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Fluoroindate glasses are attractive materials for the fabrication of mid-infrared transmitting fibers with extended spectral range. Preparation of fluoroindate glasses under different melting conditions and preform fabrication using the billet extrusion technique were investigated in this study. Experimental results showed that the fluorination of the raw materials using ammonium bifluoride reduced OH content and oxide impurities, and enhanced the crystallization stability of the glasses. In addition, a shift of the IR absorption edge to longer wavelength was observed by using ammonium bifluoride. Casting and extrusion methods were compared for application to preform fabrication. In this work, the fiber with the lowest loss (~2 dB/m at 1.55 μm) was obtained using preform extrusion at 322 °C. The significantly reduced loss of the fiber made from the extruded preform compared to the fiber made using a cast preform is attributed to the suppression of scattering centers and the better surface quality of extruded rods compared with the cast rod.
©2013 Optical Society of America
1. Introduction
Halide and chalcogenide glasses have attracted significant attention and experienced rapid developments due to their importance as infrared transmitting materials [1,2]. Current applications of these materials include bulk optical components, and optical fiber for the near and mid infrared (IR) spectral regions. Chalcogenide glasses provide transparency further into the IR region than heavy metal fluoride glasses, and they are commonly investigated for nonlinear wavelength transfer due to their high nonlinearity [3,4]. However, chalcogenide glasses have higher refractive index and weaker mechanical strength than fluoride glasses, which limits their application for high power delivery devices [5]. In the heavy metal fluoride glass family, extensive previous research has focused on fluorozirconate glasses [6], which have been successfully drawn into low-loss fibers [7]. Other glass compositions of interest are fluoroindate systems, which present good optical quality, stability against atmospheric moisture, good mechanical stability [8] as well as low optical attenuation from 250 nm to 8 μm. Due to their low maximum phonon energy of ~510 cm−1, they are characterized by transmittance further into the IR region, and significantly lower non-radiative decay rates in comparison to fluorozirconate glasses [9,10].
Several research studies have been published on the subject of fluoroindate glass fibers. Among these fluoroindate glass matrices, PbF2-InF3 based fluoride glass has good durability, high thermal stability against crystallization, and has been fabricated into a fiber with a minimum transmission loss of 0.043 dB/m at 3.33 μm [11]. In comparison, Itoh et al. have developed a single-mode fiber, using InF3/GaF3 glass as a core glass and ZBLAN glass as a cladding glass, with a low loss round 0.2 dB/m at 1.2 μm [12]. Nishida et al. systematically studied InF3/GaF3 glass fibers and obtained minimum loss of 0.025 dB/m at 3.3 μm [13]. Recently a multimode fiber with an optical loss of 0.85 dB/m at 1.3 μm was obtained from a preform made by rotational casting method, where the importance of fluorination with HF during glass melting was also discussed [14]. However, all these fibers have comparatively low InF3 (~28 mol%) and high GaF3 contents, which limits their IR transmittance.
Commercial fluoroindate glass fibers are now available with low loss in a wide spectrum (<1 dB/m for 1.5-5 µm) [15], but the glass composition is not available to the public. An earlier research study [16] showed that the glass composition 32InF3-20ZnF2-20SrF2-18BaF2-8GaF3-2CaF2 (IZSBGC) with high InF3 content was particularly promising for mid-infrared applications due to its high transmission and low rate of crystallization. Although short unstructured fibers (up to 5 m in length) were successfully drawn from a glass rod with a diameter of 9 mm from this IZSBGC glass, neither the approach used for glass rod fabrication (or preform preparation) nor the resulting fiber loss was given in their paper.
Casting techniques (e.g., rotational casting and suction casting methods) are widely used for fluoride glass preform fabrication, and have been used for making fluoroindate glass fibers [11,13,14]. Another promising alternative technique is billet extrusion, which can easily form long preforms with desired diameters and internal structures [17]. The extrusion technique has been demonstrated to be a versatile method for the production of optical fiber preforms for a wide range of soft glasses, (e.g., fluorozirconate glass) and the subsequent fabrication of low-loss microstructured optical fibers [17–21]. Itoh et al. successfully made their InF3/GaF3 core and ZBLAN cladding preform by stacked extrusion [12]. However, no details of the fabrication of fluoroindate glass preform using the extrusion technique have been reported.
The aim of this work was to explore the use of the extrusion technique to produce low-loss IZSBGC glass fiber preforms. To the best of our knowledge, for the first time, we demonstrate extrusion of fluoroindate glass and drawing of fibers from extruded preforms. In order to ensure good quality of the glass billets for the extrusions, we also studied the impact of the choice of melting conditions on the glass quality. More specifically, our aims were to reduce the hydroxyl (OH) group content in the glass and to explore the feasibility of fabricating glass billets with good optical quality.
