Thin grating couplers for integrated optics: an experimental and theoretical study.

A detailed account is given of the practical and theoretical aspects of grating or periodic couplers. Such couplers are used to launch light into thin film optical waveguides. The theoretical description stresses considerations of reciprocity. A Green's function formulation is used to calculate the radiated fields occurring when a guided wave is incident upon a grating coupler. Experiments were performed on grating couplers, formed in photoresist on top of thin film waveguides of 7059 glass deposited on glass of lower refractive index. Measurements of the coupling length and proportion of power in the various beams are presented as a function of the grating period for waveguides supporting one and many modes. Good agreement is found between the theoretical and experimental results. The methods used in the fabrication of the gratings are discussed in detail, and guidelines for the design of grating couplers are given.