Insights into draw-induced refractive index changes in intrinsically low nonlinearity MCVD preforms and optical fibers

Power scaling of fiber lasers and amplifiers is currently limited by nonlinear optical effects, such as transverse mode instability (TMI) and stimulated Brillouin scattering (SBS). Addressing optical nonlinearities through a material approach allows for such challenges to be confronted at their source - the interaction of the light and the material, lessening the need for complex fiber designs. However, effectively mitigating these issues through compositional engineering requires considerably higher dopant concentrations than are now typical for the modified chemical vapor deposition (MCVD) derived silicate glasses from which modern commercial laser fibers are made. Fibers doped with high concentrations of P₂O₅ and B₂O₃ experience additional fabrication challenges, including draw-induced refractive index changes. Reported herein are index changes of up to six milliunits from this system, compared to index changes of fractions of a milliunit common in industry-standard compositions. More specifically, a passive borophosphosilicate fiber with a core composition of approximately 10 wt.% P₂O₅ and 15 wt.% B₂O₃ is investigated to determine the potential sources of these index changes. These investigations include explorations of glass topology by NMR and Raman spectroscopy, as well as the first direct evidence of the formation of boron phosphate (BPO₄) linkages in MCVD optical fibers.