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A theoretical and experimental study is made of the grating couplers for thin-film waveguides. It is shown that the perturbation series expansion of the field based upon the Green's function approach can actually be summed to all orders to give an expression for the efficiency of grating couplers of arbitrary thickness. The results obtained by this theory are in good agreement with the experimental results obtained. It is also shown that the grating coupling efficiency depends strongly on the grating profile.