Photodarkening in large mode area fibers

Introduction Ytterbium (Yb) doped fiber devices are an interesting and viable choice for many applications ranging from wavelength conversion to materials processing in the 1.0-1.1μm wavelength region. Such devices, particularly using fibers with double cladding structures, are often realized with a large core diameter (large mode area, LMA) fiber. LMA fibers are of interest since the length of the doped fiber may be considerably shortened by increasing the core/cladding area ratio and/or by increasing the Yb concentration, which result in higher pump absorption. For many applications, shortening the length of the Yb doped fiber is considered highly beneficial, for example, in reducing non-linearities in high peak power amplifiers. On the other hand, higher concentrations and/or higher pumping rates have been known to result in certain deleterious effects, the most troublesome of which is the phenomenon of photodarkening (PD), generally acknowledged as a parameter potentially limiting both the efficiency and the lifetime of Yb doped fiber devices. Such PD, attributed to the formation of color centers, manifests as a time dependent broadband absorption at visible and near IR wavelengths.