Mahmoud R. Shahriari

Porous optical fibers for high-sensitivity ammonia-vapor sensors.

A new porous glass optical fiber has been developed for use as a sensor for the detection of ammonia vapors at low concentrations. The porous structure that remains after selective heat treatment, phase separation, and chemical leaching of a borosilicate glass imparts a high surface area to the fiber core. Ammonia vapors permeating into the porous zone, which is pretreated with a reversible pH dye indicator, produce a spectral change in transmission. The resulting pH change is measured by in-line optical absorbance and is proportional to the ambient-ammonia concentration. Ammonia-vapor concentrations as low as 0.7 part in 10(6) have been detected.

Synthesis, characterization, and potential application of highly chemically durable glasses based on AlF3

Fluorozirconate glasses are stable with respect to devitrification but have poor chemical durability and only fair mechanical strength. AlF 3 -based glasses with improved chemical durability and enhanced mechanical strength are reported here. The optical, mechanical, and thermal properties of these glasses are contrasted to the more familiar ZBLAN composition. The infrared edge of these glasses lies at shorter wavelengths than ZrF 4 -based glasses, but aluminum fluoride glasses offer some interesting opportunities for short-range IR fiber applications such as sensing, remote spectroscopy, and laser power propagation.

Active silica-gel films for hydrogen sulfide optical sensor application

We have developed a novel H(2)S-sensitive film by immobilizing thionine in a silica matrix by means of the solgel process. The film is stable, reversible, and sensitive to dissolved H(2)S to as low as parts-per-billion levels. The photochemical instability of thionine in basic solution has been improved dramatically by use of this immobilization technique and silica substrate. Based on the developed new films, sensors for monitoring H(2)S may be prepared either by a fiber-optic approach or by integrated optical circuit techniques.

New highly stabilized AlF3-based glasses

New AlF 3 -based glasses with enhanced thermal stability have been synthesized and characterized using differential scanning calorimetry, spectroscopic and viscosity measurements. The HR values for these new glasses compare favorably with those of the base glass ((mol%) 30.2AlF 3 -10.6BaF 2 -20.2CaF 2 -8.3YF 3 -3.5MgF 2 -3.8NaF-13.2SrF 2 -10.2ZrF 4 ), ZBLAN and other compositions of halide glasses. Core and cladding glasses were synthesized using these compositions and evaluated for fiber drawing

Ormosil thin films for chemical sensing platforms

Organically modified silicate (ormosil) sol-gel thin films have many advantages over their inorganic sol-gel and polymeric counterparts for sensing applications. The addition of methyltrimethoxysilane (MTMS) to tetraethyl orthosilicate (TEOS)-based gels creates a film with much greater hydrophobicity and less cracking due to replacement of hydroxyl groups by non-hydrolyzable methyl groups. The more hydrophobic thin film is advantageous in oxygen sensing applications, since it allows only gaseous interaction with the sensing element, and liquid infiltration into the gel is minimized. Organic modification of the gels is found to increase the degree of fluorescence quenching in dip-coated films, as evidenced by fluorescence lifetime measurements, due to the more open structure of the ormosil. However, hydrophilicity can still be obtained in the ormosil thin films by adding smaller amounts of MTMS and greater amounts of TEOS. This creates a partially hydrophilic film which still maintains a low degree of cracking due to the MTMS addition. Hydrophilic films are much desired in hydrogen sulfide and carbon dioxide sensing applications, where liquid interaction with the gel matrix itself is necessary for proper protonation and deprotonation reactions. While TEOS-based spin-coated thin films have been shown to quench more poorly with additions of MTMS, it is found that low levels of organic modification will prevent cracking of the spun films while still maintaining a very high degree of fluorescence quenching. Hence, ormosil thin films have strong potential in a wide array of chemical, biochemical, and environmental sensing applications.

Optical properties of AIF 3 -based glasses doped with Pr 3+ , Yb 3+ and Lu 3+

Rare-earth ions can easily be incorporated into fluoride glasses in moderate to large concentrations. Because these glasses possess low fundamental frequencies, they appear to have many advantages over oxide glasses as hosts for rare-earth ions used in optical amplifiers and lasers. We have investigated the optical properties (fluorescence, absorption, and excited-state lifetimes) of AlF(3)-based glass doped with Pr(3+), Yb(3+) and Lu(3+). The effects of different dopant levels on the optical properties of this glass system have also been investigated. These results are compared to those obtained for the same ions in other glass hosts.

Remote infrared chemical sensing using highly durable AlF 3 -based glass fibers

Unclad, low-loss AlF(3)-based glass fibers with enhanced chemical durability have been successfully used for the first time to our knowledge as intrinsic evanescent infrared sensors for monitoring liquid chemicals. Different liquids with absorption bands between 1 and 4.5 microm, such as alcohol, acetonitrile, and mixtures of alcohol/acetonitrile and water/acetonitrile, have been tested. These fibers have also been used successfully as distributed sensors for simultaneous monitoring of different chemical species.

Preliminary study of fiber drawing of AlF3-based glasses

Abstract Although AlF 3 -based glasses have shown superior chemical durability and mechanical strength relative to ZrF 4 -based glasses, devitrification problems near the drawing temperature have limited their fiber applications. We have investigated the effect of atmosphere, particularly moisture, on the surface crystallization during the fiber drawing of these glasses. The crystallization behavior has been monitored by optical microscopy, scanning electron microscopy (SEM) and differential scanning calorimetry (DSC) techniques. Preforms of AlF 3 -based glasses have also been drawn into long fibers in a tower enclosed by a unique three-stage vertical glove box in which the moisture levels are maintained below 1 ppm.

Mechanical properties of aluminum fluoride glass fibers

The effect of high temperature aqueous solutions of various pH values on the mechanical properties of polymer coated optical fibers of an aluminum fluoride-based composition are examined. It was found that such fibers retain much more strength when aged in these aqueous environments than fibers of the more common zirconium fluoride-based composition. The aging is not affected by pH unless the fiber is under stress, in which case a low pH solution decreases the time to failure of the fiber. In static fatigue, the time to failure of the aluminum fluoride-based fibers is 20 times greater than that of the zirconium fluoride-based fibers.

Evaluation Of An FTIR/Fluoride Optical Fiber System For Remote Sensing Of Combustion Products

Infrared transmitting optical fibers were combined with a Fourier Transform Infrared Spectrometer (FTIR) to perform remote quantitative gas analysis. A glass clad fluoride fiber, 1/2 meter in length, transmitted the infrared radiation to a double pass reflective gas cell, with a total pathlength of 10 cm, and a second fiber returned the absorbed characteristic spectrum back to the instrument. Methane (CH4), carbon dioxide (CO2), and carbon monoxide (CO), were readily detected to very low concentrations in nitrogen with this arrangement. The concentration range and lower detection limit are dependent upon the absorption coefficient of each gas, the pathlength of absorption, and the available energy throughput at the detection wavelength. Carbon monoxide had the largest range of detection at this pathlength, with detection capabilities from 100 vol% to 0.3 vol% in nitrogen. Methane was detected from 15 vol% to 0.2 vol% and carbon dioxide was detected from 2.5 vol% to 0.05 vol%. From this study, it was determined that the fluoride fiber/FTIR system can be utilized to remotely detect gases effectively. Proper adjustment of the absorption pathlength will enable each gas to be detected over a broad range of concentrations.