We present an in situ optical method for thickness correction during the gas-source molecular beam epitaxial growth of asymmetric Fabry-Perot cavities. Separate growth corrections are made to center the spectrum of the mirror reflectivity, and to place the cavity resonance at the desired wavelength, all by measuring the reflectivity spectrum without breaking the vacuum. We choose to use an AlInP/InGaP quarter-wave mirror to demonstrate that the correction can be made after the growth of only three periods. By adding additional GaAs cap layer thickness and thereby adjust the positions of the Fabry-Perot minima of a AlGaAs/GaAs field-effect transistor self-electro-optic effect devices, improvement of modulation contrast from 3:1 to 9:1 is demonstrated.