Amorphous Al2O3 channel waveguides with different Yb3+ concentrations are fabricated by reactive co-sputtering onto thermally oxidized silicon substrates and subsequent reactive ion etching. Pump transmission and bleaching of pump absorption, luminescence spectra, luminescence-decay curves, and the dependence of luminescence intensity on pump power are measured for the transition at 1 μm from the 2F5/2 first excited state of Yb3+ and for the green upconversion luminescence from the second excited state of the single quantum systems formed by Yb3+-Yb3+ pairs. The typical spectroscopic features of cooperative upconversion are observed. Analysis by rate equations reveals that only a microscopic two-ion-class model comprising single ions and ion pairs can quantitatively explain the measured spectroscopic data, i.e., the local chemical environment must be taken into account in the analysis. The fraction of quenched ions determined from the pump-absorption measurements and the fractions of ions forming Yb3+-Yb3+ pairs and producing cooperative upconversion (CU) are found to be equivalent and increase linearly with Yb3+ concentration. It suggests that the observed phenomena of non-saturable absorption and CU are linked to each other and evoked by the same class of ions. Internal net gain in these channel waveguides under the influence of cooperative upconversion is experimentally investigated and successfully modeled by use of the two-ion-class model. This analysis delivers important spectroscopic insight for optimizing the performance of distributed-feedback lasers in these waveguides. Particularly, it will provide a route towards optimizing the Yb3+ concentration for simultaneously ensuring sufficient pump absorption and minimizing non-saturable absorption by CU-quenched Yb3+-Yb3+ pairs that would otherwise significantly increase the laser linewidth, which is the most important performance parameter of such lasers.