruption. The reproducibility of the center wavelength and full width at half-maximum of the reflectivity stop band with a variation of <+/-0.2% and <+/-0.4% for the AlAs/GaAs and AlAs/AlGaAs mirror stacks, respectively, were achieved.