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The efficiency and the stability of As2S3 microstructured optical fibres (MOFs) are limited by the shift of their optical properties that occurs over time due to a naturally induced aging process. Such sensitivity becomes more crucial for long optical path. Among the variety of fibre designs, the MOFs are developed for promising photonics applications such as supercontinuum generation for example. In the present work, we carried out an extensive aging study on As2S3 chalcogenide MOFs in ambient atmosphere. The evolution of the fibre transmission spectrum has been studied with regards to exposure time. The analysis of the transmission line profile was performed in terms of different spectral components Gaussian in shape and the infrared absorption bands have been attributed to the corresponding chemical groups' vibration modes or overtones. The time-dependent evolution of fibre attenuation has been studied as well as its longitudinal evolution for a given exposure time. Previous knowledge of extinction coefficient inherent to vibration components allows to predict their corresponding concentration. The content of hydroxyl groups tightly bonded to the glass network of the sulphide MOF core decreases exponentially with distance away from the MOF extremity. The report results show that a deleterious aging effect occurs over the first hours and days of exposure. This have crucial implications for storage and employment techniques and requires holes airproofing technique.
© 2014 Optical Society of America
1. Introduction
Chalcogenide glasses of different compositions are progressively developed for their potentials in a widespread variety of applications in the field of optic, spectroscopy [1], telecommunication [2] or biology. The reputation of these materials is attributed to their combined properties of broad transparency window, reasonably low optical losses, nonlinear properties, reasonable chemical and physical durability [3, 4]. The performance of chalcogenide based devices is strongly related to these combined fundamental characteristics. However, the scientific reliability and economic profitability impose a long-term service of the considered device, even if exposure of chalcogenide based devices to the real operation environment will eventually result in changes in the original physical and chemical properties. These changes correspond to an aging process, through chemical and physical processes.
Glass chemical aging is defined as a process of chemical interactions between glassy materials and their environment. It refers to chemical and structural modifications over time trough different mechanisms. Thermo-oxidative [5, 6], photo-induced [7, 8] and hydrolytic [9, 10] are the most known chemical degradation mechanisms. These processes become increasingly severe by extending exposure time, as by raising temperature levels, simultaneously as well as separately. Among others, this evolution is characterized by changes in glass transmission spectrum; new absorption bands appear while others are significantly amplified. Such process could eventually be of major benefits for different applications such as chemical sensing.
Glass physical aging is a process of structural relaxation [11]. The material evolves toward thermodynamic equilibrium. This is related to the preparation procedure of inorganic glasses [6, 12]. Glasses are defined as liquids which are frozen on time scale of experimental observation. They are considered thermodynamically metastable compared to crystals. Their enthalpy, free volume and entropy levels are higher than those of most stable forms. Thus, a glass matrix is continuously relaxing toward the equilibrium state of crystals over time. During real-time aging, this continual evolution of bulk properties is too slow to be measured and frequently unnoticeable. According to [6, 12] few and up to many years are needed to detect measurable changes in the mechanical properties for example. By heating glass above Tg at sufficiently high temperature and quenching of the glass melt, initial material could be recycled. Therefore, physical aging is a thermo-reversible process.
