Abstract
The photo darkening (PD) absorption spectra from unseeded amplifier operation (by 915 nm pumping) of ytterbium / aluminum and co-doped silica fibers is after prolonged operation observed to develop a characteristic line at 2.6 eV (477 nm). This line is proposed to be due to inter center excitation transfer from type II oxygen deficiency centers ODC(II) to Tm3+ trace impurities. The ODC(II) is proposed to be the result of a displacive transition of a 4-fold silica ring hosting two 3-fold silicon units that presents two non-bridging oxygen to Yb3+ (as part of its 6-fold coordination by oxygen). The displacive transition is initiated by a charge disproportionation process which leads to NBO transfer in forming dioxasilirane (2-fold coordinated silicon with two NBO attached) next to silylene (2-fold coordinated silicon with a lone electron pair). In collaboration with a valence electron of Yb3+ a new ½ / 1½ chemical bond is formed on dioxasilirane which comprises the PD color center for the visible and near-infrared. Difference in solid acidity of the silica material co-doped with Yb/Al and Yb/P may explain the observed difference in spectral shapes by change of bond order to the formed chemical bond.
Highlights
The development of InGaAs diode lasers specific to pump erbium doped fiber amplifiers for commercial use in telecommunications industry have made cheap, reliable and steadily increasing pump diode laser power available for the 900–980 nm wavelength range
The three upper curves are spectra of the fibers operated at 46% population inversion whereas the final two spectra are for 35% and 22% population inversion of fiber #3
The photo darkening (PD) absorption spectra from unseeded amplifier operation of ytterbium / aluminum and co-doped silica fibers is after prolonged operation observed to develop a characteristic line at 2.6 eV (477 nm) along with a 2.2 eV (558 nm) dent
Summary
The development of InGaAs diode lasers specific to pump erbium doped fiber amplifiers for commercial use in telecommunications industry have made cheap, reliable and steadily increasing pump diode laser power available for the 900–980 nm wavelength range. This again has fueled the development of ytterbium co-doped silica fiber amplifiers/lasers that over. The PD absorption spectra show spectra with centers in the UV and visible stretching long tails into the near infrared These color centers give raise to increased thermal load which decreases the laser efficiency. Mitigation of PD is the key to further improvement of the performance and reliability of high power fiber lasers / amplifiers
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