At present, second-order autocorrelation represents a common and reliable technique for the characterization of ultrashort pulses. It consists of an interferometer and a nonlinear optical medium, usually a nonlinear crystal. In this letter, we propose the use of a nonlinear optical fiber instead. The method is explained using numerical simulations and experimental measurements. A comparison with the standard method is given, demonstrating the viability of the new proposal and opening the way to a new kind of autocorrelator. The use of nonlinear optical fibers presents some benefits in terms of stability and feasibility. The final device allows the implementation of an all-fiber autocorrelator.