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Chalcogenide glass fibers are very suitable to carry out mid-infrared spectroscopy by Fiber Evanescent Wave Spectroscopy (FEWS). Nowadays, selenide glasses are used for FEWS, but the reachable domain is limited in the infrared to typically 12 µm. Te-rich glasses, due to their heavy atomic weight, are better for far-infrared sensing but they crystallize easily and until now that was difficult to prepare operational optical fibers from such glasses. In this work, Te-Ge-AgI highly purified glasses have been prepared and successfully drawn into optical fiber. The minimum of attenuation is 3 dB/m around 10 μm, which is up to now the lowest value ever measured for Te-based fiber. Overall, such fibers open the sensing window up to 16 μm against 12 µm so far. Then, for the first time, tapered telluride fibers with different diameters at the sensing zone were obtained during the fiber drawing process. Chloroform and butter were used to test the fiber infrared sensing ability, and the sensitivity has been greatly enhanced as the sensing zone fiber diameter decreases. Finally, the new protocol of telluride glass preparation allows shaping them into efficient functional fibers, opening further in the mid-infrared which is essential for chemical spectroscopy.
© 2014 Optical Society of America
1. Introduction
Chalcogenide glasses, based on sulfur, selenium, tellurium and other additional elements, are good candidates for designing sensors working in the infrared range, especially as optical fibers [1–5]. The leading mechanism is to take advantage of the evanescent wave generated at fiber surface to capture the excitation of the vibrations of chemical bonds within the molecules in contact with the fiber. For numerous molecules and biomolecules, the fingerprint region, which covers a series of complex and specific absorption bands, is located between 2.5 and 25 μm. However, for S and Se-rich chalcogenide glass fibers, light atomic weight causes the multi-phonon cut-off wavelength to be around 6 μm [6, 7] and 12 μm [6, 8, 9], respectively. This is a strong and detrimental limitation. Te-rich chalcogenide glass, due to its atom heaviness, exhibits the ability to transmit light in the middle and far infrared ranges up to 28 μm [10–13] and therefore has drawn tremendous interests. In order to test the fundamental vibrations (stretching and bending) of molecules and biomolecules whose main absorption bands are beyond 12 μm, such as benzene [14] and chloroform [15], optical sensor of high selectivity and sensitivity should be developed based on telluride glass fiber. Meanwhile, development of telluride glass fiber is also relevant in the field of extraterrestrial exploration. Carbon dioxide, which is one of the markers of potential life on telluric exoplanets, can be easier analyzed by monitoring infrared absorption peak at 15 μm. Thus, telluride glass fibers have also to be fabricated for remote detection of CO2 in Darwin mission (European Space Agency) or Terrestrial Planet Finder (National Aeronautics and Space Administration) [9, 16, 17].
Nevertheless, due to the strong metallic character of tellurium, Te-based glasses are difficult to control and vitrify. Different strategies have already been considered to enhance glass stabilization by introducing some other, rather heavy, elements in the mixture, such as Te-Ge-Se [17–19], Te-Ge-I [11], and Te-Ge-Ga [10]. Nevertheless, the requirement of glass stabilization is very demanding in order to obtain a fiber without surface crystallization during fiber drawing process, and none of these systems provide glasses easy to fiber.
Very recently, GeTe4 glass containing 10% of silver iodine showed no crystallization peak by thermal analysis and appeared as an interesting candidate [20, 21]. However, it was quite difficult to obtain fibers from the glass preforms and their optical losses were larger than 20 dB/m, which is unacceptable for any application.
The aim of the present work is to show how this raw glass exhibiting poor physical properties is becoming the most promising one as soon as it is properly purified and carefully worked. Indeed, this new protocol of preparation can definitely see these glasses as stable enough to be functional. Especially, they can be drawn into optical tapered fibers for mid and far infrared applications. The fibers will be used to implement FEWS experiments to demonstrate their unique abilities and performances, upsetting the state of the art.
