BEAT-LENGTH MEASUREMENT IN DIRECTIONAL-COUPLERS BY THERMOOPTICAL MODULATION

In integrated-optical directional couplers formed by two parallel wave guides we measure the difference Delta beta = beta(even) - beta(odd) b etween the propagation constants of the supermodes. We couple them loc ally by heating a fine spot on one of the guides. When the spot is sca nned along the coupler, the output power from one of the guides is fou nd to vary periodically. The period of variation is the modal beat len gth Lambda = 2 pi/Delta beta. We demonstrate this technique with direc tional 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%. (C) 19 96 Optical Society of America