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Abstract
Low-loss optical fibers of Ge15As25Se40Te20 glass purified by combined chemical and physical methods were successfully drawn. A novel fabrication method of fiber preform was adopted by wrapping a 1 mm thick polyethersulfone (PES) plastic tube around the glass rod. The optical, thermal, and bending strength properties of the glass and fiber were comprehensively investigated. The minimum transmission loss of the fiber was measured as 0.6 dB/m at 6.05 µm, and the average bending strength was approximately 180.67 MPa with a Weibull slope of 49.18 for a 400 µm fiber diameter. Optical power transmission characteristics of a 10.6 µm CO2 laser beam were also studied. The maximum transmitting power through a 1 m long and 400 µm diameter fiber was 1.37 W, which corresponded to a power density of 1.09 kW/cm2 at the fiber output end. Therefore, this fiber is suitable for infrared applications, e.g. mid-infrared laser transmission.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Chalcogenide glass (ChG) fibers have considerable application potential in many infrared (IR) fields, such as mid-IR (MIR) supercontinuum (SC) source [1–3], fiber laser [4–6], IR sensing [7–9], and IR power delivery [10–12], because of their ultra-broad mid- and far-IR transmission window, large nonlinear optical susceptibility, and low phonon energy. Wide IR transmission windows covering numerous molecular fingerprint regions and high mechanical strength are particularly required for ChG fibers in terms of MIR sensing and far-IR laser-conducting applications. ChG fibers are divided into S-, Se-, and Te-based fibers based on their glass compositions, and their corresponding IR cut-off wavelengths are around 8, 12, and 16 µm, respectively [13]. Therefore, Te-based fibers are the first choice for the applications in the entire spectral fingerprint region. Nevertheless, Te-based fibers experienced the following disadvantages: easy crystallization, thermal instability, and low operating temperature [14]. Among numerous Te-based glass fibers, thus, quaternary Ge-As-Se-Te (GAST) fiber was selected due to its high glass transition temperature (Tg), preferable mechanical behavior, good thermal stability (strong anti-crystallization ability), compromise of the ternary Ge-As-Se (large glass-forming area), and Ge-As-Te (wide IR transmission band) systems [15].
Savage et al. first proposed that GAST glasses have the potential to become 3–5 µm and 8–12 µm IR optical materials and studied their glass-forming region and the thermooptical properties of these glasses [16]. Inagawa et al. further investigated the glass-forming area of the GAST glasses, which were roughly purified, and then successfully drew fibers for the first time [17]. Xu et al. and Nguyen et al. successively studied the effects of the introduction of halogen on the properties of GAST glasses [18,19]. Afterward, Tikhomirov et al. investigated the glass formation in the Te-enriched part of the GAST system [15]. Shiryaev et al. comprehensively studied the characteristic temperatures and crystallization behavior in GAST system [20]. Although lots of studies have been conducted on the GAST glasses, reports on GAST fibers are few. Nguyen et al. studied the effect of temperature on the loss of GAST fiber [21]. Velmuzhov et al. demonstrated a fiber sensor based on GAST glass for analysis of aqueous solutions [22]. Qi et al. reported using GAST fibers to constitute fiber bundles for long-wave IR imaging [23]. However, none of these studies pertain on the performance of the fiber itself. Quinn et al. measured the bending strength of the GAST fiber and conducted fractographic analysis for investigating the causes of fracture [24]. The average bending strength of their investigation was 427 MPa but used a 240 µm fiber diameter, which could considerably affect the fiber strength. Their Weibull slope was about 11, explaining the scattered strength distribution of their fibers and demonstrating their low quality.
