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 light-guiding 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.
The DIN standard 58145 “Measuring method for determination of solarization effect of fused silica optical fibers” was introduced over 5 years ago to standardize the quality control for UV fibers. A deuterium lamp with a lens-based imaging system, coupling a light power density of approx. 200 µW/(cm2 nm) at 214 nm into the fiber under test, was specified as light-source. With time, the availability of new powerful broadband light-sources stimulated new applications. Further, improvements in deuterium lamps and imaging coupling systems resulted in higher light power densities. It was quickly determined that these new conditions induced different spectral and temporal UV losses, which needed to be studied. As a result, the cw-power coupled into the fibers under test, referenced at 214 nm in the original set-up, increased. We observed that two measurement systems, both assembled by the DIN standards recommendations, showed significant deviations in solarization effects at 215 nm (E’ center) and 265 nm (NBOH center). Therefore, we investigated ideal lens coupling conditions, influence of lens aging (solarization) and the influence of N2 purging of light-source and detector on permanent and transient solarization effects in fibers. A closer analysis and selective change of the spectral power between 190 nm and 260 nm wavelength coupled into silica-based fibers with undoped high-OH silica core is presented and an improved system for solarization measurements on UV fibers is recommended.
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