Previous studies [6, 12–14] have shown a natural and induced physical as well as chemical aging of chalcogenide glasses. R.Ya. Golovchak and al [6]. showed that AsSe based-glasses exhibit physical aging after several months of storage. These phenomena might be accelerated by exposure to sub-band gap and above-band gap light. Herein, Se-based samples subjected to aging influences, lose their excess of configurational entropy, enthalpy or free volume (gained during melt quenching) to reach a more favourable thermodynamic state. M.F. Churbanov and al [13]. studied the stability of optical and mechanical properties of As-S-Se based chalcogenide glass fibres under different conditions. They underlined an essential preventive function of fibre coating, which allows maintaining the original properties of the fibres. Uncoated fibres have shown a tremendous decrease of mechanical and optical properties after few days of exposure to atmospheric conditions. This collapse becomes more dramatic under hostile conditions such as water [15] and acetone. The observed changes during storage are due to the nucleation and propagation of cracks on the fibre surface, impurity adsorption and other microstructural changes. Toupin and al [14]. have briefly reported an optical aging on As2S3 six holes-design microstructured optical fibre upon exposure to air. The optical aging was associated with a dynamic grow of OH and H2O attributed absorption bands. Recently, our group has demonstrated the deleterious time evolution of As2S3 suspended core MOFs upon exposure of the MOF's core to atmospheric moisture and its impact on mid-infrared supercontinuum generation [16]. The kinetic of these changes reported in the literature has been studied on time scale from few hours to several years. It demonstrates that the optical properties are highly sensitive and severely degenerated in a few days. It was reported that 24 h are sufficient to completely quench the transmission of an As2S3 fibre immersed in water [15], whereas after 64 days of immersion in water, an As2S3 bulk sample lost almost 20% of its initial transmission [9]. In the case of fibres, optical losses level remains the most critical parameter. The control of this key criterion is a complicated process. It is based on sequential operations which are not limited to the synthesis technique. It extends to glass machining and device’s storage. The recent significant improvements of synthesis strategies allow the fabrication of low loss chalcogenide fibres exhibiting standard or microstructured profile [3, 17–20]. Subsequent stages are necessary to preserve the amorphous material’s optical loss at its post-synthesis level. Therefore, an aging study is needed, especially in the case of microstructured fibres, to provide an overview of exposure effects. It allows identifying the nature, the sources and the impacts of deterioration factors.
Thus, in this work, the chemical aging of sulphide based chalcogenide microstructured optical fibres in ambient atmosphere is accurately assessed. The purpose of the paper is to correlate the detrimental effect of atmospheric moisture with the aging behaviour of sulphide MOFs. We have undertaken a detailed study of components contributing to optical aging using infrared spectroscopy and we report results on real time monitoring of this optical aging in As2S3 MOFs. The transmission spectra were systematically registered until sixteen days of exposure under ambient thermal and hygrometric conditions. An estimation of content of absorbing species was calculated based on their attenuation measurements and their extinction coefficient. Finally, a preventive solution allows to reduce aging kinetics and control resulting effects.
2. Samples preparation
The highly pure As2S3 ingots were prepared by the conventional melt quenching method under vacuum inside silica ampoule using elemental highly pure starting products. Sulphur and arsenic were loaded into silica glassware and subjected to purification process under vacuum prior to their enclosing in the synthesis ampoule. Synthesis ampoule was loaded into a rocking furnace and heated up to the synthesis temperature for several hours. The final glass rod was obtained after quenching the melt and annealing the glass around the glass transition temperature. The mechanical drilling technique previously developed on chalcogenide glass in our lab permits to prepare the preform which is then drawn into suspended core MOF with specific parameters adapted for ease handling and technical requirements. The detailed used procedures have been already described elsewhere [19, 21, 22]. The fibre has 310 µm outer diameter and 10 µm of core diameter. No external coating is applied. Figure 1shows a cross section picture of tested fibre captured by means of Scanning Electron Microscopy (SEM).
3. Experimental measurements
The transmission spectra were recorded for wavelengths from 1.0 µm to 3.9 µm on 2.80 m of suspended core MOF sample. Measurements were carried out over 16 days at temperature ranging from 20 °C to 26 °C and relative humidity (RH) varying between 50 and 60%. A NICOLET 6700 Fourier Transformed InfraRed (FTIR) spectrometer was used to obtain the transmission spectra. Its source is an external halogen lamp emitting in the 0.1-5 µm range, providing higher output power than the optical source initially integrated in the spectrophotometer and allowing performing successfully this study on MOF sample within the range of interest. The MOF was mounted onto 3 axis holder in order to optimize light coupling exclusively in the MOF core. An infrared camera is used at the output end of the fibre to ensure that light is effectively guided in the core of the MOF only. This is important to stultify the possibility of any significant contribution of the fibre clad in the aging process since the fibre is not coated. At the input end a reflective objective is used to avoid chromatic effect. The beam transmitted along the fibre core is focused into an In-Sb detector using an optimal aspheric AMTIR lens. Note that previous in situ experiments have proved the stability of the measurement system through the experimental period.
