Comparison of Photodarkening in Ytterbium-doped fibers

Development of photodarkening in two similar large-mode-area ytterbium doped fibers from different sources is compared. The excess loss induced by photodarkening is derived from transmission loss measurements of pristine and pumped or photodarkened samples. To accelerate the photodarkening process, cladding pumping is used so as to achieve high and uniform inversion through the sample. Further, intensity profiles are measured and compared in effort to detect possible radial variations in the induced losses. Two ytterbium-doped LMA fibers from different sources were used in the comparison. Both fibers feature 20 µm diameter low numerical aperture core and 400µm diameter cladding. The two sample fibers, Fiber1 and Fiber2, were first characterized by their core and cladding diameters using a scanning electron microscope (SEM). The diameter data was used to convert measured cladding absorption to respective core absorption values. The ytterbium absorption was measured from the cladding propagating light using the cut-back method. Light from an incandescent lamp was directly coupled to the 400 µm cladding, with the other end of the fiber connected to the optical spectrum analyzer (OSA). Transmission of the fiber was then measured while cutting back on the fiber. Figure 1 presents the characteristic ytterbium absorption spectra for Fiber1 and Fiber2. Cladding absorption measured at the secondary peak at 915-920 nm is 0.61 dB/m for Fiber1 and 0.47 dB/m for Fiber2. Estimated core absorption values are reported in the summary Table 1