We present a novel coupling scheme using a collimating gradient-index (GRIN) element provided with a high frequency grating to couple light from a single mode optical fiber directly to planar thin-film waveguides. The waveguide devices are used, for example, for an efficient fluorescence excitation in biosensor applications. The external coupler can be multiply reused and supersedes the conventional internal gratings. FEM simulations and experimental results show that the new technique can provide similar coupling efficiencies as common internal grating couplers.