An approach to the quantum phase, taking into account the process of light generation and extending in this way the operational approach, is proposed. The cosine and sine operators of the phase difference of the two circularly polarized modes are determined with the aid of a polar decomposition of the angular momentum of radiative transition and conservation of the total angular momentum. Application of the approach to the Jaynes-Cummings model for an electric dipole transition shows consistency with the classical definition of the phase difference and with the results of standard operational approach as well.