2. Experimental section
The Ge-Te-AgI glass with its composition to be (Ge21Te79)90AgI10 was synthesized by chemical- distillation method. During the chemical purification step, Te (6N), Ge (5N) were weighed in the adequate proportion and melted with a small amount of Al at 750 °C for 10 hours. As a kind of oxygen getter, Al acted as a reducing agent and thoroughly reacted with the oxide in homogenization period. AgI, as an unstable compound, is not proper for distillation, and therefore should not be added into Te and Ge during this chemical purification procedure. The amorphous material was obtained by quenching and annealing at 150 °C for 3 hours. For distillation, Te-Ge mixture with 100 ppm of Al was distilled [22, 23] through a filter to a reaction silica tube and was mixed with AgI (5N) placed in advance. During distillation, the filter and the reaction silica tube were kept at 1000 °C and room temperature respectively. The reaction tube was then sealed under vacuum and homogenized at 750 °C for 10 hours. After quenching and annealing, a typical glass preform, with its diameter and length to be around 7 and 150 mm respectively, was obtained. To justify the effectiveness of the two-steps purification method, Ge-Te-AgI glass was also prepared by simply melting the raw elements in a sealed vacuum silica tube.
The Differential Scanning Calorimetry (DSC) were performed using a DSC Q20 (TA Instruments) at a heating rate of 10 °C/min in order to determinate the temperature difference (∆T) between glass transition temperature (Tg) and crystallization temperature (Tx), which is the criteria of glass stability. In order to confirm the quality of our base glass, X-ray diffraction (XRD) and energy-dispersive X-ray spectroscopy (EDS) were also executed. To measure the bulk optical transmittance, the optical polished glass discs were prepared with thickness of approximately 1mm. The transmission spectra of the samples with and without purification were recorded in the wavelength range from 1.25 to 25 μm (8000-400 cm−1) using a Bruker Tensor 27 FT-IR spectrometer with DTGS detector. In order to reduce the influence of preform surface defects on the fiber optical and mechanical properties, Ge-Te-AgI glass preform was well polished using polishing papers (1200 and 4000 grits) and alumina powder successively. The polish depth can reach 400 μm depending on the removal degree of surface defects. In our case, the polish depth was controlled to be 200 μm. The fabrication of the fibers from polished preforms were carried out under a He controlled atmosphere thanks to a home-made fiber tower. During the drawing step, the diameters of the fiber were controlled to be 350 and 450 μm by using the fixed preform feed speed and coordinating drum speed. Tapered fibers with their diameters at sensing zone to be 160 and 200 μm respectively were also obtained by a sudden drum speed increase during the drawing process. Fiber attenuation coefficients were measured by Bruker Tensor 27 FT-IR spectrometer with MCT detector using the cutback technique [24] by comparing the optical power transmitted through a long piece of fiber to the optical power transmitted through a short piece of fiber. Meanwhile, in order to check the fiber sensing property beyond 12 μm and as well as the relationship between sensitivity and sensing zone diameter, chloroform (CHCl3), melted butter (80% fat, wt.) and light butter (15% fat, wt.) were put in contact with the fiber sensing zone with a contact length to be 50 mm.
3. Results and discussions
3.1 Glass purification
Glass formation in the Te-Ge-AgI ternary system [21] and the fiber drawing feasibility [20] have been reported. At that time, the quality of these first achievements is poor and far from meeting the requirements needed for any efficient use, especially as mid-infrared sensing device. Therefore, we have properly reinvestigated this glass system in that direction.
The DSC study shows that the (Ge21Te79)90AgI10 glass has no obvious crystallization peak before 300 °C, which is consistent with the previous study on bulk glass. XRD pattern displays no sharp peaks, confirming amorphous state of the samples.
To prepare high quality optical fiber with low optical losses, it should be known that, due to the long-distance light transmission in fiber, any tiny absorption caused by the presence of impurities in bulk glass can be significantly amplified in fiber. Therefore, preforms with high purity have to be prepared. This is the key operation, of course for the optical quality of the glass, but also to avoid any nucleation of microcrystals in the steps of shaping the preform. The bulk transmittance spectra of the Te-Ge-AgI glasses with and without purification were tested and compared in order to verify the efficiency of the chemical-distillation method (Fig. 1). After purifications, the Ge-O absorption peak located at 13.3 μm has been totally removed. To achieve this goal, aluminum has been introduced in the batch. Indeed, Al acts as an oxygen getter and reacts with the oxygen in oxides and water. The Al2O3 generated has an intrinsic low equilibrium vapor pressure at high temperature, and is mostly kept in the distillation chamber. The peaks at 2.70, 4.26 and 6.30 μm are caused by the concentration differences of –OH, CO2 and H2O vapor in air between sample and reference tests. After purification, the glass shows a very flat transmission region from 2.2 to 19.0 μm. Here, it should be emphasized that the low transmittance, less than 60%, is caused by the Fresnel losses caused by the high refractive index of the glass.
