Visible and near infra-red up-conversion in Tm3+/Yb3+ co-doped silica fibers under 980 nm excitation

The spectroscopic properties of Tm(3+)/Yb(3+) co-doped silica fibers under excitation at 980 nm are reported. Three distinct up-conversion fluorescence bands were observed in the visible to near infra-red regions. The blue and red fluorescence bands at 475 and 650 nm, respectively, were found to originate from the (1)G(4) level of Tm(3+). A three step up-conversion process was established as the populating mechanism for these fluorescence bands. The fluorescence band at 800 nm was found to originate from two possible transitions in Tm(3+); one being the transition from the (3)H(4) to (3)H(6) manifold which was found to dominate at low pump powers; the other being the transition from the (1)G(4) to (3)H(6) level which dominates at higher pump powers. The fluorescence lifetime of the (3)H(4) and (3)F(4) levels of Tm(3+) and (2)F(5/2) level of Yb(3+) were studied as a function of Yb(3+) concentration, with no significant energy back transfer from Tm(3+) to Yb(3+) observed.

Experimental evidence for molecular chaos in granular gases

We present measurements showing the presence and the absence of molecular chaos in a two-layer vertically vibrated granular media where a plate drives a horizontal layer of massive grains, which, in turn, drives a second horizontal layer of lighter grains above the first. In the first layer driven by the plate, the velocities are spatially correlated. In the second layer, we find uncorrelated velocities consistent with the presence of molecular chaos. In this experiment, energy injection that is randomized in both space and time throughout the shaking cycle is necessary for observing molecular chaos and "kinetic theory"-like behavior. At higher densities, excluded volume effects force velocity correlations in the system which is no longer "gaslike" in behavior.

Analysis of changes in optical fibers during arc-fusion splicing by use of quantitative phase imaging

A non-interferometric imaging technique in conjunction with Abel inversion is used to directly and quantitatively examine the changes in optical fibers due to the heating produced during arc-fusion splicing as a function of fusion arc parameters. Phase images in the vicinity of a fusion splice are obtained using Quantitative Phase Microscopy, allowing the refractive-index change to be reconstructed with high spatial resolution. This simple, nondestructive method confirms that, for a fixed arc current, while the fusion time increases, the refractive-index of both fiber cores within the fusion region decreases in magnitude, the core region broadens, and the axial gradient decreases.

High-temperature-resistant chemical composition Bragg gratings in Er3+-doped optical fiber

Chemical composition gratings (CCGs), unlike standard fiber Bragg gratings (FBGs), do not suffer a significant decrease in reflectance or an irreversible wavelength shift when they are exposed to elevated temperatures. To date, the growth of CCGs has been related to the fluorine content of the fibers in which they are written. It is shown that FBGs with high thermal stability, resembling CCGs, can be fabricated in Er3+-doped optical fibers that do not contain any fluorine.

Refractive-index profiling of optical fibers with axial symmetry by use of quantitative phase microscopy

The application of quantitative phase microscopy to refractive-index profiling of optical fibers is demonstrated. Phase images of axially symmetric optical fibers immersed in index-matching fluid are obtained, and the inverse Abel transform is used to obtain the radial refractive-index profile. This technique is straightforward, nondestructive, repeatable, and accurate. Excellent agreement, to within approximately 0.0005, between this method and the index profile obtained with a commercial profiler is obtained.

Single-mode near-infrared optical parametric oscillator amplifier based on periodically poled KTiOPO4

A singly resonant, single-axial-mode, optical parametric oscillator (OPO) based on periodically poled KTiOPO4 (PPKTP) is reported. Signal (1.68 micron) and idler (2.90 micron) optical bandwidths have been narrowed to <400 MHz by use of a diffraction grating at grazing incidence. The OPO generates 370 microJ of signal radiation when pumped by 3.1 mJ of 1.064-micron radiation. We implemented a single-pass PPKTP amplifier to yield 2.15-mJ signal and 1.17-mJ idler radiation without broadening the spectral bandwidths.

