Material-dependent properties influencing the performance of fiber amplifiers are reviewed together with the available data for Er/sup 3+/. The major glass types potentially useful in this application are considered and compared to silica. The topics addressed include quenching processes and the solubility of rare-earth ions, transition strengths and bandwidths at the 1500-nm gain transition, and the characteristics at the 800-, 980-, and 1480-nm pump bands. Aluminum is shown to be an extremely useful codopant for silica, improving its ability to dissolve rare-earth ions and providing desirable spectroscopic properties for Er/sup 3+/. For some of the attributes considered, other glasses have advantages over Al silica, but only with respect to gain bandwidth and pumping performance at 800 nm is significantly better than expected from other glass compositions. >

Erbium-doped glasses for fiber amplifiers at 1500 nm

In vivo fluorescence imaging in the near-infrared region between 1500–1700 nm (NIR-IIb window) affords high spatial resolution, deep-tissue penetration, and diminished auto-fluorescence due to the suppressed scattering of long-wavelength photons and large fluorophore Stokes shifts. However, very few NIR-IIb fluorescent probes exist currently. Here, we report the synthesis of a down-conversion luminescent rare-earth nanocrystal with cerium doping (Er/Ce co-doped NaYbF4 nanocrystal core with an inert NaYF4 shell). Ce doping is found to suppress the up-conversion pathway while boosting down-conversion by ~9-fold to produce bright 1550 nm luminescence under 980 nm excitation. Optimization of the inert shell coating surrounding the core and hydrophilic surface functionalization minimize the luminescence quenching effect by water. The resulting biocompatible, bright 1550 nm emitting nanoparticles enable fast in vivo imaging of blood vasculature in the mouse brain and hindlimb in the NIR-IIb window with short exposure time of 20 ms for rare-earth based probes. Fluorescence imaging in the near-infrared window between 1500–1700 nm (NIR-IIb window) offers superior spatial resolution and tissue penetration depth, but few NIR-IIb probes exist. Here, the authors synthesize rare earth down-converting nanocrystals as promising fluorescent probes for in vivo imaging in this spectral region.

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Boosting the down-shifting luminescence of rare-earth nanocrystals for biological imaging beyond 1500 nm

Sol-gel silica/titania-on-silicon Er/Yb-doped waveguides for optical amplification at 1.5 μm

Highly (up to 4 mol% Er2O3) Er-doped phosphate bulk glasses have been prepared by common glass-melting techniques. Afterwards, a heat treatment was performed on the as-melted samples. The photoluminescence lifetime of Er ions for the 4I132–4I152 transition increases substantially, typically from 3 ms up to 7 ms for a sample doped with 2 mol% Er2O3, due to the heat treatment. The increase of the lifetime is ascribed to a decrease in concentration of hydroxyl groups incorporated in the glass, which is confirmed by IR absorption spectroscopic measurements. The photoluminescence peak intensity also increases because of drying by a factor of 3 to 7 depending on glass composition. Based on electric dipole-dipole interaction theory, the luminescence concentration quenching mechanism by hydroxyl groups is modelled. The model predicts that more than half of the hydroxyl groups in the glass is coupled to Er ions. The influence of the glass structure and role of Al3+ on the Er3+ luminescence is studied by infrared spectroscopy.

/pdf/luminescence-quenching-by-oh-groups-in-highly-er-doped-48hxdnfthx.pdf

Luminescence quenching by OH groups in highly Er-doped phosphate glasses

Systematic studies of cooperative upconversion and Yb3+–Er3+ energy transfer in newly developed phosphate glasses were performed by a rate-equation formalism. The cooperative-upconversion coefficients of the  4I13/2 level for different Er3+ concentrations were determined from the luminescence-decay curves for high pump intensities. A small cooperative-upconversion coefficient of 1.1×10-18 cm3/s was obtained for a high Er3+ concentration of 4×1020 ions/cm3.Yb3+–Er3+ energy-transfer coefficients for an Er3+ concentration of 2×1020 ions/cm3 codoped with different Yb3+ concentrations were calculated from the lifetime measurements of the  2F5/2 level of Yb3+ ions. For Er3+ codoped with an Yb3+ concentration of 6×1020 ions/cm3, an energy-transfer coefficient of 1.1×10-16 cm3/s and an energy-transfer efficiency higher than 95% were determined from our measurements under weak excitation. The cooperative-upconversion coefficients of Yb3+–Er3+-doped samples were found to be consistent with that of an Er3+-doped sample with the same Er3+ concentration. The weak cooperative-upconversion effect of high Er3+ concentrations and efficient Yb3+–Er3+ energy transfer indicate that these newly developed Er3+- and Yb3+–Er3+-doped phosphate glasses are excellent for active device applications.

Cooperative upconversion and energy transfer of new high Er 3+ - and Yb 3+ –Er 3+ -doped phosphate glasses

In this paper results of experimental investigations into erbium glass lasers and their likely applications are discussed. Regularities of the inversion energy accumulation, also as channels of energy losses, are covered in detail. Different erbium lasers, both flashtube and neodymium laser-pumped, are compared. Parameters of some erbium laser glasses are presented, including the new LGS-E7, which requires a smaller pumping energy density (less than 100–200 J cm-2). Test data from experimental models are summarized.

V.B. Kravchenko

Papers

Erbium glass lasers and their applications