Fig. 1 The bulk optical transmittance of (Ge21Te79)90AgI10 glasses with (black line) and without (red dotted line) chemical-distillation purification. The thickness of sample is controlled to be 1.0mm. After purification, Ge-O absorption are totally removed, the glass show a very flat transmission region from 2.2 to 19.0 μm.
3.2 Fiber preparation
Based on this purification method, a Te-Ge-AgI glass preform with its diameter and length to be 7 and 150 mm approximately was prepared using the chemical-distillation purification method as described in the experimental section. Nevertheless, the glass surface always showed lots of defects after annealing. In the past, this phenomenon has already been observed for glasses, especially for tellurium glasses containing iodine. Some Energy Dispersive Spectrometry (EDS) measurements showed that the composition of the inclusion is GeI4. This surface defects could act as nucleating agents and accelerate surface crystallizations when the preform is heated during the fiber drawing process for example. Meanwhile, the defects themselves could also generate a large light scattering during the reflection at the glass-air interface. Therefore, the Te-Ge-AgI glass preform was mechanically polished step by step in order to get a perfect shiny surface. The images of the glass preform together with the final fibers are shown in Fig. 2.
Fig. 2 Macroscopic and optical microscopic photos of (Ge21Te79)90AgI10 glass preform before (a and b) and after (c and d) mechanical polishing; the optical fibers with and without a tapered zone obtained from polished preform (e) and the optical microscopic graph of one typical fiber cross-section (f). Mechanical polishing can effectively remove the preform surface. After drawing, fiber surface remains shiny and shows no visible crystallization. In the meantime, the fibers had a nice brilliant circular cross-section, which showed a typical morphology of glass. The fiber diameter is precisely controlled.
From both macroscopic and optical microscopic scale, it could be observed that the preform surface defects are totally removed by optical polishing. After drawing, fiber surface remains shiny and shows no visible crystallization. In the meantime, the fibers had a nice brilliant circular cross-section, which showed a typical morphology of glass.
The drawing of the preform is implemented using a specific drawing tower suitable for low melting temperature glasses [25, 26]. By changing the processing parameters, fibers with various diameters can be prepared in1 the range 500 to 100 µm typically. For FEWS experiments, 350 µm is a good diameter which enables to efficiently focus the beam light at the input of the fiber. Moreover, during fiber drawing process, by a sudden acceleration of the rotational motion on the fresh fiber which still maintained its appropriate viscosity, the fiber has been locally tapered down to a very small diameter. This geometry considerably increases the evanescent wave intensity along the reduced-diameter zone [27–30] which is then used as sensing probe of the final device. Thus, some tapered optical fibers with the sensing zone diameters ranging from 160 μm to 200 μm were successfully prepared. It is a challenging operation, as the viscosity versus temperature curve is very short for telluride glasses. To our knowledge these are the first tapered tellurium based glass fibers ever prepared, giving birth to efficient spectroscopic tools. Also, the optical losses of the fibers were measured using the cut-back method [24], which compares the output powers of different fiber lengths by cutting the tested fiber step by step.