2. Experimental procedure
All glass fabrication steps including batching, melting at 900 °C for 3 hours and casting were conducted in a controlled dry N2 atmosphere (99.99%) melting facility. The glasses were prepared using commercially available fluorides as starting materials which were well mixed into 30~70 g batches and then melted in a platinum alloy crucible containing 5% gold (internal volume: 100 ml). It is well known that oxide and OH impurities in the raw fluoride starting materials can cause an increase in transmission loss, and that a fluorination process during fabrication can reduce the concentration of these impurities in the resulting glass [22,23]. Therefore, small glass blocks (30 g) (Table 1 ) were initially fabricated to optimize the melting conditions and investigate the effect of different fluorination techniques on the OH content in the glass blocks. Two approaches were considered; ammonium bifluoride (NH4HF2) was incorporated into the melting process as a fluorination reagent [24], and SF6 was used as a secondary atmosphere to study the dehydration of the fluoride glasses [25]. The glass batches with excess NH4HF2 (6.7 wt% of the batch weight) were first heat-treated at 235 °C, and then melted at 900 °C, and the glass melts were cast into a pre-heated mold after melting. The SF6 procedure included the application of an SF6 reactive atmosphere at 700 °C for 15 minutes, followed by the melting process at 900 °C. SF6 was introduced at 700 °C, rather than 900 °C, because we have observed that SF6 can attack Pt metal to form black particles at the higher temperature. Table 1 summarizes the processes used to fabricate the glasses under investigation.
Table 1. Glass blocks melting conditions
Electron probe micro-analysis (EPMA) (CAMECA SX51) was carried out to determine the glass composition of sample C after melting. The analysis was undertaken using an accelerating voltage of 15 kV and a beam current of ~20 nA with wavelength-dispersive X-ray spectrometers (WDS).
To study the effect of the fluorination processes, glass blocks were cut into slides with a thickness of ~15 mm which were then polished for IR transmission measurements using a commercial spectrometer (PerkinElmer Spectrum 400). The absorption coefficients were calculated from the transmission data, where background absorption due to the Fresnel reflection at the interfaces was subtracted. The characteristic temperatures; glass transition temperature (Tg), the temperature of crystallization (Tx) and peak of glass crystallization (Tp) of samples A to C (Table 1), prepared under different melting conditions, were also measured using a Setaram Differential Scanning Calorimetry (DSC) 131 equipment with experimental errors of ± 2 °C.
For fiber fabrication, larger glass billets (up to 70 g) were prepared using fluorination with NH4HF2. One glass melt was cast into a rod mold with an inner diameter of 10 mm and a length of 120 mm. After annealing, this cast rod was directly used as a preform for fiber fabrication. The other two glass melts were cast into a billet mold with an inner diameter of 30 mm and a length of 30 mm, which were used for preform extrusion (Fig. 1 ).
Fig. 1 Fluoroindate glass in the form of: (Left) Cast billets. (Top) Cast rod. (Centre) Preforms cast from cast billets.
Graphite has been successfully used as a die material for fluorozirconate glass extrusion due to the relatively low friction between the die and glass surfaces [21]. Using a graphite die, fluorozirconate glass could be extruded at a relatively higher viscosity (~108 Pa·s), i.e. lower temperatures, which allows the fluorozirconate preforms to be extruded below the onset of glass crystallization temperature (Tx) [21]. Therefore, in this work, we chose a graphite die to extrude the glasses at temperatures within ~70 °C of Tx. Prior to extrusion the cylindrical billets were polished using 1 μm Al2O3 powders. The billets were then extruded into rod-shape (cylindrical) preforms of 8 mm in diameter at temperatures within 20 °C above the glass transition temperature (Tg = 310 °C) using graphite dies.
The quality of the surface of a glass preform has a significant impact on the loss and strength of a fiber drawn from the preform. For fluorozirconate glasses, chemical etching of the preforms can prevent surface crystallization during fiber drawing due to the removal of pre-existing surface defects, thereby reducing the loss and increasing the mechanical strength of fluorozirconate fibers [26,27]. HCl has been widely used as an etchant for soft glasses as it is a corrosive acid which can remove a surface layer from matrice [28,29]. In this work, preliminary chemical etching was performed by suspending Preform 2 (Fig. 1) in a 15 wt% HCl(aq) solution (stirring applied) at a reasonable etching rate and room temperature for about 25 min to remove a ~0.5 mm thick outer layer, followed by a short rinse with methanol in an ultrasonic bath. The glass surfaces before and after etching were probed by an Atomic Force Microscope (AFM) (NT-MDT Ntegra Solaris AFM) to determine their roughness, which allowed the quality of the etch to be quantified. During etching, some white precipitate was formed on the glass rod surface. Identification of this white precipitate will enable optimization of the etching components in future work (i.e., help to suppress the formation of the white precipitate). In this study, we employed a scanning electron microscope (SEM; Philips XL30 field emission SEM) equipped with an energy dispersive X-ray spectrometer (EDS) to study the microstructures and compositions of the white precipitate on the glass surface after etching.
