Measurements of the magnitude and the sign of certain quadratic electro-optic coefficients of potassium dihydrogen phosphate (KDP) and ammonium dihydrogen phosphate (ADP) were made with an actively stabilized Michelson interferometer. The results obtained for these coefficients are, in units of 10 Ϫ20 m 2 V Ϫ2 (as opposed to literature values of order 10 Ϫ18 m 2 V Ϫ2 ), as follows: (KDP)g xxxx ϭ Ϫ3.4 Ϯ 0.5, g yyxx ϭ Ϫ0.2 Ϯ 0.4, and g zzxx ϭ Ϫ0.7 Ϯ 0.4; (ADP)g xxxx ϭ Ϫ7.4 Ϯ 1.0, g yyxx ϭ Ϫ1.7 Ϯ 0.9, and g zzxx ϭ Ϫ1.4 Ϯ 0.9. The quadratic Faust-Henry coefficient describing the lattice and the electronic contributions to the quadratic electro-optic effect in KDP and ADP is estimated from our results. These show that the nonlinear susceptibility responsible for the quadratic electro-optic effect in these crystals is due mainly to nonlinear interactions of the lowfrequency electric field with the crystal lattice.

We demonstrate enhanced performance of a single-ended spontaneous-Brillouin-intensity-based distributed-temperature sensor with a sensing length of 50 km and a spatial resolution of 15 m by use of Raman amplification of the probe pulse within the sensing fiber. The Raman amplification was achieved with a copropagating pump pulse at 1450 nm. The standard deviation error of the temperature resolution was 1 degree C at the front end and increased to less than 13 degrees C at 50 km with Raman pulse amplification.

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