ZBLAN is the most stable, and consequently the most used, fluoride glass, a subcategory of the heavy metal fluoride glass (HMFG) group. Typically its composition is 53% ZrF4, 20% BaF2, 4% LaF3, 3% AlF3 and 20% NaF. ZBLAN is not a single material but rather has a spectrum of compositions, many of which are still untried. The biggest library in the world of ZBLAN glass compositions is currently owned by Le Verre Fluoré, the oldest company working on HMFG technology. Other current ZBLAN fiber manufacturers are Thorlabs and KDD Fiberlabs. Hafnium fluoride is chemically similar to zirconium fluoride, and is sometimes used in place of it. ZBLAN glass has a broad optical transmission window extending from 0.22 micrometers in the UV to 7 micrometers in the infrared. ZBLAN has low refractive index (about 1.5), a relatively low glass transition temperature (Tg) of 260–300 °C, low dispersion and a low and negative temperature dependence of refractive index dn/dT.
History The first fluorozirconate glass was a serendipitous discovery in March 1974 by the Poulain brothers and their co-workers at the University of Rennes in France. While looking for new crystalline complex fluorides, they obtained unexpected pieces of glass. In a first step, these glasses were investigated for spectroscopic purposes. Glass formation was studied in the ZrF4-BaF2-NaF ternary system while the fluorescence of neodymium was characterized in quaternary ZrF4-BaF2-NaF-NdF3 bulk samples. The chemical composition of this original glass was very close to that of the classical ZBLAN, on the basis of a simple La/Nd substitution. Further experimental work led to major advances. First, ammonium bifluoride processing replaced the initial preparation method based on heat treatment of anhydrous fluorides in a metallic sealed tube. This process was already used by K. H. Sun, a pioneer of beryllium fluoride glasses. It offers significant advantages: preparation is implemented at room atmosphere in long platinum crucibles, zirconium oxide can be used as a starting material instead of pure ZrF4, synthesis time is reduced from 15 hours to less than one hour, and larger samples are obtained. One of the problems encountered was the devitrification tendency upon cooling the melt. The second breakthrough was the discovery of the stabilizing effect of aluminum fluoride in fluorozirconate glasses. The initial systems were fluorozirconates with ZrF4 as the primary constituent (>50 mol%), BaF2 main modifier (>30 mol%) and other metal fluorides LaF3, AlF3 added as tertiary constituents, to increase glass stability or improve other glass properties. Various pseudo-ternary systems were investigated at 4 mol% AlF3, leading to the definition of 7 stable glasses, such as ZBNA, ZBLA, ZBYA, ZBCA that could be cast as multi-kilogram bulk samples and resulted later in the classical ZBLAN glass composition that combines ZBNA and ZBLA. Further development on preparation method, scale-up, improvements of the manufacturing process, material stability and formulations was largely motivated by the experiments in French telecom at that time that found that intrinsic absorption for ZBLAN fibers was quite low (~10 dB/km) which could lead to an ultra-low optical loss solution in the mid-infrared. Such optical fibers could then become an excellent technical solution for a variety of systems for telecommunications, sensing and other applications.
Glass preparation Fluoride glasses have to be processed in a very dry atmosphere in order to avoid oxyfluoride formation which will lead to glass-ceramic (crystallized glass) formation. The material is usually manufactured by the melting-quenching method. First the raw products are introduced in a platinum crucible, then melted, fined above 800 °C and cast in a metallic mold to ensure a high cooling rate (quenching), which favors glass formation. Finally they are annealed in a furnace to reduce the thermal stresses induced during the quenching phase. This process results in large transparent pieces of fluoride glass.
Material properties
Optical The most obvious feature of fluoride glasses is their extended transmission range. It covers a broad optical spectrum from the UV to the mid-infrared. The polarisability of fluorine anions is smaller than that of oxygen anions. For this reason, the refractive index of crystalline fluorides is generally low. This also applies to fluoride glasses: the index of ZBLAN glass is close to 1.5 while it exceeds 2 for zirconia ZrO2. Cationic polarisability must also be considered. The general trend is that it increases with atomic number. Thus in crystals, the refractive index of lithium fluoride LiF is 1.39 while it is 1.72 for lead fluoride PbF2. One exception concerns fluorozirconate glasses: hafnium is chemically very close to zirconium, but with a much larger atomic mass (178 g vs 91 g); but the refractive index of fluorohafnate glasses is smaller than that of fluorozirconates with the same molar composition. This is classically explained by the well known lanthanidic contraction that results from the filling of the f subshell and leads to a smaller ionic radius. Substituting zirconium by hafnium makes an easy way to adjust the numerical aperture of optical fibers. Optical dispersion expresses the variation of the refractive index with wavelength. It is expected to be low for glasses with a small refractive index. In the visible spectrum it is often quantified by the Abbe number. ZBLAN exhibits zero dispersion at about 1.72 μm, compared with 1.5 μm for silica glass. Refractive index changes with temperature because the polarisability of the chemical bonds increases with temperature, and because thermal expansion decreases the number of polarisable elements per unit volume. As a result, dn/dT is positive for silica, while it is negative for fluoride glasses. At high power densities, refractive index follows the relation :
n = n0 + n2I where n0 is the index observed at low power levels, n2 the nonlinear index and I the average electromagnetic field. Nonlinearity is smaller in low-index materials. In ZBLAN n2's value lies between 1 and 2×10−20 m2W−1.
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