Jonathan D. Weiss

Using Fiber Optics to Detect Moisture Intrusion into a Landfill Cap Consisting of a Vegetative Soil Barrier

Abstract The intrusion of moisture into landfills can pose a health hazard because of the possibility that the moisture will carry harmful substances into the groundwater. Early detection of moisture anywhere within these landfills is essential if corrective action is to be taken well before an occurrence of this kind. This paper presents the results of a field-scale simulation test of the use of fiber optics to detect the presence of moisture within landfill covers, using a detection method based on the thermal response of soils as a function of their moisture content. By sending electrical current through an embedded stainless-steel tube, soils of varying moisture content were heated and time-dependent temperature measurements were obtained with a fiber-optic distributed temperature sensor system. The optical fiber itself lay within the tube, but its temperature was a function of how rapidly heat was conducted into the surrounding medium. The results of this experiment, which are in agreement with those obtained using more traditional “point” sampling and laboratory analysis, are presented along with the strengths and limitations of the thermal-response method of detecting moisture.

Fluorescent optical liquid level sensor

A liquid level sensor comprising a transparent waveguide containing fluorescent material that is excited by light of a first wavelength and emits at a second, longer wavelength. The upper end of the waveguide is connected to a light source at the first wavelength through a beveled portion of the waveguide such that the input light is totally internally reflected within the waveguide above an air/liquid interface in a tank but is transmitted into the liquid below this interface. Light is emitted from the fluorescent material only in those portions of the waveguide that are above the air/liquid interface, to be collected at the upper end of the waveguide by a detector that is sensitive only to the second wavelength. As the interface moves down in the tank, the signal strength from the detector will increase.

The radiation response of a Selfoc microlens

The radiation-induced darkening of SLW Selfoc microlenses at 810 nm is measured using 100-ns pulses from the Hermes II X-ray simulation machine. Their transmission loss versus total dose is displayed immediately after exposure and 45 mu s later, up to a dose of about 32 krds. >

Impurity-doped fiber-optic shock position sensor

We have demonstrated the operation of a fiber-optic device originally proposed to continuously measure the location of a shock front, but which also has potential for use as a crack location sensor. It consists of one or a group of fibers uniformly doped along their length with neodymium, which fluoresces at about 1060 nm in response to being optically pumped at a shorter wavelength. In our case, the pump source was a laser diode emitting at 810 nm. As the length of the fiber is reduced, the strength of the fluorescence signal diminishes along with the number of fluorescing atoms. Laboratory and explosives experiments were performed on sensor fibers containing various concentrations of dopants. We also investigated the benefits, in an explosives application, of adding phosphorus to narrow the fluorescence line. >

Vacuum feedthrough for optical fiber cables

A vacuum feedthrough for optical fiber cables is constructed and used in a nuclear-radiation environment.

Trapping efficiency of fluorescent optical fibers

Abstract. Fluorescent optical fibers are used in nuclear detection and other forms of fiber-optic sensors. The trapping efficiency of a fluorescent optical fiber is defined by the optical energy trapped (or guided) by the fiber divided by the total energy emitted within it by the fluorescers that dope the fiber core. This characteristic is clearly important in determining the size of signals from these devices. A calculation of the trapping efficiency has been performed under the assumption that the fluorescence radiation is emitted isotropically by the individual fluorescers that are uniformly distributed throughout the core and are equally likely to be excited by particles or shorter-wavelength light. At the price of increased complexity, nothing in the analysis precludes the lifting of these restrictions. What is included in this analysis is the contribution of skew rays, which, to the author’s knowledge, is not presented elsewhere. A very simple expression for the trapping efficiency as a function of the cladding-to-core index ratio is derived. Also important in determining signal size is the transmission loss of the fluorescence radiation to either end of the fiber from the point of its generation. However, as it is a separate matter, it is not discussed here.

Piezooptic behavior of certain fluids

In this paper we present an analysis of pressure-volume data for certain optical fluids, which characterizes them by two parameters: their bulk moduli and the pressure derivative of their bulk moduli, both evaluated at zero pressure. We then relate their refractive-index changes to density and pressure using this analysis and the Lorentz-Lorenz equation with a density-dependent polarizability. An example of the use of such fluids in a fiber-optic pressure gauge being developed at Sandia is also discussed.

Using optical absorption to measure the state of charge of lead-acid batteries

Proposed here is a method of measuring the state of charge of lead-acid batteries based on the assumption that the addition of sulfuric acid to water will reduce the optical absorption at certain absorption peaks of pure water. This reduction is expected to result from the accompanying drop in the concentration of water molecules. Measurements of the absorption in the vicinity of 0.97, 1.20, and 1.45 ?m indicate that this is indeed the case, although deviations from a linear dependence on water concentration are noted. Two ways of implementing this phenomenon in a lead-acid battery are presented; one involves an absorption cell in a flooded battery, while the second involves an optical fiber woven into an absorbed-glass-mat battery. In the second case, the absorptive electrolyte serves as the cladding of the optical fiber and introduces attenuated total internal reflection into what would otherwise be perfectly guided modes.

Gallium arsenide as an optical strain gauge

We have investigated the use of gallium arsenide (GaAs) as an optical strain gauge, presuming the shift in its absorption edge with uniaxial stress as the principle of operation. In our experiments, optical fibers guided light of the GaAs samples from a laser diode source and from the samples to an optical detector, as might be the case in a practical application of such a device. Compressive and tensile strains were developed in the samples by flexing a cantilevered beam to which they were bonded. An important understanding of this strain distribution was obtained with the help of a finite‐element calculation. The effects of sample width and the input optical energy distribution on the strain‐induced change in optical transmission were measured. In the case of the distribution with the shortest median wavelength, over 80% change in transmission was observed for a compressive strain of only 0.05%. In addition, we calculated the strain sensitivity of this device, assuming only a rigid, strain‐induced shift in its absorption edge. A comparison between experiment and calculation suggests that strain also caused the band edge of these samples to steepen in compression and broaden in tension. Another calculation suggests that this phenomenon is not intrinsic to the material.

Dielectric-waveguide strain gauge: a theoretical study

We present a theoretical analysis of the strain dependence of bend loss from a multimode slab dielectric waveguide formed in the shape of an arc of a circle. This dependence suggests its use as a strain gauge. We calculate the strain sensitivity of this gauge as a function of the core–cladding index difference, bend radius, and arc angle. The method that we use is a geometrical-optics analysis modified to account for the curvature of the boundary between the core and the cladding.