Experience with mode-field radius measurements

The transverse offset, and variable-aperture far-field methods have been applied to the measurements of the mode-field radii of two types of single mode optical fibre. Comparison with the results of calculations based on the measured refractive index profiles shows that discrepancies between the experimental results by the two methods may be confidently attributed to the deviation of the electromagnetic field in the fibre from a Gaussian form. A possible alternative test method for measuring the numerical aperture of a parabolic-index multimode fibre is described. The paper includes a number of comments on the technical problems associated with the measurement methods.