Experimental characterization of coupled waveguides by normal-mode excitation

We present a novel method for measuring the coupling coefficient of a symmetric directional coupler, based on the direct excitation of the normal modes of the entire structure. The output-power distribution at different coupling lengths was related to the experimentally measured normal modes in order to find the coupling coefficient. The device was alternatively characterized by means of the individual-waveguide modes according to coupled-mode theory in the strong-coupling regime. Both methods were implemented on single-mode directional couplers in Ti:LiNbO/sub 3/. The validity and accuracy of both approaches are discussed.