Measurement of fourth-order aberration in a lens-like medium

Deformation of the spot of an off-axis Gaussian beam after passing through a lens-like glass fiber (SELFOC®) with the fourth-order aberration is investigated by using first-order perturbation theory and numerical calculation. A method for determining the coefficient of the fourth-order aberration is proposed. By this method, it is found that a SELFOC sample (0.5 mm in diameter and 150 mm in length) has a coefficient
h
of about 1.1, if the dielectric constant in the meridional plane of the sample is expressed as
\epsilon(x) = \epsilon(0) (1 - g^{2}x^{2} + hg^{4}x^{4})
, where
x
is the distance from the axis of the medium and
g
is a positive constant.