Two-dimensional refractive index profile measurement of optical fiber preforms by the digital holographic microtomography

A method for measuring the two-dimensional (2D) refractive index (RI) profile of optical fiber preforms is proposed based on digital holographic microtomography. By the large field Mach–Zehnder interferometer , the digital hologram which contains the core region information of optical fiber preform is recorded. The phase distribution of the core region is extracted at high accuracy using the spatial domain and frequency domain hybrid reconstruction algorithm. Then by scanning the preform, the phase distribution of the cladding region is obtained with the above process. The core phase and cladding phase are registered via a linear least squares algorithm in order to acquire the complete phase distribution of the preform. Based on computer tomography , the preform is rotated through 360° at 5° intervals, and the phase projections of the preform are obtained at different angles. The filtered back projection (FBP) algorithm is used to reconstruct the 2D RI profile of preform. We demonstrate a 2D RI profile measurement of a few-mode optical fiber preform, and the results are compared with PK2600, which is a commercial preform analyzer by Photon Kinetics, Inc, the RI measurement accuracy of the proposed method reaching 2 × 1 0 − 4 , and repeatability reaching 1 0 − 5 . The 2D RI profile of an active Er-doped fiber preform with nonuniform cross section is also successfully measured. In addition, based on 2D RI profile, edge detection and elliptical fitting are applied to extract the geometric parameters of the optical fiber preforms . The proposed method provides a new approach for measuring the 2D RI profile of complex optical fiber preforms. The non-destructive measurement technique will be useful for developing high-performance optical fibers. • A method for measuring the 2D refractive index profile of optical fiber preforms is proposed based on digital holographic microtomography. • A large field Mach–Zehnder microscopic tomographic interferometry measurement system is successfully established. • The results measured agree with those measured by PK2600 Preform Analyzer . • The RI measurement accuracy of the proposed method reaching 2×10 -4 , and repeatability reaching 10 -5 . • The proposed method is suitable for measuring the RI profile of preform with nonuniform cross section.