Propagation in graded-index fibers: comparison between experiment and three theories.

We compare the measured impulse response of a graded-index fiber by three different theories: the WKB method applied for an arbitrary profile; the alpha-profile approximation; and perturbation theory. We find that the WKB method gives both a qualitatively and quantitatively good agreement. The perturbation theory gives a qualitatively good calculation of the width of the impulse response (the dispersion). The alpha-profile approximation gives a very poor result. We also investigate the influence of the launching conditions on the impulse response and find that the dispersion for this fiber decreases with the excitation of higher order modes. This is found to be in agreement with the WKB theory.