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In integrated-optical directional couplers formed by two parallel waveguides we measure the difference Deltabeta = beta(even) - ss(odd) between the propagation constants of the supermodes. We couple them locally by heating a fine spot on one of the guides. When the spot is scanned along the coupler, the output power from one of the guides is found to vary periodically. The period of variation is the modal beat length Lambda = 2pi/Deltabeta. We demonstrate this technique with directional couplers fabricated by K-ion exchange in glass. Beat lengths in the range 0.6 -2.2 mm are measured with an accuracy of +/-0.3%.

Beat-length measurement in directional couplers by thermo-optic modulation