The calculated optical losses of Te-Ge-AgI optical fiber are shown in Fig. 3. This Te-based glass fiber shows a large signal transmission region up to 16 μm. This is much broader than for selenide glass fibers which stop to transmit beyond 12 µm for the best of them. Moreover, the minimum attenuation value is now 3 dB/m around 10 μm. This result has been repeatedly verified in the following six months after preparation. Note that previous pure telluride glass fibers exhibit minimum optical losses larger than 20 dB/m [31]. Thus the fiber attenuation has been significantly reduced and is up to now the minimum value ever measured for a telluride glass. In order to view the strong progress that has been done, the attenuation curve of previous achievements is also shown for comparison in the Fig. 3. No doubt that this results from the combination of the purification steps with the careful preform polishing. Indeed, the significant improvement of optical loss value is due to the surface defect removal from the preform using optical polishing. Moreover, no parasitic absorption band is visible showing that the polishing seem not to induce embedding of additional impurities. Also, for such tellurium glasses, the residual optical absorptions in the transmitting range are due to the charge carriers’ concentration inherent to the semi-conducting behavior of the tellurium. As electronegative element, it is clear that iodine plays a benefit role to trap the electronic charges [11, 12], which was one motivation to introduce AgI in the glass composition. It is finally much more efficient to introduce iodine as a salt rather than in its elemental form, because it remains in the glass network and would not escape as previously observed. The small absorption peak at 5.1 μm is caused by Te-H bonds. The hydrogen could be introduced by the water molecules attached to the surface of the elements during the distillation when Al reacted with H2O and generated Al2O3. Another possible source is the hydrogen absorbed on silica set-up by chemical bond during the tube fabrication and cleaning process. Such impurities are also classically observed in selenide glasses (Se-H bonds) and are difficult to be totally removed by vacuum drying.
Fig. 3 The optical losses of (Ge21Te79)90AgI10 single index fibers obtained from the preforms with (black line) and without (red dotted line) optical polishing. By purification and careful preform polishing the fiber attenuation has been significantly reduced and is up to now the minimum value ever measured for a telluride glass. A large signal transmission region up to 16 μm provides a potential for the fiber to act as a sensing probe between 12 μm and 16 μm.
3.3 Mid-infrared sensing
At this time, mid-infrared FEWS experiments are carried out with the TAS glass, Te20As30Se50. Compared with the Te-Ge-AgI glass, TAS glass fiber shows a rather lower minimum attenuation value because, although it contains 20% of tellurium, it is basically a selenide glass. However, due to its light atom weights, TAS glass fibers are totally opaque beyond 12 μm, making it is impossible to test the vibration absorption band located at longer wavelength, such as the strongest absorption peaks of chloroform and benzene, for examples. Thus, Te-Ge-AgI optical fiber is especially proper for testing the signal located between 12 μm and 16 μm.
To check the sensing efficiency of Te-Ge-AgI glass fiber, particularly the sensing ability beyond 12 μm, chloroform was selected due to its strong C-Cl stretching and C-Cl bending absorption peaks located at 13.3 μm and 14.9 μm respectively [15]. Pure chloroform (2N) was put in contact with the sensing zone of a tapered Te-Ge-AgI glass fiber in a silica evaporating dish. Figure 4 shows the related FEWS absorbance spectra of chloroform. The sensitivities of the fibers with different sensing zone diameters were studied and also compared with TAS glass optical fiber. It can be observed from the spectra that Te-Ge-AgI glass fiber is able to monitor the chloroform absorption peaks at 3.3 μm, 8.2 μm, 13.3 μm and 14.9 μm at the same time, which is, by the way, another evidence of the fiber broader transmission range. For comparison, the equivalent chloroform FEWS spectra collected with a TAS fiber (with the same tapered sensing zone diameter) is also presented in Fig. 4. Besides the largest transmission windows giving access to absorption band beyond 12 µm, it is noticeable that for the C-H bending absorption at 8.2 μm, the Te-Ge-AgI fiber even shows a larger absorbance value than the TAS fiber. This indicates that the optical attenuation value in the transmitting domain does not have a direct consequence on the sensitivity of FEWS experiments which are short distance applications. This sensitivity also depends on the chemical nature of the optical glass fiber. Clearly the physical interaction between the fiber and the liquid sample is very good, as shown by the very high signal to noise ratio of the FEWS spectra. Besides, as the fiber diameter at sensing zone decreases, the sensitivity increases significantly due to the much larger number of internal reflections that results in many absorption events at the interface between the glass and the sample.
