Refractive-index profiling of azimuthally asymmetric optical fibers by microinterferometric optical phase tomography

Abstract

Accurate nondestructive refractive-index profiling is needed in the modeling, design, and manufacturing of optical fibers and fiber devices. Most profile measurement techniques cannot correctly characterize fibers with small or irregular refractive-index variations over their cross sections. Microinterferometric optical phase tomography (MIOPT) is a technique that allows measurement of fiber refractive-index profiles exhibiting such variations. We present the first demonstration, to our knowledge, of MIOPT. The profile of a polarization-maintaining fiber is measured by MIOPT and shown to be in agreement with (destructive) fiber end-face measurements. MIOPT is also applied to the limiting case of a symmetric single-mode fiber.

Measuring the spatial distribution of rare-earth dopants in high-power optical fibers

“…CT was first applied to the linear absorption of a collimated probe beam, for example a "CAT" scan of a medical patient using an X-ray beam (termed absorption CT). When imaging a phase object, such as an optical fiber, the phase delay accumulated through a sample, such as an optical fiber, is acquired from a diversity of projection angles to reconstruct the refractive index profile 18,19 or the birefringence distribution 20 (termed phase CT). The fiber measurement technique described here involves a third type of CT, termed emission CT 17,21 , in which a heterogeneous distribution of emission (rather than absorption or phase delay) is acquired at a multiplicity of angles and processed to reveal the spatial distribution of that emission.…”

Multifocus tomographic algorithm for measuring optically thick specimens

“…Recently, tomographic algorithms originally developed for these situations have been applied by many research groups to the important problem of measuring the refractive index distribution of nonazimuthally symmetric optical fibers [2][3][4][5][6][7][8][9]. The transverse projection of features in an azimuthally symmetric optical fiber, such as conventional single-mode optical fiber, is unchanged by any amount of rotation about its central axis whereas the transverse projection of features inside a nonaximuthally symmetric fiber, such as a polarization-maintaining (PM) optical fiber, will change substantially when it is rotated about its central axis.…”

“…Quantifying the refractive index and geometry of such fibers is critically important for understanding their performance and optimizing their manufacture [11]. Such fibers have been measured by transverse tomographic interference microscopy [3,4,7,8], quantitative phase microscopy [2,6,9], or diffraction tomography [5]. The tomographic reconstruction algorithms used for these investigations assumed that the depth-of-field of the imaging system encompassed the transverse dimension of the fiber.…”

“…However, the core separations of multicore fibers, the transverse diameter of PCF microstructure, and the separation of PM fiber stress-applying members are all typically of the order of several tens of micrometers, exceeding the depth-of-field of relevant microscope objectives by as much as 2 orders of magnitude. Figure 1 schematically illustrates the effect of limited depth-of-field on optical path length measured on a threecore fiber, for example with an interference microscope [3,4,8,12]. It is seen that at each illustrated focal position, cores outside the focal plane are smeared.…”

“…Effect of focal position on measured optical path length when the specimen is larger than the imaging depth-of-field. [3,4,7,8,12] and other transverse measurement techniques [2,5,6,9].…”