Accurate Magnified Near-Field Measurement of Optical Waveguides Using a Calibrated CCD Camera

A fast and accurate magnified near-field (NF) measurement technique that employs a charge-coupled device (CCD) camera is described to measure the two-dimensional (2-D) intensity distributions of optical waveguides with arbitrary refractive-index profiles. Calibration of the InGaAs CCD camera used in the system is essential in terms of correction for bad pixels, linearity, and uniformity in order to obtain accurate results. The magnification factor of the system is obtained using a calibrated optical dimensional standard. The NF measurement system with the calibrated CCD camera is validated from the mode field diameter (MFD) measured for six single-mode fibers with different refractive-index profiles. The MFD results from the magnified NF technique exhibit good agreement with those obtained from the far-field scanning technique to within 2.3%. An error of about 15% in the MFD would result if corrections to the CCD camera were not applied to the raw measured data. The refractive-index profiles computed from the measured data for a single-mode fiber and a Ti:LiNbO₃ waveguide are also presented. The radial-index profile of the fiber exhibits good agreement with the profile measured from the refracted NF technique