Gnewuch, H., Román, J.E., Hempstead, M., Wilkinson, J.S. and Ulrich, R.
Beat length measurement in directional couplers by thermo-optic modulation
Optics Letters, 21, (15), . (doi:10.1364/OL.21.001189).
In integrated optical directional couplers formed by two parallel waveguides, we measure the difference Delta beta = beta(even) - beta(odd) between the propagation constants of the 'supermodes'. They are coupled 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/Delta beta. We demonstrate this technique with directional couplers fabricated by K+-exchange in glass. Beat lengths in the range of 0.6 ... 2.2 mm are measured with an accuracy of ±0.3%.
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