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Precise intracavity phase interferometry in an optical parametric oscillator with two pulses per cavity round trip.

Phase shifts of less than 10(-7) rad are measured through intracavity phase interferometry, a figure that corresponds to cavity length changes of the order of femtometers. The technique is demonstrated with a synchronously pumped optical parametric oscillator. By applying the technique to the measurement of the nonlinear index of refraction of lithium niobate, it is shown that higher accuracy and sensitivity than the traditional z-scan method can be achieved.