Determination of dopant concentration in single-mode step-index optical fiber based on measured numerical aperture and mode field diameter

Numerical modeling of many optical-fiber-based devices' operation requires an exact knowledge of fiber's parameters like core diameter and dopant, usually GeO₂, concentration. While diameters are typically specified in the fiber's data sheet, material composition, including dopant in the core, is rarely available. We present a procedure utilizing a reverse engineering approach to find GeO₂ concentration in single-mode step-index optical fiber. Our method consists of several stages. First, we measured the numerical aperture NA for several commercially available fibers employing a onedimensional far-field scan. The far-filed mode intensity was acquired by a Ge detector placed on a rotation stage with a stepper motor for fiber end-face positioned on the motor's axis of rotation. We calculated NA for the angular detector position when the light intensity reached 13.5%, 5%, and 1% of its maximum value. Then, taking the corresponding values of core and cladding diameters and using the Sellmeier formula for pure (cladding) and GeO₂ doped (core) silica glass, we found the concentration of GeO₂ numerically matching calculated NA to the experimental data. We found that dopant concentration equals 9.0, 18.0, 34.0, 34.5, and 39.8 mol% for the fibers, respectively, 980-HP, UHNA1, UHNA3, UHNA4 and UHNA7 produced by Coherent. To verify the correctness of our method, we performed this procedure for several fibers with a known level of GeO₂ concentration in the core fabricated by the Laboratory of Optical Fiber Technology, Maria Curie-Sklodowska University. The results of this simulation coincide with expectations with great accuracy.