Characterization of multi-clad fibers: approaches to measure the beam divergence angle and its degradation in the fiber

Abstract

Multi-cladding (MC) fibers with undoped silica core and alternating highly fluorine doped and undoped silica cladding layers are a new standard for high power laser beam transmission. Such MC-fibers allow tailoring the output beam profile of a single laser source to the desired application.An important factor for these applications is to avoid additional scattering of the guided light in the different regions leading to an increased output divergence, so-called focal ratio degradation (FRD).In this work, we compare two possible measurement concepts to determine the beam divergence angle under controlled launch conditions to quantify FRD of MC fibers. One approach is the pinhole setup historically used in astronomy; another approach is the inverse far field method with selective excitation. The objective is to define a reliable setup for quality inspection.We show measurement results for FRD of MC fibers with both methods and evaluate the differences between the lightguiding sections. To our knowledge, this is the first time such data are presented. The objective described above is best fulfilled by a modified far field method with defined broad angular excitation.