Abstract

Emission spectra in the $\sim 240–1100\text{\hspace{0.17em} nm}$ wavelength region as well as the temporally resolved decay of ${\text{Yb}}^{3+}$ and point defect spontaneous emission have been recorded when aluminosilicate optical fibers doped with Yb are irradiated with $\sim 160\text{\hspace{0.17em} fs}$ laser pulses having a central wavelength of $\sim 250\text{\hspace{0.17em} nm}$ $\left(\hslash \omega =5\text{\hspace{0.17em} eV}\right)$. Photoexcitation of the fibers in this region of the deep ultraviolet (UV) provides access simultaneously to the Type II Si oxygen deficiency center (ODC), the non-bridging oxygen hole center (NBOHC: an oxygen-excess defect), and the Ge ODC. Emission from all of these defects in the ultraviolet and/or visible is observed, as is intense fluorescence at 976 nm from ${\text{Yb}}^{3+}$. Absorption measurements conducted in the $\sim 230–265\text{\hspace{0.17em} nm}$ region with a sequence of UV light-emitting diodes reveal a continuum peaking at $\sim 248$ nm and having a spectral width of $\sim 18\text{\hspace{0.17em} nm}$ (FWHM), confirming that the 250 nm laser pump is photoexciting predominantly the ODC. The temporal histories of the optically active defect and rare earth ion emission waveforms, in combination with time-integrated spectra, suggest that the Si ODC(II) triplet state directly excites ${\text{Yb}}^{3+}$ as well as at least one other intrinsic defect in the silica network. Prolonged exposure of the Yb-doped fibers to 250 nm radiation yields increased ${\text{Yb}}^{3+}$, NBOHC, and Si ODC(II) singlet emission which is accompanied by a decline in Si ODC(II) triplet fluorescence, thus reinforcing the conclusion—drawn on the basis of luminescence decay constants—that the triplet state of Si ODC(II) is the immediate precursor to the NBOHC and is partially responsible for Yb ion emission at 976 nm. This conclusion is consistent with the observation that exposure of fiber to 5 eV radiation slightly suppresses ODC absorption in the $\sim 240–255\text{\hspace{0.17em} nm}$ region while simultaneously introducing an absorption continuum extending from 260 nm to below 235 nm $(\hslash \omega \approx 5.28\text{\hspace{0.17em} eV})$. These results suggest that $\mathrm{ODC}\to E^{\prime }$ center conversion assumes a role in excitation transfer to ${\mathrm{Yb}}^{3+}$.

© 2010 Optical Society of America

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