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In contrast to volume holographic material where 1-to-many fanouts are realized using multiplexed volume holograms, we report in this paper the first Si-based surface-relief polygonal gratings aiming at optical clock signal distribution application. Surface-relief gratings with 1-μm period (0.5 μm feature size) were fabricated using reactive ion beam etching (RIE). Both hexagonal and square gratings were demonstrated for 1-to-4 and 1-to-6 fanouts. Surface-normal input and output coupling schemes were carried out with an combined coupling efficiency of 65%. Employment of substrate modes in silicon greatly releases the required grating spacing for the demonstrated two-way surface-normal coupling. 7.5 GHz 1-to-4 clock signal distribution operating at 1.3 μm was demonstrated with a signal-to-noise ratio as high as 60 dB. The intensity fluctuation among fanout beams was measured to be within 1 dB. Generalization of 1-to-many fanout can be realized by implementing a polygonal grating with an equivalent number of facets.