Design of a large field-of-view immersion microscope for optical fiber preform measurement

A large field-of-view (FOV) immersion double-sided telecentric microscope with long working-distance (WD) is designed for optical fiber preform measurement, which corrects the phase distortion caused by the cylinder lens effect of preforms. An immersion ray transmission model is constructed to analyze the aberration distribution. Cooperating with spherical lens of high refractive index (RI), large size and a double-Gaussian structure, a composite aberration compensation method is proposed to design the large FOV immersion double-sided telecentric microscope with a FOV 10mm, a magnification 5×, a numerical aperture 0.33, a WD 55mm, a telecentricity within 0.1°. Based on the ray tracing theory, an optical phase distortion model for the cylindrical lens effect of preforms is establish. An analysis is conducted regarding the variation of phase distortion in the emitted optical field from the preform with distance. The phase distortion caused by the cylinder lens effect of preforms is corrected by the designed microscope. The microscope is fabricated and subsequently utilized to capture images of the USAF-1951 resolution chart, yielding a resolution of 1.5 μm. The microscope is applied to the RI profile measurement of optical fiber preforms. Using a multi-layer structured preform as a sample, the measurement results are consistent with those obtained by PK2600, which is a commercial preform analyzer by Photon Kinetics, Inc. The designed large FOV immersion double-sided telecentric microscope with long WD in this paper provides a novel optical tool for measuring optical fiber preforms.