Strain-independent temperature measurement by use of a fluorescence intensity ratio technique in optical fiber

The strain sensitivity of the fluorescence intensity ratio temperature-sensing technique has been measured to be (2 +/- 3) x 10(-4)%/muepsilon in Yb3+-doped fiber, implying a temperature-to-strain cross sensitivity of (2 +/- 3) x 10(-4) degrees C/muepsilon. The near-zero strain sensitivity means that this optical-fiber sensor technique is well suited for temperature measurement in strain-affected environments.

The rotating bucket of sand: Experiment and theory

The surface shape of a bucket of sand rotating about its cylindrical axis is studied experimentally and theoretically. Focusing on fast time scales on which surface shape is determined by avalanches, we identify three regimes of behavior. At intermediate and high frequencies, the surface shape is always at its critical shape determined by the Coulomb yield condition. The low frequency behavior displays an unexpected subcritical region at the center of the bucket. To understand this central region, we adapt a continuum model of surface flow developed by Bouchaud et al. and Mehta et al. The model indicates that the subcritical region is due to a nonlinear instability mechanism. (c) 1999 American Institute of Physics.

Self-referenced point temperature sensor based on a fluorescence intensity ratio in Yb(3+)-doped silica fiber

An optical fiber temperature sensor, based on the fluorescence intensity ratio from the (2)F (5/2)(a) and (2)F(5/2)(b) Stark sublevels in ytterbium-doped silica fiber, has been investigated. Results of a sensor prototype demonstrate an accuracy near 1 degrees C in a 600 degrees C temperature range. Changes in the fluorescence intensity ratio because of variation in pump power, pump wavelength, and induced fiber bending loss are demonstrated to be small, supporting development of a practical sensor based on the technique described.

Erbium-doped silica fibers for intrinsic fiber-optic temperature sensors

The variation in the green intensity ratio ((2)H(11/2) and (4)S(3/2) energy levels to the ground state) of Er ions in silica fibers has been studied as a function of temperature. The different processes that are used to determine the population of these levels are investigated, in particular 800-nm excited-state absorption in Er-doped fibers and 980-nm energy transfer, in Yb-Er-codoped fibers. The invariance of the intensity ratio at a fixed temperature with respect to power, wavelength, and doped fiber length has been investigated and shown to permit the realization of a high-dynamic-range (greater than 600 °C), autocalibrated fiber-optic temperature sensor.

1.2-µm transitions in erbium-doped fibers: the possibility of quasi-distributed temperature sensors

We propose the principle of a high-dynamic, quasi-distributed temperature sensor based on the behavior of the 1.13- and the 1.24-µm emission lines in erbium-doped silica fibers. The ratio of fluorescent intensity of these lines presents a temperature dynamic of more than 11 dB between room temperature and 600 °C. As the lower level of these transitions is not the fundamental, the emission lines are absorption free, and no dependence of the intensity ratio of the two lines has been observed, with power and wavelength pump variations permitting the realization of self-calibrated quasi-distributed sensors.

Single-mode 1.34-microm Nd:YVO(4) microchip laser with cw Ti:sapphire and diode-laser pumping

We report cw single-mode output at 1.34 microm in a Nd:YVO(4) microchip laser pumped separately with a Ti:sapphire laser and a diode laser in the region of 809 nm. The 0.5 mm x 1.0 mm x 1.0 mm Nd(3+)-doped YVO(4) crystal produced output powers as high as 60 mW with slope efficiencies as high as 40% and laser thresholds between 155 and 275 mW by use of a pumped volume of approximately 150-microm diameter and 0.5-mm length. Data are also presented for the laser's performance as a function of pump wavelength and for the spectral variation of the output with crystal temperature.

Performance of a Nd:YVO(4) microchip laser with continuous-wave pumping at wavelengths between 741 and 825 nm

We report the performance of a Nd:YVO(4) laser, which uses cw Ti:sapphire pumping, for a range of pumping wavelengths, polarizations, and crystal temperatures.