4. Experimental results and discussions
4.1. Optical aging and speciation of absorbing species in As2S3 MOF
Figure 2 shows the ratio of As2S3 MOF transmission spectra measured over time in conditions as described above. The transmission spectra (Tt) registered at different time are normalized to the initial one (T0) plotted shortly after fibre drawing. Therewith, dramatic degradation of the optical transmission is observed in the whole transparency window over time. Furthermore, for 16 days of exposure, a significant decrease at a rate of 90% (resp. 40%) is registered at 1.0 µm (resp. 2.4 µm) transmission wavelength, whereas for the same 16 days exposure period, 15% average decrease is noted in the region above 3.3 µm. This phenomenon has been correlated to local structural modifications that occur on the surface of the fibre core such as cracks and roughness [9, 23]. In addition to the surface defects, nucleation and growth of nanometre and micrometre-sized particles at the exposed surface may induce additional scattering losses. The impact of these effects is more severe in the short wavelengths range because of the dimensions of concerning defects [10, 24, 25]. The growth and the contribution of these defects were unequivocally demonstrated in our previous study [16].
Fig. 2 Transmission spectra registered on As2S3 MOF with amplification of absorption bands for an exposition time of (a) 0 to 46 h (b) 46 to 384 h.
The OH extrinsic absorption bands are inherent to chalcogenide glasses as for many other glasses. The combination of different vibration modes (stretching and bending) of weakly H-bonded OH groups and free OH in the glass, provide absorption bands located at well-defined wavelengths. Figure 2 shows the set of fundamental as well as combinations of OH and SH absorption peaks evolving in time. Moreover, new time-dependent absorption peaks arise overtime. For sake of clarity, the results are selected according to their time evolution (Fig. 2(a)-2(b)).
Figure 2(a) shows a highly deleterious impact of moisture on fibre over the first 24 h. The transmissions severely decreases at characteristic absorption bands centred on 2.77 µm and 2.84 µm. These characteristic bands can be assigned to the vibration modes of OH bonds in isolated adsorbed H2O molecules [26–34]. At these wavelengths, OH absorption reaches 40% (resp. 33%) in less than 6 h and continues to rise until 90% (resp. 85%) after 24 h before reaching the detection limits after 46 h of storage in ambient conditions. These facts reflect a severe water-sorption on the surface of the guiding medium. Consequently, the first aging process corresponds to the adsorption of water molecules on the surface of the core of the MOF, where water molecules are weakly bonded to glass network. The water comes from the ambient atmosphere present in the holes surrounding the core of the MOF. Note that at this stage, no dramatic decrease is registered on transmission window despite the almost 20% of transmission loss below 1.3 µm. Following this early extinction of light signal around 2.8 µm, a gradual broadening of the absorption band occurs upon exposure time to natural atmosphere (Fig. 2(b)). This enlargement progress to the short wavelength side more than the opposite side. It reaches a maximum width of 800 nm centred at 2.8 µm after 384 h. This phenomenon is explained by the participation of the combination of different vibration modes of weakly H-bonded OH and free OH bonds, which are embedded under the overall peak. It is also reported in the literature that SH and OH absorption bands may occur above 3.0 µm [26]. Thus, the shift of the absorption signature towards 3.2 µm is the results of the implication of SH vibration modes at 3.11 µm [33] and vibration modes at 3.18 µm that we assume due to molecular water [9].
