The coherent light interacting with a direct band-gap semiconductor is modelled as two coupled harmonic oscillators with Kerr-type nonlinearity in one of the oscillators. By considering the nonlinearity in the exciton mode, we derive approximate analytical solutions of the coupled differential equations involving two bosonic modes. For both photon and exciton modes which are in initial coherent states, we calculate the phase fluctuation parameters due to Carruthers and Nieto. We have used the Pegg–Barnett formalism for defining the useful phase operators. We report the increase and decrease of phase fluctuation parameters of input radiation field compared to its initial values. Effects of vacuum field on the phase fluctuation parameters are found distinctly different from their counterparts without vacuum. The complete analytical approach of the present investigation is substantiated by the numerical calculations based on QuTip.