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 M<sup>2</sup> 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.
Erbium-ytterbium co-doped phospho-silicate double-clad fibers are used in many applications were powerful 1.5 μm sources are needed, such as telecommunication systems, LIDAR, medical lasers and much more. These fibers are typically pumped with diodes emitting at 915, 940 or 976nm to excite Ytterbium ions, which in turn transfer their energy to erbium ions through a phonon-assisted mechanism, thus leading to 1.5 μm emission. This energy transfer requires a large phosphorous content in the core of the fiber and therefore these fibers exhibit typically high numerical apertures. Properly optimized, the ytterbium to erbium ratio will minimize parasitic emission at 1 μm which provokes system failures through non-controlled spurious laser effects. We have recently observed, on such optimized fibers exhibiting 12 μm core diameter and 0.20 numerical aperture, that long term operation in CW mode in both amplifier and laser configuration, leads to a slow and irreversible decrease of the output power. This phenomenon starts at moderate signal power of just 7W and increases rapidly with the output power. This phenomenon is also observed in polarization maintaining version of the very same fibers. We have studied this phenomenon which resembles the well-known photodarkening effect in Ytterbium doped fibers. Our experiments show that all the commercially available fibers tested exhibit the same behavior. We will present a tentative explanation of the phenomenon and some solutions we implemented to drastically stabilize the output powers up to 20W enabling the use of such fibers in many industrials applications.
One key parameter in the race toward ever-higher power fiber lasers remains the rare earth doped optical core quality. Modern Large Mode Area (LMA) fibers require a fine radial control of the core refractive index (RI) close to the silica level. These low RI are achieved with multi-component materials that cannot be readily obtained using conventional solution doping based Modified Chemical Vapor Deposition (MCVD) technology. This paper presents a study of such optical material obtained through a full-vapor phase Surface Plasma Chemical Vapor Deposition (SPCVD). The SPCVD process generates straight glassy films on the inner surface of a thermally regulated synthetic silica tube under vacuum. The first part of the presented results points out the feasibility of ytterbium-doped aluminosilicate fibers by this process. In the second part we describe the challenge controlling the refractive index throughout the core diameter when using volatile fluorine to create efficient LMA fiber profiles. It has been demonstrated that it is possible to counter-act the loss of fluorine at the center of the core by adjusting the core composition locally. Our materials yielded, when used in optical fibers with numerical apertures ranging from 0.07 to 0.09, power conversion efficiency up to 76% and low background losses below 20 dB/km at 1100nm. Photodarkening has been measured to be similar to equivalent MCVD based fibers. The use of cerium as a co-dopant allowed for a complete mitigation of this laser lifetime detrimental effect. The SPCVD process enables high capacity preforms and is particularly versatile when it comes to radial tailoring of both rare earth doping level and RI. Large core diameter preforms - up to 4mm - were successfully produced.