Geometrical parameter measurement of optical fibers based on an autofocus zoom imaging system

A zoom imaging system is designed, and an autofocus algorithm based on the three-neighborhood operator is proposed. Based on the autofocus zoom imaging system, the geometrical parameters of the optical fibers with different diameters are measured using the transmitted near-field method without the replacement of lenses and the adjustment of the object plane. A four-component positive group compensation method is employed to design and implement a lens that provides continuous magnification ranging from 10× to 40× with a resolution of 3.48 µm. A three-neighborhood sharpness evaluation operator is proposed, which uses the sum of squares of gray differences as the criterion. Combined with a threshold-based hill-climbing algorithm with variable step size, autofocus imaging of the fiber end face is achieved. The geometrical parameters of optical fibers with diameters of 400, 310, 220, and 125 µm are measured using the proposed autofocus zoom imaging system. The maximum deviation for repeated measurements of cladding diameter and concentricity is 0.02 and 0.01 µm, respectively.