By comparison of calculations and measurement of the farfield and the pulse response from a single-mode fiber, it is found that some limitations are given on the shape of the index profile. These limitations are used to fit the index profile, and it appears that the equivalent step profile is not a useful approximation in designing singlemode fibers with zero dispersion.By integration of the product of refractive index, group index and intensity we obtained the group delay [6]. Here, it was found also that the integration can be performed analytically. Differentiation of the group delay gives the dispersion. The convergence of this method with respect to group delay and dispersion was examined by increasing the number of layers in the index profile to 20, 40 and 80. The agreement between results indicates that the dispersion is correct within 0.2 ps/(km · nm).

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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.

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The propagation in a fibre which does not exhibit any mode coupling is investigated by varying the launching conditions. It is shown that for this fibre there exists a trade-off between dispersion and power coupling efficiency. The measurements are compared to theoretical calculations taking leaky modes and material dispersion into account and good agreement is obtained.