The cast rod was polished using 1 μm Al2O3 powders, and Preform 1 was cleaned by isopropyl alcohol in an ultrasonic bath for 30 min prior to fiber drawing while Preform 2 was etched as described above. Both cast rods and extruded rods were then pulled into unstructured fibers with diameters of 130~180 μm.
To investigate the impact of extrusion and other fabrication conditions on fiber loss, we selected 1550 nm as the wavelength for loss measurements as both light source and detector are readily available for this wavelength compared with the mid-infrared >2 µm. The fiber losses were measured using the cutback method and 1550 nm laser as light source. The output powers of the fibers under tests were recorded by a power meter.
3. Results and discussions
Sample C (glass block) was selected for EMPA analysis to determine its final chemical composition (Table 2 ). Although we used 3 hours for glass melting at 900 °C, there was no significant difference between measured and calculated concentrations for each element, suggesting that evaporation loss during glass melting is negligible.
Table 2. Electron probe microanalysis of sample C (glass block)
Figure 2 shows the absorption coefficient spectra of the three glasses prepared under different conditions (Table 1). It is clear that fluorination of the raw materials using NH4HF2 significantly reduced the absorption of OH groups (at ~2.9 μm) from 0.00106 ( ± 0.0012) to 0.0023 ( ± 0.0002) cm−1 (Fig. 2a). The infrared absorption edges of our glass samples are shifted to longer wavelengths, compared to ZBLAN fluorozirconate glass (Fig. 2b). For example, at the wavelength where the absorption coefficient is 2.3 cm−1 (corresponds to ~10% transmission for 1cm sample thickness), IZSBGC glass (sample C) has a wavelength of 8.7 μm whereas ZBLAN has a wavelength of 7.4 μm. As the OH content increases, relatively strong hydrogen bonds between OH and fluorine ions can form, which shifts the OH resonance (~2.9 μm) to longer wavelengths [30]. In our IZBSGC glasses (Fig. 2b), additional absorption >7.5 μm is observed for samples A and B, which were made without NH4HF2. In particular, a shoulder at 7.8 μm and a shift of the IR edge to the shorter wavelengths is found. Similar IR absorption features were observed in ZrF4-BaF2-LaF3 glasses (7.1 μm), and BaF2-ZrF4-YbF3-ThF4 glasses (9.1 μm) due to oxide impurities [31,32]. Therefore, the shoulder at 7.8 μm in the absorption spectrum in samples A and B is attributed to oxide impurities in the glasses caused by incomplete fluorination without NH4HF2. The absorption at 7.8 μm was suppressed using NH4HF2 for sample C, which indicates effective reduction of oxide and OH impurities in sample C. Drexhage [33] found that an incomplete conversion of the oxide starting materials by the fluorinating agent (NH4HF2) during glass melting could alter the slope of the IR multiphonon edge, which corresponds to a shift of the IR absorption edge. Hence, the significant shift of the IR absorption edge to shorter wavelengths for samples A and B compared to sample C (Fig. 2b) is also attributed to incomplete conversion of oxides to fluorides due to the lack of using NH4HF2 for samples A and B.
Fig. 2 Absorption coefficient for the glass samples prepared under the three different fabrication conditions (a) 2.75-6.00 μm; (b) 5.00-8.75μm.
DSC measurements showed that melting with NH4HF2 for sample C increased ΔT (Tx-Tg) and flattened the crystallization peak, suggesting a lower crystallization rate [16] compared with samples A and B (Table 1 and Fig. 3 ). The crystallization peak of sample C shows a small shoulder at ~400 °C. Based on the crystallization theory by [34,35], the shoulder indicates a slight change in the crystallization behavior at this temperature.
It was found that oxide inclusions stimulated crystallization in zirconium fluoride-based glasses [36]. Therefore, we attribute the enhanced crystallization stability (reflected by increased ΔT and reduced crystallization rate) for sample C to a lower amount of oxide impurities in this sample as a result of the fluorination with NH4HF2. Sample C has a slightly lower glass transition temperature than the other two samples A and B, which we also attribute to the lower oxide impurity content of sample C. Similar results were observed when oxide impurities were present in ZrF4-based ternary glasses [37,38].
