Characterization of optical fiber profile using dual-wavelength diffraction phase microscopy and filtered back projection algorithm

We present a measurement scheme of the refractive index profile of optical fibers which does not require accurate index oil match using dual wavelength diffraction phase microscopy. Using dual wavelength algorithm, we solved phase wrapping problem caused by large optical path length difference between the optical fiber and its medium and final phase image was obtained without 2π ambiguity. The index profile was obtained by the filtered back projection algorithm that applies inverse Radon transformation of the phase profile matrix. As an example, a single mode fiber with a known index profile was measured by using this method. The measured profile is well agreed with index profile of the preform, which had been designed by a preform analyzer before the fiber being fabricated from the preform.