The spectral properties of an active single-mode microring resonator are investigated in the frame of the generalized transfer function (GTF) approach, as derived from extended scattering and/or transfer matrix formalism. Spontaneous emission, looked upon as the driving source of the radiation, is described in a semi-classical way in the spectral domain. The internal and emitted fields are filtered into the resonance modes of the whole structure. The generalized transfer function expresses the spectral density of internal saturating intensity and includes all essential mechanisms at work in a laser oscillator: gain, losses and sources. The active zone is saturated through amplified spontaneous emission (ASE), integrated over its whole spectral range. Continuously valid across threshold, the method enables one to derive in a simple way the main steady-state properties of the laser oscillation, with the pumping rate as the only external parameter.