The survey of transmission spectra of tested MOF over time verifies thus a dramatic deviation of molecular water absorption band. However, additional vibration components remain embedded under the whole band. In order to discern and to attribute the set of vibration components involved in the aging process, a careful fitting of experimentally plotted spectra was conducted. The whole spectra have been fitted, assuming different peaks specific to different vibration modes previously reported for As2S3 glasses [9, 26–37] and summarized in Table 1.
Each spectrum was subjected for background correction by subtracting a linear background between the long and short wavelength edges of concerned peaks. Figure 3 shows a typical ratio (Tt/T0) of transmission spectrum after 32 h and 103 h respectively. Figure 4 shows the evolution of spectra over time between 4 h and 32 h, where spectra have been subjected to the fitting process with Gaussian components. We arranged different scales related to the spectra for better contrast. In Fig. 4(a) corresponding to 4 h of MOF exposure to ambient atmosphere, the overall spectrum essentially consists in three peaks. The 2.77 µm and 2.84 µm absorptions are caused by the fundamental symmetric stretching vibration bands of OH bonds in molecular-adsorbed H2O [37]. The 2.91 µm absorption band is related to OH groups tightly linked to the glass network trough arsenic (Table 1) [33]. Figures 4(a)-4(b) show resolved spectra for respectively 4 h and 6 h of exposure, where the absorption bands are unequivocally identified. In order to keep a good agreement between the experimental spectra and the fitting results for longer exposure times (24h and 32h, Figs. 4(c)-4(d)), it was necessary to take into account the peak at 3.0 µm attributed to OH absorption bands in As2S3 glass [31]. The contribution of this absorption peak is clearly seen on Figs. 4(c)-4(d).
Fig. 3 Ratio of Near-Infrared spectra after 32 h (a) and 103 h (b) of exposure, showing sloping background and the linear correction adopted for background subtraction.
Fig. 4 Deconvoluted absorption spectra over time scale (a) 4 h, (b) 6 h, (c) 24 h and (d) 32 h respectively.
The results of fitting allow estimating the peak height of each Gaussian component and consequently estimating the contribution of each vibration mode in the overall spectrum. With increasing exposure time, new absorption peaks arise and over 24 h of exposure, peaks at 2.54 µm, 2.63 µm, 2.70 µm and 3.0 µm are present (Fig. 4(c)). The 2.63 µm [35, 36], 2.70 µm [35, 36] and 3.0 µm [31] bands are caused by OH vibrations modes (Table 1). The 2.54 µm absorption band is assigned to the combination of vibrations modes of hydrogenous tightly linked with the sulphur atoms of the glass network [32]. The spectrum continues to evolve upon exposure. Eight hours later (Fig. 4(d)), new peaks centred at 3.11 µm and 3.18 µm have appeared. Similar behaviour is registered for 1.92 µm absorption peak simultaneously, which appears after 24 h of exposure and then continues to grow. The 3.11 µm absorption band is assigned to the stretching vibration mode of S-H bond [33], the 3.18 µm peak is related to molecular water absorption [9], whereas the 1.92 µm corresponds to the combination of the fundamental OH symmetric stretching vibration and the vibration due to hydroxyl strongly bonded to arsenic of the glass network (Table1) [33]. It appears that an active dissociation mechanism occurs at the glass surface. It leads to produce = As-O-H and = As-S-H bonds with their absorption signature at characteristic wavelengths.
From all these considerations, the proposed mechanism of chemical aging is the following: in the first hours of As2S3 MOFs exposure to ambient atmosphere, growing water adsorption occurs at the surface of the MOF core, because of the atmospheric steam present in the holes surrounding the fibre core. This adsorption leads progressively to a chemical reaction between adsorbed water and the sulphide glass in which = As-S-As = bridges are broken to form As-OH and H-S-As bonds.
