John M. Jewell

Interferometric method for concurrent measurement of thermo-optic and thermal expansion coefficients

A Fabry-Perot-type interferometer is described that permits the concurrent measurement of the coefficient of thermal expansion (alpha) and the thermooptic coefficient (dn/dT) of transparent materials. Measurements of the a and the dn/dT of vitreous silica and a heavy-metal fluoride glass show that the technique is accurate, reproducible, and easily performed. In addition, the technique is used to show that the dn/dT of heavy-metal fluoride glasses is negative and temperature dependent. The magnitude of dn/dT is found to increase with increasing temperature.

Separation of Intrinsic and extrinsic Optical-Absorption in a Fluoride Glass

The contribution of impurity ions to the total optical absorption of a heavy metal fluoride glass has been determined at 532 and 1064 nm. Four ZrF4‐BaF2‐LaF3‐AlF3‐NaF glasses were prepared from various purity raw materials. The absorption coefficients of these glasses range from 0.92 to 45.4×10−4 cm−1 at 1064 nm and from 7.43 to 11.1×10−4 cm−1 at 532 nm as determined by laser calorimetry. The concentrations of Fe, Ni,Cu, and Co ions in each glass were determined by graphite furnace atomic absorption spectroscopy. These two measurements enable the absorption, due to transition metal ions to be differentiated from the intrinsic absorption of the glass. At 1064 nm, the absorption coefficient of these glasses is controlled entirely by the transition metal ion content. However, at 532 nm, the absorption by the transition metal ions accounts for 4–42% of the total absorption depending on impurity concentration. The intrinsic absorption of this fluoride glass calculated from these data at 532 nm is (7.69±0.99)×10−4 cm−1.

Thermal lensing in heavy metal fluoride glasses

The effect of glass composition on the theoretical thermal lensing of a ZrF 4 BaF 2 LaF 3 AlF 3 NaF (ZBLAN) heavy metal fluoride glasses (HMF) has been examined. A number of composition substitutions, i.e., HfF 4 for ZrF 4 , PbF 2 for BaF 2 , and LiF and NaCl for NaF, have been made, and the effects of these substitutions on critical properties have been determined. These substitutions have significant effects on the refractive index, coefficient of thermal expansion (CTE), and the thermo-optic coefficient (d n /d T ). The changes in properties can be used to develop glass compositions that will show no thermal lensing when heated by laser fluences. The elimination of thermal lensing by composition changes is evaluated in terms of changes in heat capacity, glass stability and processing parameters that might be detrimental to large scale window production.

Transparent heavy metal fluoride glass-ceramic

Abstract A CdF 2 LiFAlF 3 PbF 2 (CLAP) glass was doped with ZrF 4 and crystallized using a controlled heat treatment. The heat treatment resulted in the formation of a glass-ceramic which is > 95% crystalline and transparent (> 80%) from the visible (∼ 400 nm) to the mid-IR (> 7 μm).

Raman and infrared spectra of lead gallosilicate glasses

Abstract Infrared and Raman spectra for two series of lead gallosilicate glasses were measured. Vibrational spectra are fully consistent with a previous structural model for these glasses consisting of two ‘sub’-networks of polyhedral chains, i.e., a gallium tetrahedral network and a lead pyramidal network, intertwined in various configurations. Silica enters the glass substituting directly for GaO 4 tetrahedra in the gallium sub-network. Vibrational bands associated with the bonds necessary to form this type of structure are present in both infrared and Raman spectra. Discrete sub-networks are indicated by the presence of separate bands attributable to both GaOGa bonds and PbO bonds (contained within lead chains). The intensities and frequencies of these bands as well as those attributable to intra-network bonds vary with composition in a manner predicted by the structural model. The SiO vibration frequency is close to that reported elsewhere for the vibrational band associated with SiOGa bridges.

Optical properties of BaO-Ga2O3-GeO2 glasses for fiber and bulk optical applications

Barium gallogermanate glasses are a relatively new family of glasses with tremendous potential for both fiber and bulk optical applications. This ternary system has a broad region of glass forming ability, excellent stability with respect to crystallization, and transmission beyond 5 micrometers . This paper reports the effects of composition and processing on properties critical to both fiber and bulk optical applications of these glasses.

Structural influences on the hydroxyl spectra of barium gallogermanate glasses

Abstract The infrared absorption of hydroxyl in three series of barium gallogermanate glasses — 10 mol% Ga2O3, 60 mol% GeO2, and BaO/Ga2O3 = 1 — have been measured. Four separate bands associated with the OH stretching vibration dominate the absorption spectra of these glasses between 4000 and 1500 cm−1. Two of these bands are attributed to hydroxyl with vibrational frequencies perturbed by hydrogen bonds between the hydroxyl and adjacent non-bridging oxygens. The other two bands arise from the vibration of ‘free’ hydroxyl ions associated with either germanium or gallium tetrahedra. Hydrogen-bonded hydroxyl absorptions are present in the spectra of all compositions. This observation leads to a modification of the previously proposed structural model for these glasses. In addition, the relative concentration of various types of hydroxyl is correlated to the changes in structure associated with changes in composition. Specifically, the fraction of hydrogen-bonded hydroxyl increases at the expense of ‘free’ hydroxyl when the number of non-bridging oxygens in the glass increases.

A structural model for PbOGa2O3SiO2 glasses

Abstract The effect of composition on the glass transformation temperature, viscosity, coefficient of thermal expansion, and density/molar volume of PbOGa2O3SiO2 glasses was investigated. Trends in these data cannot be fully accounted for by previous structural explanations of the properties and glass-forming ability of the PbOGa2O3-based heavy metal oxide glasses. A new structural model is proposed which incorporates what is known about the structures of both PbOSiO2 and PbOGa2O3 glasses to explain the anomalies in composition-property relationships demonstrated in this paper. In addition, this model is able to account for the discontinuities in the properties of PbOGa2O3 binary glasses which occur near the 75PbO25Ga2O3 composition.

PHOTOSENSITIVITY OF RARE EARTH-DOPED GLASSES

Rare earth-doped glasses exhibit high initial photosensitivity but their response saturates at relatively modest values of (Delta) n (approximately 5 X 10-7), which greatly limits their usefulness for device applications. In the context of our model, saturation results from either exhaustion of photosensitive rare earth sites, trap sites, or through competition between two photon creation and one photon bleaching processes. In this paper we report the results of new experiments designed to further elucidate the photosensitivity process with specific emphasis on the saturation mechanisms(s). Based on these new experimental results we present a refinement of our earlier model.

Dynamics of the 5 eV optical absorption in SiO2 glass

The optical absorption at 5 eV in SiO2 glass was observed, using laser calorimetry, to change reversibly depending on the intensity of UV light. The generation and bleaching of an absorption band at 5 eV by two‐ and one‐photon absorption processes, respectively, can explain these reversible changes. This observation supports the structural model of unrelaxed oxygen deficiency center for the 5 eV absorption band in silica.