ChG fibers would inevitably be bent into a U shape when they applied in many MIR sensing experiments [25–27]. Consequently, a good bending strength of the fiber is essential. Considering the compromise between mechanical property and light coupling, the fiber diameters used in the sensing experiments were mostly 400 µm. However, the bending strength of 400 µm GAST fiber has not been reported so far. Therefore, optimization of the glass preparation and fiber drawing process was conducted in this work to obtain 400 µm GAST fibers of considerable bending strength. Second, reports on the transmission characteristics of CO2 laser power in the GAST fibers were also few. For example, Nishii et al. obtained an output power of 2 W using a 1.5 m long and 440 µm diameter fiber comprising a GeSeTe core and a GAST cladding, indicating the laser beam was propagated through the GeSeTe core [12]. Then, Busse et al. reported using core-cladding GAST fibers with the diameter of 270 µm and length of 1 m to deliver CO2 laser power, achieving a maximum output of 0.6 W [11]. Until now, reports on CO2 laser transmission in the single-index GAST fiber are unavailable. Thus, optical power transmission characteristics of 10.6 µm CO2 laser beam through the single-index GAST fiber were also investigated in this study.
A high-purity ChG glass with the composition of Ge15As25Se40Te20 was fabricated in this work, and then was successfully drawn into fiber form. The thermal and optical properties of the glass and fiber were comprehensively studied. The bending strength of the fiber was studied for their future applications in MIR sensing. Furthermore, the transmission performance of 10.6 µm IR laser in the fiber was also investigated.
2. Experimental
2.1 Glass and fiber preparation
In this study, the glass composition of Ge15As25Se40Te20 was chosen as fiber host. The reasons for selecting this composition are demonstrated as follows. First, GAST glasses, which comprise more than 10 mol% Ge and high content of As (>20 mol%), have better glass-forming ability [17]. Second, higher content of Se than Te and the increase in Ge content could play an important role in the enhancement of mechanical properties of the GAST fibers [24]. In addition, with the increased content of Te, which has a strong metallic property, making it impossible to form a stable covalent bond alone to constitute a stable glassy state, the multi-phonon absorption edge of the GAST fiber shifts to long wavelength and increases scattering loss synchronously [17,28]. Ultimately, subsequent experiments need PES-coated fibers; therefore, the Tg must be around 200 °C, requiring Ge content of below 20 mol% [16].
A special technology comprising two steps was taken to purify the fiber matrix glass. First, the 6N purified commercial raw materials of As, Se, and Te were prior heat-treated at 350 °C, 220 °C, and 320 °C, respectively, under vacuum (10−3 Pa) for 2 h to eliminate surface oxide impurities. Second, the method of full distillation dynamic purification was adopted, and the corresponding technological process is shown in Fig. 1. An appropriate mass of initial purified raw materials together with the getters were loaded into Section I of a tailor-made tube, which comprises two portions connected with a fine quartz tube. Moderate magnesium strips (10‰ wt) and GaCl3 (5‰ wt) with purity of 5N were used as chemical getters to remove the oxygen impurities, water molecule, and hydrogen impurities (removed in the form of volatile HCl) during melting. All these ampoules were preliminarily washed out in HF acid for 30 min, subsequently cleaned with deionized water, and then heated at 180 °C within 12 h. The tailor-made tube was evacuated down to 10−4 Pa and then sealed at position A. Afterward, the entire tube was put into a horizontal furnace maintained at 680 °C for 10 h to fully form the mixture compound, facilitating the easy and thorough distillation from Section I into II. A small part of Section II was then taken out from the furnace when the tube temperature reached 900 °C, and the furnace was allowed to be slightly gradient for another 10 h. Impurities remained in Section I because of their low vapor pressures, with the tube subsequently resealed at position B. The remaining part of Section II containing the purified materials was melted at 850 °C for 12 h in the rocking furnace. Finally, the tube containing the melted part was quenched in water at 650 °C and annealed at 175 °C for 4 h before slowly cooling to room temperature.
The obtained glass rod was precisely polished to obtain a perfect shiny surface (see Fig. 2(a)) with the aim of decreasing fiber loss [29]. Then, commercially customized PES plastic tube with internal diameter of 10 mm, length of 80 mm, and thickness of 1 mm was wrapped around the glass rod (see Fig. 2(b)). Compared with the method of wrapping PES film on the periphery of the glass rod [30], adopting PES plastic tubes can effectively reduce the fluctuation of fiber diameter because the latter cannot guarantee the uniformity of the preform diameter along the entire length. Finally, the fiber preform was drawn into fibers with a diameter of 400 µm (±2 µm) and length of 50 m (see Fig. 2(c)) using a high-precision fiber drawing tower (SGC, Customized, UK) under argon gas protection. Additionally, 4 m long bare fibers for bending tests had been drawn before the drawing of the PES-coated fibers.