4.2. Time evolution of OH attenuation
Figure 5 illustrates the temporal evolution of additional attenuation due to aging, added to the initial optical loss spectrum of the as-drawn As2S3 suspended core fiber, during storage in ambient atmosphere ranging from 1 h to 384 h, for 2.8 meters of fiber. The attenuation at any time (t) is estimated using the relationship:
where It and I0 are the transmitted intensity at (t) and (t0) respectively.Fig. 5 Additional attenuation for 2.8m of As2S3 MOF as a function of exposure time to ambient conditions.
In the long wavelength range, the attenuation is dominated by the absorption of extrinsic impurities. After 6 h of exposure, 2.5 dB are added to the initial attenuation at 2.77 µm. This value increases to reach 11dB, 23 h later. Finally, the first 24 h are sufficient to severally attenuate the signal.
These dynamics continue to develop and reach the detection limits of our detection device after 46 h, thus plotted spectra become noisy and follow evolution of peaks at 2.77 and 2.84 µm becomes impossible. However, the overall peak steadily extends over time due to the emergence of new absorption components as described above. Previous fitting procedure can be exclusively performed for resolved peaks. This explains the possible treatment of absorption peaks around 2.8 µm until 29 h, before they become noisy after 46 h of exposure to aging factors. Since 1.92 µm and 1.44 µm absorption peaks remains resolved until the end of the study, they allowed a continuous survey of their evolution. The fitting of experimentally plotted spectrum allows then to estimate the contribution of each vibration under the cumulative absorption peak. Resulting attenuations of each component are listed in Table 2.
Table 2. Attenuations for each deconvoluted peak after background corrections, for 2.8 meters of As2S3 MOF.
Figure 6 shows regularity in the evolution of the fundamental OH peak attenuation related with molecular water at 2.77 µm as a function of exposure time to atmospheric conditions. It can be used as an analytical tool for estimation of evolution over time of weakly bonded molecular water accumulated on the surface of the core fiber. Similar evolution is reported for the peaks of OH tightly bonded to the glass network at 2.91 µm and 1.92 µm. Then, during exposure of MOF to ambient atmosphere, the total content of molecular water adsorbed on the 2.8 meters of MOF core increases continuously and lead at 2.77 µm to an attenuation contribution above 11 dB after 29h. The amount of OH groups tightly linked to the glass network increases in a similar way leading to an attenuation contribution around 3 dB at 2.91 µm after 29 h and around 9 dB at 1.92 µm after 384 h. This is in agreement with the previous proposed mechanism of aging, starting with the adsorption of water on fiber's core, then followed by a consecutive chemical reaction and incorporation of OH groups in the glass material which forms the core of the fiber.
Fig. 6 Attenuation evolution with time of different OH groups' vibration modes for 2.8 m of an As2S3 MOF: (a) OH vibrations at 2.77 µm and 2.91 µm; (b) OH vibrations at 1.92 µm.
4.3. Distribution of absorbing species along the fibre
The previous analyses indicate that aged As2S3 MOF contains hydroxyl groups from water adsorbed on the MOF core as well as tightly bonded hydroxyl groups coming through chemical reaction between adsorbed water and the sulphide glass. The water comes from atmospheric steam which diffuses from the MOF's extremities in the holes along the core. We therefore investigate the spatial distribution of absorbing species along the fibre. To that purpose, the following experiment was carried out on the MOF exposed to ambient atmosphere during 384 h. Taking advantage of the cut back technique, a series of absorption spectra were registered along the tested fibre at different point from the output edge (Fig. 7).Admitting that the diffusion dynamic is similar from both ends, our study has been limited to one half of the concerned fibre. From these spectra we obtain the attenuation αSn of different sections (Sn = Ln-1 - Ln) along the MOF from the output to the middle, using the relationship:
L0 = 2.8 m is the initial length of the fibre, Ln-1 and Ln are the considered length where n varies between 1 and 4, Tn-1 and Tn correspond to the transmission registered for Ln-1 and Ln respectively. The attenuation spectra of fibre sections S1 and S4 are presented Fig. 8.One can note that even if the different removed fibers sections are only of several centimeters long, the evolution of the recorded intensities is significant because of the large attenuation effect of OH groups, which makes the cut back technique relevant in this case.Fig. 7 Schematic representation of fibre sections considered for studying the longitudinal distribution of absorbing species.
