Nondestructive method for measuring the coupling between optical waveguides

A new method for direct measurement of the coupling coefficient between parallel and identical optical waveguides is presented. The method is based on imaging and recording the field emerging at the output facet of the device, while the excitation at the input facet is performed by illuminating it with a planar wavefront. The angle of incidence of the wavefront is scanned with respect to the input facet. A useful and simple relationship exists between the coefficients of the normal-modes excited by this procedure and the spatial Fourier Transform of these modes. From this relationship, the coupling coefficients are readily deduced. Only one pair of coupled guides running from facet-to facet is required for characterization. The method is nondestructive, simple, and requires a microcomputer assisted CCD camera and minimal standard optical equipment. The coupling coefficient of two nearby Ti indiffused LiNbO₃ waveguides was experimentally measured by this technique and the results were compared to those furnished by other methods.