That is in essence what Feng et al. do in this article, which results from a collaboration between the Key Lab for All Optical Network (Beijing, China) and the Georgia Institute of Technology (Atlanta, GA, USA). Using the three-dimensional concurrent stress-index (3D-CSI) measurement method, they provide a detailed perspective of residual stress and refractive index perturbations in large mode area ytterbium-doped optical fibers that results from manufacturing and subsequent arc fusion splicing.
While sounding somewhat academic, such perturbations can have marked impact on the performance of optical fibers, particularly fiber-based lasers for high energy applications. As power levels have continued to increase, the deleterious effects of nonlinearities---particularly stimulated Brillouin scattering (SBS)---have become increasingly problematic and performance-limiting. One of the more common approaches to reducing the impact of such effects is to enlarge the core size of the fiber, often while reducing the numerical aperture, so that the intensity distribution of the propagating light is spread over a larger cross-sectional area. Such large mode area (LMA) fibers exhibit higher thresholds for SBS and permit scaling to higher optical powers.
However, LMA fibers come in many flavors of shapes and sizes and splicing to them is non-trivial. Additionally, the combination of temperature and time involved in the splicing can lead to modifications to the fiber’s compositional and stress profile, both of which can influence its optical behavior. Feng et al. employ a state-of-the-art 3D-CSI measurement method to investigate residual stress (RS) and refractive index (RI) perturbations in LMA EDFs resulting from manufacturing, cleaving, and arc fusion splicing. As shown in their work, the 3D-CSI method is especially well-suited for such studies and, indeed, the effects of fusion splicing are significant for LMA fibers, particularly active ones. Specifically evaluated was the arc-fusion splicing of a commercial LMA fiber (LIEKKI Yb1200-10/125-DC) to a commercial single mode fiber (Corning SMF-28). Splicing using arc-fusion is clearly shown to relax both the anisotropic and isotropic components of (frozen-in) viscoelasticity of the glass as well as the mechanical component of the residual stress. Splicing at high temperatures also results in significant diffusion of the core dopants. Taken together, these perturbations decrease the core/cladding index difference by as much as 1.74×10−3, which represents a 75.8% change from as-sold fiber.
Combining these measured results with electromagnetic modeling, the team further showed that these combined perturbations led to about 21% of incident power being lost when these RS/RI effects are considered. A 'transition region' whereby the RS/RI perturbations are present due to the splice yield an expansion of the mode field diameter (MFD) by nearly 40%. If such modifications are not taken into account in the design of the spliced system, such a MFD, expansion can result in significant additional slice losses. Because the performance of high-power all-fiber lasers and amplifiers depend heavily on these (and other) losses, the results of this work are critically important for the design and optimization of present and future active and passive fiber-based devices.
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