Fig. 8 Attenuation spectra of sections S1 and S4 of the 384 h aged As2S3 MOF, located at different distances from the fibre's output: (a) Attenuation of section S1, the first 7 cm of the MOF; (b) Attenuation of section S4 located at ~100 cm from the MOF end.
The spectra illustrate a non-homogenous repartition of the hydroxyl groups along the aged MOF. By moving deeper in the fibre from an edge, the absorption in the 2.6-3.2 µm region (due to molecular water, Table 1) as well as absorption around 1.92 µm (due to OH groups tightly linked to the glass network, Table 1) decrease. After 384 h of exposure to room atmosphere, near the MOF edge in the first 7 centimetres (section S1), the attenuation due to OH groups reaches around 50 dB/m at 1.92 µm, whereas it exceeds the 60 dB/m threshold in the 2.6-3.2 µm range. It decreases to ~3 dB/m (resp. ~6 dB/m) at 1.92 µm (resp. in the 2.6-3.2 µm region), near the middle of the MOF, in section S4 at ~1 m from its edge. In both cases, absorptions in 2.6-3.2 µm region are strong, and the transmitted light signal is weakly detected by the detection device, explaining thus the noisy aspect of the attenuation spectra in this wavelength range. The distance-dependent nature of the hydroxyl-groups attenuation at 1.92 µm is more clearly presented on Fig. 9, where the corresponding variation of OH groups' concentration with the position in the MOF is also indicated. The quantitative estimation of the hydroxyl absorbing species is given by Eq. (3):
Where β (dB.m−1.ppm−1) is the characteristic extinction coefficient of the corresponding absorption band. A previous knowledge of hydroxyl groups' extinction coefficient of corresponding vibration modes remains mandatory. Numerous research activities were devoted to elucidate the OH absorption and determine its absolute content in silica glasses [38, 39]. Such detailed studies on sulphide glasses were not reported. Thus, in order to estimate the OH content in As2S3 glasses, we use the values of β (dB.m−1.ppm−1) calculated from the molar absorptivity ε (l.mol−1cm−1) of OH groups in silica glasses. According to Stolper and al [37], the molar absorptivity of OH groups around 1.9 µm calculated in silica glass from absorbance measurements is ε = 1.8 l.mol−1cm−1. We assume the same value for ε in As2S3 glass. The extinction coefficient β (cm−1.ppm−1), corresponding to an OH concentration of 1 ppm weight in As2S3 glass is then given by:Where ρAs2S3 = 3.2.103 g/l is the density of As2S3 glass and MOH = 17 g.mol−1 is the OH molar mass. Here β = 339.10−6 cm−1.ppm−1. For optical fibers, the extinction coefficient β is preferably expressed in dB.m−1.ppm−1 and directly from attenuation and absorbance definitions:Here β = 339.10−3 (dB.m−1.ppm−1). This value allows to calculate the OH groups' concentration in the different section Sn of the fibers according Eq. (3) and reported on Fig. 9.Fig. 9 (a) Measured attenuation (black squares) of 1.92 µm absorption band and calculated absolute content (blue stars) of corresponding OH groups in one half of the fibre as a function of distance from the fibre edge; (b) Similar evolution for the second half of the fibre. Results reported for an As2S3 MOF exposed for 384 h to room atmosphere.
