Digital holographic interferometric characterization of bent optical fibers

The cross-sectional refractive index distribution is the most important parameter for optical fibers. If fibers are bent, birefringence occurs and the refractive index distribution is changed, which may lead to malfunction. Thus there is great interest in the exact knowledge of this parameter. Here we describe the precise characterization of bent optical fibers by digital holographic interferometry. In a Mach-Zehnder-like arrangement, phase shifting digital holography is employed. By means of a mutual tilt of the mirrors, a carrier of controllable orientation and spatial frequency is introduced. A strategy for an evaluation of the phase shifted holograms is presented, which contains image enhancement and noise reduction steps. It is shown how different orientations of the carrier fringes relative to the fiber orientation lead to a variation of the fringes, but, in each case, to an accurate measured refractive index distribution.