We present a study that demonstrates the limits for achieving net optical gain in an optimized waveguide where Si nanoclusters in SiO2 codoped with Er3+ are the active material. By cross correlating absorption losses measurements with copropagant pump (lambda(pump) = 1.48 mu m) and probe (lambda(probe) = 1.54 mu m) experiments we reveal that the role of more than 80% of the total Er3+ population present on the material (intended for optical amplification purposes) is to absorb the propagating light, since it is unfeasible to invert it. (C) 2011 American Institute of Physics. [doi:10.1063/1.3665950]