The cast rod contained relatively large bubbles (up to 1 mm in diameter) that formed during the rod casting procedure, while bubbles were not observed in preforms extruded from the glass billets (Fig. 1). This is consistent with our general observation that casting into long thin moulds leads to bubbles in the cast glass. This can probably be explained by the more turbulent flow of molten glass when it is poured into long thin moulds, which generates bubbles in the cast glass.
Our results show that the extrusion method is suitable for the preparation of bubble-free preforms, and that extruded rods typically exhibit a better surface finish than cast rods. The different surface finish between the extruded and cast rods can be understood by their different fabrication techniques: extruded preforms have a fire polished surface as the hot glass cools down in free space, whereas the hot cast glass rod during casting procedure is in contact with the mold when cooling down (Fig. 1).
The extrusions were conducted at 317 °C, 322 °C or 330 °C, requiring at least 8 hours. The higher viscosity at the relatively low temperature of 317 °C required higher extrusion force (~35 kN) compared with the other two higher temperatures, which caused the graphite die to crack. The extrusion force was greatly reduced by increasing the extrusion temperature, more specifically the force decreased to ~10 kN at 322 °C and 7-8 kN at 330 °C, respectively. The reduced extrusion force successfully prevented the graphite die cracking and decreased the friction between the glass surface and graphite die. Although no obvious crystallization was observed for the rods extruded at temperatures in the range of 322 °C to 330 °C under the optical microscope, we used thin polished glass slides made from the fluoroindate glass in this study to explore whether surface crystallization could occur under such conditions. The polished glass slides had lower surface roughness and thickness than our extruded preforms. Therefore, it is easier to find crystallization on the glass slide under microscope. Two identical glass slides were treated at elevated temperatures comparable to those used for extrusion. There was no observable change before or after annealing at 322 °C, while significant crystallization in the form of wrinkles occurred on the edge of the glass sample after annealing at 330 °C (Fig. 4 ). This suggests that a temperature of 330 °C for preform extrusion can cause potential surface crystallization. By contrast, 322 °C extrusion temperature is sufficiently low to avoid potential surface crystallization while being sufficiently high to prevent graphite die cracking.
Fig. 4 Glass surface images: (a) before annealing at 330 °C less than 8 hours; (b) the edge of the glass after annealing at 330 °C less than 8 hours; (c) before annealing at 322 °C for 24 hours; (d) after annealing at 322 °C for 24 hours.
Note that the potential crystallization temperature of 330 °C is much lower than Tx measured by DSC due to the different thermal treatment during extrusion in comparison to the DSC measurement. In the extrusion process, the glass was kept at elevated temperature for about 8-10 hours, whereas during the DSC measurement the glass was heated at a rate of 10 °C/min. The long dwell time during extrusion can result in crystallization at temperatures below Tx.
The annealing test showed that crystallization or nucleation were most likely to occur at 330 °C, even though no crystals were identified on the extruded Preform 2 on inspection with an optical microscope. The potential crystallization for the Preform 2 extruded at 330 °C would result in increasing amounts of crystals at the elevated temperature during the fiber drawing procedure, which would ultimately increase loss and decrease mechanical strength of the fiber made from this rod. To remove the potential crystallization outer layer, chemical etching was applied by suspending Preform 2 (Fig. 1) in a 15 wt% HCl(aq) solution (stirring applied) at room temperature for about 25 min to remove a ~0.5 mm thick outer layer.
To evaluate the efficacy of preform etching, we determined the surface roughness of the preform before and after etching using AFM (Fig. 5 ). AFM images of 50 × 50 μm2 preform area were processed to determine the roughness parameters, Sa (Average Roughness), and Sq (Root mean square (RMS) Roughness) defined in Eqs. (1) and (2), respectively [39]:
where M and N are the numbers of the measured points and Z(xk, yl) is the distance from each measured point to the central plane on the surface of the tested preform sample. Sp is the maximum peak height. During the extrusion procedure, extrusion stripes, which can be seen on the preform surfaces (shown in inset optical micrograph of Fig. 5a), were most likely caused by the friction between the glass surface and graphite die. On the preform surface without white precipitate, the maximum roughness in the stripe regions dramatically decreased after etching, while the average roughness did not change obviously after etching (Fig. 5). SEM-EDS analysis (Fig. 6 ) showed that the white precipitate was composed of metal halides (F- and Cl-). EDS analysis also suggests that the chlorine ions from the etchant (i.e., HCl(aq)) can co-precipitate with ions leached out from the glass matrix. Thus we can conclude that etching using solely 15 wt% HCl(aq) solution is not an effective way to improve the surface quality. However, the reduction of the surface roughness in the stripe regions (when no precipitate was present) indicates that etching will improve the surface quality and remove defects associated with preform extrusion once the etchant is further optimized, overcoming the issue of precipitate formation.Fig. 5 AFM morphology of preform surface (a) before etching; (b) surface without white precipitate after etching by 15 wt% HCl(aq). Insets are optical microscope images of the preform surfaces before and after etching.
