Erbium-doped fiber amplifiers are modeled using the propagation and rate equations of a homogeneous two-level laser medium. Numerical methods are used to analyze the effects of optical modes and erbium confinement on amplifier performance, and to calculate both the gain and amplified spontaneous emission (ASE) spectra. Fibers with confined erbium doping are completely characterized from easily measured parameters: the ratio of the linear ion density to fluorescence lifetime, and the absorption of gain spectra. Analytical techniques then allow accurate evaluation of gain, saturation, and noise in low-gain amplifiers (G >

Modeling erbium-doped fiber amplifiers

Tellurite glass: a new candidate for fiber devices

Fibre lasers in the mid-infrared regime are useful for a diverse range of fields, including chemical and biomedical sensing, military applications and materials processing. This Review summarizes the different rare-earth cations and host materials used in mid-infrared fibre laser technology, and discusses the future applications and challenges for the field.

Towards high-power mid-infrared emission from a fibre laser

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

Lanthanide-doped upconversion nanoparticles (UCNPs) have attracted considerable interest due to their superior physicochemical features, such as large anti-Stokes shifts, low autofluorescence background, low toxicity and high penetration depth, which make them extremely suitable for use as alternatives to conventional downshifting luminescence bioprobes like organic dyes and quantum dots for various biological applications. A fundamental understanding of the photophysics of lanthanide-doped UCNPs is of vital importance for discovering novel optical properties and exploring their new applications. In this review, we focus on the most recent advances in the development of lanthanide-doped UCNPs as potential luminescent nano-bioprobes by means of our customized lanthanide photophysics measurement platforms specially designed for upconversion luminescence, which covers from their fundamental photophysics to bioapplications, including electronic structures (energy levels and local site symmetry of emitters), excited-state dynamics, optical property designing, and their promising applications for in vitro biodetection of tumor markers. Some future prospects and efforts towards this rapidly growing field are also envisioned.

Lanthanide-doped upconversion nano-bioprobes: electronic structures, optical properties, and biodetection

Nonradiative decay of excited rare-earth ions by multiphonon emission has been investigated in a series of oxide glasses. Various rare-earth electronic levels were selectively excited by short-duration laser pulses and multiphonon relaxation rates were determined from measurements of fluorescence rise and decay times. Time resolution for fluorescence measurements was 3 nsec, so excited states were probed for which the decay was predominantly nonradiative. Excited states of ${\mathrm{Nd}}^{3+}$, ${\mathrm{Er}}^{3+}$, and ${\mathrm{Tm}}^{3+}$ with energy gaps to the next-lower $J$ state ranging from 1300 to 4700 ${\mathrm{cm}}^{\ensuremath{-}1}$ were studied. The multiphonon relaxation rates for each glass investigated exhibited an approximately exponential dependence on energy gap. Evidence of breakdown of this dependence was observed in the region of small energy gaps. The measured temperature dependences of the decay rates establish that the relaxation occurs predominantly by excitation of the highest-frequency vibrations associated with stretching modes of the glass network former. Borate, silicate, phosphate, germanate, and tellurite glasses were studied. From Raman spectra, the highest-frequency vibrations for these glasses ranged from 700 to 1400 ${\mathrm{cm}}^{\ensuremath{-}1}$. The corresponding multiphonon relaxation rates for a given energy gap differed by three orders of magnitude. The strength of the ion-phonon coupling was found to be approximately equal for all glasses.

Multiphonon relaxation of rare-earth ions in oxide glasses

Relaxation of excited electronic states of rare-earth ions by multiphonon emission was investigated in a beryllium-fluoride glass. Decay rates were measured from the transient fluorescence following pulsed laser excitation of selected states. The multiphonon emission rates exhibit an approximately exponential dependence on the energy gap to the next-lower state and are in general agreement with the phenomenological treatment of multiphonon processes.