Fig. 4 The chloroform sensing property of (Ge21Te79)90AgI10 fibers with different sensing zone diameters. The inset is chloroform absorbance of Te20As30Se50 (TAS) fiber for comparison. (Ge21Te79)90AgI10 fibers can monitor the vibrations from 3.3 μm to 14.9 μm clearly. By decreasing the sensing zone diameter, the fiber sensitivity could be greatly enhanced. It is also noticeable that for the C-H bending absorption at 8.2 μm, the Te-Ge-AgI fiber even shows a larger absorbance value than the TAS fiber.
Both the schematic diagram of light propagation in a tapered fiber and the absorbance at 13.3 μm are shown in Fig. 5.
Fig. 5 The relationship between absorbance caused by asymmetric C-Cl stretch and fiber sensing zone diameter, inset is the schematic diagram of light propagation in a tapered fiber. Absorbance, which is proportional to the number of reflections at the interface (N), shows a significant linear relationship with 1/d.
Absorbance shows a significant linear relationship with 1/d, in agreement with previous theoretical models and observation, which definitively validates the benefit of such geometry for the sensing device.
Food safety and medical diagnosis has become hot issues in recent decades. Selenide glass optical fibers has already been successfully used to discriminate tissues types [32, 33], monitor the contamination of pathogens [34] in food, and others bio-medical applications [35]. In order to show the potential of Te-Ge-AgI optical fiber for rapid in situ monitoring on food quality, the FEWS spectra of butter (with around 80% wt. butterfat) and light butter (with around 15% wt. butterfat) were collected from 2 μm to 16 μm and are compared in Fig. 6. The band assignment of the IR vibrational absorption peaks of butter are listed in Table 1.
Fig. 6 The infrared absorption spectra of butter (80% wt. butterfat) and light butter (15% wt. butterfat) obtained from (Ge21Te79)90AgI10 fibers sensing. Due to the butterfat and water concentration difference, the absorbance of the two beyond 12 μm shows an obvious dissimilarity, showing a potential of (Ge21Te79)90AgI10 fiber to control the chemical content and the quality of food.
Table 1. The band assignment of the IR vibrational absorption spectrum of butter.
It can be observed that due to a large butterfat and water content difference between the regular butter and the light butter, the absorbance beyond 12 μm of the two shows an obvious dissimilarity. Normal butter, due to its high butterfat content, has a distinct absorption peak at 13.8 μm caused by CH2 rocking. On the contrary, light butter shows a wide absorption shoulder owning to the librations of liquid water due to the restrictions imposed by hydrogen bonding [36]. Therefore, by collecting additional infrared signal beyond 12 μm, we clearly get richer information on a complex biological material such as butter which could be benefit for controlling the chemical content and the quality of food. More generally, this permits to focus on the potential of the Te-Ge-AgI glass optical fiber for food identification and quality verification based on FEWS which could be extended to various applications in the health field. Besides, for future application in energy sector, the monitor of benzene in gasoline and the toluene absorption variation along with concentration in 2,2,4-trimethylpentane solution is also planned.
4. Conclusion
The aim of this paper is to change the status of pure-tellurium glass from curiosity for material scientist toward effective functional material for mid and far-infrared optical devices. To achieve this goal, an efficient method of glass purification, together with a careful operation of polishing, has permitted to obtain GeTe4-AgI high quality glass preforms. Some optical fibers have been prepared exhibiting the lowest optical losses ever get in telluride glasses. By tapering them, the fibers were designed for mid-IR FEWS experiments, which have been carried on chloroform detection and butter monitoring, taking as an example of a complex bio-system. In term of sensitivity, these new tellurium glass fibers supplant the currently in-service optical fibers made from selenium based glasses. Above all, they give access to a range of wavelengths that was previously inaccessible with chalcogenide glass fibers. This will be essential for mid-infrared spectroscopy in particular for applications involving complex bio-molecules for which the 12 to 16 µm spectroscopic range could be rich in information. Also, due to its large infrared sensing range, Te-Ge-AgI fiber is also applicable to other daily and military fields, such as quantitative analysis of benzene in gasoline and CO2 analysis in the Darwin project. In this last frame, via substituting a few percent of Te by Se, double index fiber will be prepared using casting method or alternative method under development.
Acknowledgments
Financial support from European Community's Seventh Framework Programme through Marie-Curie Action: “Initial Training Networks” (GlaCERCo GA 264526) is gratefully acknowledged.
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