Error analysis of optical fiber refractive-index profiling using the focusing method

The cause of measurement error in refractive-index profiling by the focusing method is investigated by comparison of intensity distributions and reconstructed refractive-index profiles obtained by the experiment, ray tracing, and diffraction integral. It is shown that the variations of intensity distribution and refractiveindex profile experimentally obtained are between those obtained by ray tracing and diffraction integral. Therefore, the focusing method based on ray optics generates a great deal of the measurement error for the small size fiber sample due to the diffraction. In particular, the diffraction produced by the index depression of the MCVD fiber causes much degradation of the measurement accuracy not only in the reconstruction of the index depression but also in that of the whole refractive-index profile.