A wide range of FROG geometries have been used for ultrashort pulse characterisation, based on X (2) nonlinearities such as second-harmonic generation (SHG), and X (3) nonlinearities such as polarisation rotation, self-diffraction, and third harmonic generation (THG). The most sensitive technique is SHG-FROG, currently able to characterize pulses with pJ energy, but it does possess a fundamental ambiguity in the direction-of-time of the retrieved pulse. In this Chapter we describe the Fiber-FROG geometry  , which is based on the X (3) (Kerr) nonlinearity in an optical fiber. Using a fiber waveguide increases the interaction length so that a X (3)-based geometry gives a sensitivity comparable to SHG-FROG, but without any direction-of-time ambiguity.
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