We report on a newly built output grating coupler instrument. We have used it successfully as a refractometer for liquid samples, to monitor the adsorption of proteins (avidin) on the waveguide surface, to determine the refractive index and thickness of the adsorbing (mono)layers with high precision, and to investigate the affinity reaction between adsorbed avidin molecules as receptors and biotinylated proteins (bovine serum albumin) as the analyte in the sample, as a model of a direct affinity biosensor with a low detection limit.