Anomalous nonradiative decay in Dy-doped glasses and crystals.

Nonradiative relaxation in Dy-doped fluoroindate glass is measured and characterized in terms of the well-known exponential energy gap law. It is found that although three low-lying Dy transitions fit well with an exponential dependence on an energy gap, the overall magnitude of the Dy decay rate is an order of magnitude higher than that of other rare earth ions. It is also found that the nonradiative decay rates in InF3 glass are comparable to those in ZrF4 glass, contrary to expectations based on maximum phonon energy. These unexpected results have important implications for a potential 4 µm Dy:InF3 fiber laser.

High quantum efficiency of near-infrared emission in bismuth doped AlGeP-silica fiber.

A self-calibrating method is described for measuring the radiative quantum efficiency (QE) in doped optical fibers. The method uses an integrating sphere to collect the fluorescence from the fiber, with pump light transmitted through the fiber end serving as a reference. QE measurements for a 780 or 808 nm pump were made on bismuth-doped AlGeP-silica fibers prepared by aerosol deposition. For both wavelengths, a value of QE=1.0+/-0.05 was obtained. Fluorescence was observed in two bands centered around 800 and 1300 nm, and the relative strength of these bands was found to vary with the pump wavelength.

Self-calibrating quantum efficiency measurement technique and application to Pr(3+)-doped sulfide glass.

A measurement technique is described in which the radiative quantum efficiency of certain transitions in rare-earth-doped glasses can be determined based only on relative fluorescence measurements. We calibrate the emission from the level of interest by measuring emission into that level from a higher excited level. Application of the technique to Pr(3+)-doped sulfide glasses yields quantum efficiencies for the (1)G(4) ? (3)H(5) transition as high as 60%, in good agreement with measurements using the integrating sphere technique. Calculated efficiency values based on the Judd-Ofelt technique are shown to be subject to inherent uncertainties.

General procedure for the analysis of Er(3+) cross sections.

We have applied the theory of McCumber to the (4)4I(13/2) ? (4)I(15/2) transition of Er(3+) -doped glasses and confirmed its ability to provide both spectra and accurate values of cross sections. For an Al/P-silica fiber, the exact McCumber treatment is within 3% of the measured emission cross section, whereas an Einstein analysis is off by >50%. An approximate treatment that eliminates the McCumber theory's requirement of detailed electronic structure has been developed and found to usually provide more accurate values than an Einstein analysis.

Output saturation in a 980-nm pumped erbium-doped fiber amplifier.

A fiber amplifier model is developed which includes a finite lifetime for the pumped level. Pump excited state absorption (ESA) from the pumped level is also included. The model is solved numerically for a erbiumdoped fiber amplifier pumped at 980 nm. The signal output power at 1.5 microm is found to be limited by the finite lifetime of the (4)I(11/2); ESA of the pump from the (4)I(11/2) further limits the signal output. Lifetimes of the (4)I(11/2) were measured for Er(3+) in a series of oxide and fluoride glasses. Output saturation is most important in the fluoride based glasses, where the (4)I(11/2) lifetime is longest.

Output saturation in fiber lasers.

An analytical theory is developed for a four-level cw fiber laser which accounts for saturation of the output due to bleaching of the pump transition. A simple closed form expression is obtained which relates the pump power and output power. Output saturation is most important for systems having at least one long lived excited state other than the upper laser level. The model is used to optimize the fiber length, core radius, and mirror reflectivities for an Er(3+) doped fluorozirconate fiber laser operating at 2.7 microm. The optimized efficiency is found to be independent of pump power. Maximum efficiencies of 9.8 and 12.9% are calculated for fibers with attenuation coefficients of 100 and 10 dB/km, respectively.

Photoacoustic effect in a flowing liquid.

It is found experimentally that the photoacoustic signal in a gas microphone cell depends on the velocity of a flowing liquid sample. The signal is constant up to a threshold velocity, above which there is a characteristic oscillatory behavior to the velocity dependence. The threshold velocity and characteristic oscillations vary systematically with both modulation frequency and the position of the incident beam. A simple theory which considers only the temperature dependence in the direction of fluid flow successfully accounts for the principal features of this velocity dependence. It is found that the cell boundaries are primarily responsible for the structure in the observed velocity dependence. Simple approximate expressions for the velocity dependence are derived, and the various simplifications made in the theory are discussed. The results have implications for in vivo photoacoustic measurements on a flowing blood stream.

Photoacoustic measurement of absolute quantum efficiencies in solids.

We have developed a simple new technique for measuring absolute radiative quantum efficiencies of fluorescing ions in solids that can be applied to systems exhibiting concentration quenching of the fluorescence. The technique requires measurement of the photoacoustic signal and fluorescent lifetime as a function of ion concentration. Application of this method to the upper laser level of Nd(3+) in ED2 glass yields a value for the quantum efficiency somewhat smaller than that found in previous estimates and measurements.