A phase contrast heterodyne interference optical microscope for the precise characterization of optical path distribution across a transversely illuminated single or multi mode fiber is described. Using the Abel inversion algorithm in conjunction with this measurement, an exact profile of the refractive index can be extracted. The fiber is immersed in an index-matching liquid to ensure small optical path phase variation. This scheme is valid for any refractive index profile, assuming the fiber is cylindrically symmetric and its index of refraction is a function of the radial distance only. The technique is attractive since it requires minimum fiber preparation and has the potential of on-line index monitoring without breaking into the fiber. A brief theoretical analysis is given to illustrate the feasibility and potential of the proposed method.
"Nondestructive Refractive Index Characterization Of Optical Fibers Using A Phase Contrast Scanning Optical Microscope," Optical Engineering 24(3), 243518 (1 June 1985). https://doi.org/10.1117/12.7973518