Optical properties of directly excited erbium (Er3+) ions have been studied in silicon rich silicon oxide materials codoped with Er3+. The spectral dependence of the direct excitation cross section (σdir) of the Er3+ atomic 4I15/2→4I11/2 transition (around 0.98 μm) has been measured by time resolved μ-photoluminescence measurements. We have determined that σdir is 9.0±1.5×10-21 cm2 at 983 nm, at least twice larger than the value determined on a stoichiometric SiO2 matrix. This result, in combination with a measurement of the population of excited Er3+ as a function of the pumping flux, has allowed quantifying accurately the amount of optically active Er3+. This concentration is, in the best of the cases, 26% of the total Er population measured by secondary ion mass spectrometry, which means that only this percentage could provide optical gain in an eventual optical amplifier based on this material.