Calculations on the optical index ellipsoid yield that in LiNbO3, a quadratic electrooptic effect free of its linear counterpart exists only for propagation along theX-crystal axis with a longitudinal electric field and in that case the quadratic coefficientsRc=R12–R31andR41are exclusively involved. Because of their low values, we have measured these coefficients using a specific novel variation of the accurate frequency-doubling electrooptic modulation (FDEOM) method. After a presentation of the theoretical formulation and practical implementation of this method, results forRcandR41are given and discussed in relation to theoretically expected value
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