Conventionally, the beam-propagation method for solving the generalized nonlinear Schrödinger equation, including the slowly varying envelope approximation, has been used to describe the ultrashort-laser-pulse propagation in an optical fiber. However, if the pulse duration approaches the optical cycle regime (<10 fs), this approximation becomes invalid. Then, it becomes necessary to use the finite-difference time-domain (FDTD) method for solving the Maxwell equation with the least approximation. In order to both experimentally and numerically investigate nonlinear femtosecond ultra-broad-band-pulse propagation in a silica fiber, the FDTD calculation of Maxwell's equations has been extended with nonlinear terms to that including all exact Sellmeier-fitting values. The results of this extended FDTD method are compared with experimental results for the nonlinear propagation of a very short (12-fs) chirped laser pulse in a silica fiber. The fiber output pulse compressed to 7 fs by the simulation of group-delay compensation was obtained under the assumption of using a spatial light modulator. To the authors' knowledge, this is the first comparison between FDTD calculation and experimental results for nonlinear propagation of a very short (12-fs) chirped pulse in a silica fiber.
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