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A modified near field (MNF) method is proposed for measuring the refractive index profile (RIP) and geometrical parameters of multi-core fibers. The near field method is based on the near field intensity at the end face of optical fibers that are illuminated by a Lambertian source to get the RIP, but the methods proposed before require a leaky-mode correction factor or additional geometrical parameters to correct the measurement results. The MNF method presented in this paper offers a simplified approach, when the power of the source is constant, the distance between the source and the incident end face of optical fibers is given, and the near field intensity distribution is linear to the RIP of the de-coated optical fibers. By using SMF-28e&#x002B; as a sample to obtain the linear coefficient, the proposed method is validated by measuring the RIP of SMF-28R with an accuracy reaching   ${{10}^{ - 4}}$  . Finally, the RIP of 4-core fiber and 7-core fiber are measured by the MNF method accurately and effectively. Also, the geometrical parameters of optical fibers can be extracted through edge extraction and ellipse curve fitting for the distribution of near field intensity.

A Modified Near Field Method for Measuring the Refractive Index Profile and Geometrical Parameters of Multi-Core Fibers