Modeling the distributed gain of single--(1050 or 1410 nm) and dual-wavelength--(800 + 1050 nm or 800 + 1410 nm) pumped thulium-doped fiber amplifiers.

The distributed gain of single- and dual-wavelength-pumped thulium-doped fiber amplifiers is modeled. The excellent agreement between the model and coherent optical frequency domain reflectometry measurements enables us to estimate intrinsic loss, branching ratios of fluorescence originating from the 3H4 level, and cross sections of upconversion pumping at 1050 and 1410 nm for the Tm3+ ions in the fiber. With the branching ratios obtained it is possible to describe induced signal absorption when pumping at 800 nm.

Ligand-field analysis of an Er(III) complex with a heptadentate tripodal N4O3 ligand.

Polarized absorption and emission spectra of trigonal single crystals of an Er(III) complex coordinated to a heptadentate tripodal ligand are reported at temperatures between 8 and 298 K. The assigned energy levels below the onset of ligand absorption (< 25 000 cm(-1)) are fitted to a parametrized electronic Hamiltonian. The C(3) site symmetry of the Er(III) ion requires eight parameters for a full description of the ligand field within a one-electron operator description. This compound shows unusually large splittings of the multiplets, and the fitted parameters imply that this heptadentate ligand imparts the largest ligand field reported for an Er(III) complex. The ligand field was also interpreted within the angular overlap model (AOM). We derive the AOM matrix to include both sigma and anisotropic pi bonding and show that a useful description of the C(3) ligand field can be made using only five parameters. The success of the AOM description is encouraging for applications on isomorphous complexes within the lanthanide series and in describing the ligand field of low-symmetry complexes with less parameters than in the usual spherical harmonic expansion.

Ni(II)-doped CsCdBrCl2: variation of spectral and structural properties via mixed-halide coordination.

The mixed-halide compound CsCdBrCl2 is studied by X-ray diffraction and by using the Ni(II) ion as an optical probe. Low-temperature absorption, luminescence, and EXFAS spectra of the Ni(II) impurity are recorded. The structure of CsCdBrCl2 is shown to consist of corner-sharing [Cd3X12]6- trimers. Each trimer has a structure of three face-sharing octahedra Cl3CdX3CdX3CdCl3 where X has an equal probability of being a Br- or a Cl- ion. This equal occupancy on the bridging halide positions occurs over the whole crystal rather than within each [Cd3X12]6- trimer unit. Optical spectroscopy shows that the Ni(II) ion exists in all possible [NiBrxCl6 - x]4- isomeric forms (x = 0, 1, ..., 6). The energy of the 3A2g-->1A1g transition is a linear function of x, due to the change in inter-electron repulsion through the differing covalencies of the ligand compositions. The energy of this transition can be varied over 2750 cm-1. The inhomogeneous broadening that results from the halide disorder is discussed from the point of view of the variation of ligand-field strength and inter-electron repulsion. A model including a differential nephelauxetic effect is required to explain the energies of the ligand-field states.