Towards large-mode-area fibers fabricated by the full vapor-phase SPCVD process

Abstract

Large-Mode-Area (LMA) fibers are key elements in modern high power fiber lasers operating at 1 μm. LMA fibers are highly ytterbium-doped and require a fine control of the core refractive index (RI) close to the silica level. These low RI have been achieved with multi-component materials elaborated using a full-vapor phase Surface Plasma Chemical Vapor Deposition (SPCVD) process, enabling the fabrication of large core diameter preforms (up to 4 millimeters). Following the technology demonstration, presented in Photonics West 2017, with results on 10/130 (core-to-clad diameters (in μm) ratio) fibers, this paper aims to present updated results obtained for double-clad 11/130, 20/130 and 20/400 LMA fibers, with numerical apertures at, respectively, 0.08 and 0.065. The study is based on aluminosilicate core material co-doped either with fluorine or phosphorus to achieve optimal radial RI tailoring. The fiber produced exhibit low background losses (<20dB/km at 1100nm) and high power conversion efficiencies, up to 74% for output powers of 100W limited by our test setup. The Gaussian beam quality has been evaluated using the M2 measurement. Photodarkening behavior will be discussed for both fluorine and phosphorus-doped aluminosilicate materials and particularly the use of cerium as co-dopant. The SPCVD technology can indeed be used for the production of Yb-doped LMA fibers. Current development is now focused on other rare-earth doped fibers.