The optical cochlear implant (oCI) utilizes optogenetic stimulation of the spiral ganglion neurons (SGNs) in the cochlea to improve hearing restoration. Preclinical data has shown that this may improve bionic hearing in CI users by targeting smaller areas of the cochlea compared to electrical stimulation.This study investigates the influence of the light emitter characteristics (one/two laser-coupled glass fibers) on the spread of excitation, through a combination of computer simulations and in-vivo multiunit recordings of inferior colliculus activity in Mongolian gerbils that postnatally received gene therapy to render their SGNs light sensitive.Prior to in-vivo recordings, we used 3D modelling based on x-ray data of nine cochleae with sham fiber insertions to simulate the anticipated light spread and explore relationships between insertion parameters and the irradiance values reaching the SGNs. After optimization of the simulated insertion parameters, we 3D printed guide tools for the in-vivo recordings.During the in-vivo recordings, we systematically varied fiber diameter, position within the cochlea (apex, mid, or base), insertion angle, and, inter-fiber distance, and recorded the spread of excitation in the inferior colliculus for varying intensity levels. Our preliminary results (4 animals) show, that smaller fibers have a more confined excitation pattern and that apical stimulation activates neurons in low-frequency regions while basal stimulation activates high- frequency regions, in agreement with previous studies. Further expansion of this dataset will help in establishing the design criteria for the future optical cochlear implant.