Fig. 6 (a) SEM secondary electron image and (b) EDS analysis of the white precipitate formed after 15 wt% HCl(aq) etch.
Fiber losses of the two unstructured fibers pulled from extruded preforms were lower than that of the fiber made from the cast rod (Table 3 ). This can be explained by reference to the improved surface finish and the lack of larger bubbles in the extruded preforms. The fiber drawn from Preform 1, extruded at 322 °C, exhibited the lowest fiber loss of ~1.9 dB/m measured at 1550 nm (Fig. 7 ). This (un-etched) preform was successfully pulled into a bare unstructured optical fiber with a diameter of approximately 140 μm without any evidence of crystallization (Fig. 8 ). Both Preforms 1 and 2 were made using the extrusion technique. The lower loss of the fiber drawn from Preform 1 compared with the fiber drawn from Preform 2 is attributed to the lower extrusion temperature used for Preform 1, thus avoiding potential crystallization. Although chemical etching was applied to Preform 2 extruded at 330 °C to remove the potential crystallization surface layer, the resultant fiber exhibited a relatively high loss, which is attributed to the occurrence of the white precipitate on the etched preform. The precipitate produced an inhomogeneous surface crystallization during fiber drawing, which caused scattering centers on the fiber surface and is reflected by the relatively poor linear regression (R2 = 0.96).
Table 3. Glass rod / preform preparation conditions and results of the fiber loss
Fig. 7 Loss measurements at 1550 nm for fibers made using different conditions. Linear fits of the data are also shown in the figure.
4. Conclusions and future work
In summary, the absorption of OH groups at 2.9 μm in 32InF3-20ZnF2-20SrF2-18BaF2-8GaF3-2CaF2 (IZSBGC) glasses was reduced by employing a fluorination method with NH4HF2. Fibers drawn from preforms (prepared using the extrusion technique) had a lower fiber loss (i.e., 1.9 dB/m), compared to the fiber drawn from the cast rod. This significant decrease in fiber loss is attributed to the better surface finish and reduced quantity of the bubbles in the extruded preforms. In contrast to 330 °C, 322 °C did not cause surface crystallization, which indicates that 322 °C is a suitable temperature for rod extrusion without crystallization. The absence of crystallization in the preform extruded at 322 °C resulted in the lowest loss of the corresponding fiber. The formation of the white precipitate during etching procedure shows that 15 wt% HCl(aq) solution is not an effective etching method to improve the IZSBGC glass preform surface quality and hence reduce fiber loss. The fiber loss value of our lowest-loss fiber is still higher than that of commercially available fluoroindate glass fiber. This is potentially due to the purity of the commercially available raw materials (99.9~99.99%) used for fiber fabrications. However, we provide a novel and feasible approach to fabricate fluoroindate glass rod preforms suitable for fiber drawing using the extrusion technique for the first time.
Future work will focus on the reduction of fiber loss, including further investigation of etching procedures to avoid surface precipitates and improve surface finish. Stacked extrusion of core and cladding billets [12] will also be investigated to prepare a glass-glass interface, which may further reduce loss by improving the quality of the guiding interface.
Acknowledgments
We acknowledge support from DSTO for funding this work. We also acknowledge the facilities, and the scientific and technical assistance (Mr Angus Netting and Mr Ken Neubauer) of the Australian Microscopy & Microanalysis Research Facility at Adelaide Microscopy Centre of The University of Adelaide. This work was performed in part at the Optofab node of the Australian National Fabrication Facility, a company established under the National Collaborative Research Infrastructure Strategy to provide nano and microfabrication facilities for Australia’s researchers. We thank Dr. Guang Yang (University of Rennes, France) for DSC analysis, and Mr Roger Moore (University of Adelaide) for fiber drawing. Jiafang Bei acknowledges the International Postgraduate Research Scholarship (IPRS) supported by The University of Adelaide. H. Ebendorff-Heidepriem acknowledges the support of a DSTO Fellowship and T.M. Monro acknowledges the support of an ARC Federation Fellowship.
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