Finally, for 384 h of exposure, the most deleterious effect of atmospheric moisture occurs, from an end, over the first centimetres of the As2S3 MOFs which contain ~147 ppm of OH tightly bonded to the sulphide glass. The OH concentration then decreases exponentially towards the middle of the fibre to reach a minimum of ~9 ppm at the fibre mid length. Figure 9(a) illustrates the experimentally measured fiber losses and calculated OH content cumulated over exposure time for one half of the fiber. It is worth to note that throughout the experiment, both ends of the tested fiber were exposed to the same atmospheric conditions (pressure, relative humidity, temperature). In addition, the fiber exhibits constant holes dimensions along its length. Then, we can wisely assume that the diffusion mechanism is the same from both sides and Fig. 9(b) reproduces the similar evolution of losses and OH content for the second half of the fibre. Finally, the distribution of all OH impurities along the fiber exhibits two maxima located at the input and the output ends of the fiber and a minimum in the middle.
One of the main goals of As2S3 MOFs studied here lies in the possibility to achieve an efficient nonlinear interaction as the light signal propagates in the fibre, and especially to enable an important spectral broadening known as supercontinuum generation. These nonlinear optical processes take place within the first centimetres, even the first millimetres of the As2S3 MOFs [21, 40]. To achieve such efficient interactions, low optical losses are then required in this part of the fibre. However, the rapid and important contamination of the edge of these waveguides by OH absorbing groups, that we demonstrate in this work, sharply limit their efficiency and explains for IR SC extension limitation we previously observed [16, 40]. Thus, precautions and protection of the core MOFs, immediately after drawing process are necessary.
4.4. Prevention of aging
Based on the foregoing, a control of the diffusion of atmospheric steam in the holes of the MOF is required in order to limit the aging process. For that purpose we have airproofed the fibre ends by means of a methacrylate-based polymer. The fibre ends were soaked in the liquid polymeric solution, and then let to polymerize in free atmosphere at room temperature. Thereafter, rigorous polishing of the fibre facets allows to get rid of the thin polymer layer formed on the cross section of the fibre and to reach the core of the fibre. Herein, the diffusion of the polymer through the holes is ensured by capillarity. In order to enhance this process, polymer viscosity was adjusted using water-free solvent. Thanks to this technique, the holes of the MOF were sealed while the core of the fibre remains free for MOF's infrared transmission measurements as a function of time to investigate the evolution of absorption peaks related with OH pollution. Figure 10 presents several absorption spectra registered on 50 cm length of an As2S3 MOF exposed until 216 h to room atmosphere.
Fig. 10 IR transmission spectra of airproofed As2S3 MOF as a function of time exposure to ambient atmosphere.
These spectra maintain a remarkable stability and no evolution of OH related absorption bands is noticed. Note that exposure conditions are similar to those reported for the previous aging study. These results indicate that it is possible to protect the MOF core from OH aging. What's more, they confirm that the aging of sulphide MOF is due to atmospheric moisture diffusion in the holes of the fibre.
5. Conclusion
We report a detailed IR spectroscopic study of unsuitable process of atmospheric moisture adsorption and reaction onto the core surface of As2S3 chalcogenide microstructured optical fibres, leading to incorporation of absorbing OH groups weakly as well as tightly bonded to the MOFs core, which finally results in important additional losses. A detailed assignment of absorption bands and corresponding vibration modes is presented. The continuous evolution of the concentrations of hydroxyl groups in the MOFs as a function of exposure period is proved at least until 384 h of exposure. The observed changes upon exposure period show that the deleterious effect of moisture, which corresponds to diffusion of atmospheric steam in the holes of the MOF, occurs rapidly over the first few hours and is the most important in the early centimetres of the MOF. This early part of the fibre is of major importance for implementation of non-linear optical effects such as supercontinuum generation. In order to protect the MOF core from this aging process a first successful attempt to airproof the MOF is presented herein.
Acknowledgments
We acknowledge the financial support from the Conseil Régional de Bourgogne through the Photcom PARI program, as well as from the French DGA and the ANR Program Holigrale. This project has been performed in cooperation with the Labex ACTION program (contract ANR-11-LABX-0001-01).
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