Multiphonon relaxation of rare-earth ions in beryllium-fluoride glass

Laser-induced fluorescence-line-narrowing techniques are extended to measure site-to-site variations in the spectroscopic properties of excited states of optically active ions in glass The crystalline Stark splitting associated with the initia excited state, rather than the terminal fluorescence state, is observed The process is illustrated for ${\mathrm{Eu}}^{3+}$ in a lithium-borate glass by exciting the $^{5}D_{1}$ state and observing $^{5}D_{0}\ensuremath{\rightarrow}^{7}F_{J}$ fluorescence The measured crystal-field splitting of $^{5}D_{1}$ is in good agreement with that predicted from the splitting of $^{7}F_{1}$ Conditions necessary for obtaining fluorescence line narrowing in excited states are discussed

Spectroscopic properties of excited states of ions in glass measured using laser-induced fluorescence line narrowing

$^{9}\mathrm{Be}$ and $^{19}\mathrm{F}$ NMR techniques are used to gain information about the short-range order in two beryllium fluoride glasses: A simple ${\mathrm{BeF}}_{2}$ glass and a NaF\ensuremath{\cdot}${2\mathrm{B}\mathrm{e}\mathrm{F}}_{2}$ glass. The magnitude of the average quadrupole coupling constant for $^{9}\mathrm{Be}$ sites is deduced from the location of the first-order satellites and from low-frequency second-order broadening of the central transition of the $^{9}\mathrm{Be}$ spectrum. The average quadrupole coupling constant for the binary glass was a factor of 4 larger than for ${\mathrm{BeF}}_{2}$ glass. It is argued that this is consistent with the existence of non-tetrahedrally-coordinated Be ions in the sodium fluoroberyllate glass. The central transition of the $^{9}\mathrm{Be}$ NMR spectrum for the nominally pure ${\mathrm{BeF}}_{2}$ glass exhibited an anomalous narrow line which may be due to the presence of Be metallic clusters. The linewidth of the $^{19}\mathrm{F}$ resonance and the $^{19}\mathrm{F}$ spin-lattice relaxation time, ${T}_{1}$, of the two glasses were also studied as a function of temperature. These measurements indicate that there is some motion of the fluorines in the binary sample slightly above its transition temperature of 388 K. The two glasses were simulated by methods of molecular dynamics. It was found that whereas in simple ${\mathrm{BeF}}_{2}$ glass virtually all of the Be ions were tetrahedrally coordinated by four F ions, in multicomponent glasses a large number of Be ions have five F ions in the first coordination shell. This is qualitatively consistent with the experimental observations. Meaningful quantitative predictions of the effective $^{9}\mathrm{Be}$ quadrupole coupling constants for the two glasses failed, however, because the simulated glasses are more disordered than glasses prepared in the laboratory. There is a broad distribution of F--Be--F angles in the simulated ${\mathrm{BeF}}_{2}$ glass. The calculated electric field gradients at these nonideal tetrahedral sites were comparable to those for the non--fourfold-coordinate Be sites in the alkali fluoroberyllate glass. The limitations of molecular-dynamics simulations for providing angular distributions of ions are discussed.

Nuclear-magnetic-resonance studies and molecular-dynamics simulations of the structure of sodium fluoroberyllate glasses: Evidence of non-tetrahedrally-coordinated beryllium.

Vibronic sidebands associated with the $^{6}P_{\frac{7}{2}}\ensuremath{\rightarrow}^{8}S_{\frac{7}{2}}$ transition of ${\mathrm{Gd}}^{3+}$ in Be${\mathrm{F}}_{2}$ glass are observed with the use of line-narrowed fluorescence techniques. Comparison of the relative intensities of peaks in vibronic, polarized Raman, and infrared (${\ensuremath{\epsilon}}_{2}$) spectra demonstrate that different selection rules are operative. No variation of vibronic spectra with laser excitation wavelength was detected.

M. J. Weber

Papers

Multiphonon relaxation of rare-earth ions in oxide glasses

Multiphonon relaxation of rare-earth ions in beryllium-fluoride glass

Spectroscopic properties of excited states of ions in glass measured using laser-induced fluorescence line narrowing

Nuclear-magnetic-resonance studies and molecular-dynamics simulations of the structure of sodium fluoroberyllate glasses: Evidence of non-tetrahedrally-coordinated beryllium.

Vibronic spectrum of Gd/sup 3 +/ in BeF/sub 2/ class