Fig. 2. (a) Precision-polished glass rod. (b) Preform with PES tube wrapping. (c) Fabricated fibers with a diameter of 400 µm (±2 µm) and length of 50 m.
2.2 Properties of measurement
The glass rod with the size of Ø10 mm × 85 mm was cut into a 3.5 mm thick glass disk for measuring optical properties. The IR absorption and transmission spectra of Ge15As25Se40Te20 glass were measured by Fourier transform infrared (FTIR) spectrometer (Nicolet 380). The refractive index (n) of the single-side-polished glass sample was determined by infrared spectrum ellipsometer (J.A. Woollam IR-Vase II). Thermal properties were tested by differential scanning calorimetry (DSC) (TA, Q2000) and thermal–mechanical analysis (TMA) (Netzsch, DIL402). The optical loss of fiber with the length of 2 m was measured by cut-back method using a FTIR spectrometer (Nicolet 5700). The Young’s modulus of the glass was measured with a 10 mm long cylinder by Resonant Ultrasound Spectrometer (RUSpec, Los Alamos National Laboratory, USA).
The bending strength of the fiber was determined by the method of two-point bending between parallel plates using a self-built platform exhibited in Fig. 3 [31]. All the measurements were conducted under the following conditions: temperature was maintained at 20 °C, ambient environment was air, the driving rate of the plate was 1 mm/s, and the number of fiber samples was 20. The fracture stress σ was determined by the equation
where E (12.218 GPa) is Young’s modulus of the glass, r is the radius of the fiber without coating, D is the distance between the plates during fiber failure, and d is the total diameter of the fiber. The measurement results of fiber bending strength were represented in the form of Weibull plots, showing the dependence of probability of failure of the optical fibers upon the applied stress. The accuracy in bending measurements was approximately 1%.Fig. 3. Schematic diagram of the device for measuring bending strength (1-Platform controller, 2-Fiber fixing fixture, 3-Moving platform, 4-Slideway, 5-Measured Ge15As25Se40Te20 fiber).
The Ge15As25Se40Te20 fibers coated by PES preventing the fiber from oxidization by air in the course of laser transmission with the diameter of 400 µm for rapid diffusion cooling were studied for delivering 10.6 µm CO2 laser power. Figure 4 shows the experimental setup which comprise a tunable 10.6 µm CO2 laser (Synrad TI100) with a maximum output power of 75 W, a ZnSe beam splitter, a focusing ZnSe lens (f = 40 mm), 1 m long fiber, and two power meters (Coherent PowerMax). In this experiment, the duration time of the laser irradiation was limited to 60 s due to the laser heating. Free air convection was adopted to cool the fiber. The ZnSe beam splitter was used to split the input beam into two beams of equal power for input power detection. The ZnSe lens can focus the 1 mm diameter laser beam to a diameter of 110 µm before injection in the fiber. The beam focused by the lens would then be transmitted in the fiber, and the output power would be detected by the power meter.
3. Results and discussion
3.1 Optical properties of the glass and fiber
The IR absorption spectrum of a 3.5 mm thick glass disk is shown in Fig. 5(a), wherein some absorption bands around 2.85, 3.53, 4.12, 4.57 and 5.02 µm are ascribed to the vibration of OH, Se-H, Se-H, Se-H and As-H [32], respectively, because hydrogen impurities are difficult to be completely removed. However, only one oxygen impurity absorption bands such as Ge-O (7.8 µm) can be observed [33], indicating that oxygen getters (Mg) play an essential role in the purification process. The illustration of Fig. 5 (a) is the IR transmission spectrum of the glass disk in the wavelength range of 2.5-8.5 µm with an average transmittance of about 50%. All the impurity absorption bands mentioned above can be found in the illustration, but some bands located at 2.85, 3.53, 4.12 and 7.8 µm are quite weak because the glass disk used in the FTIR measurement was too thin to display some impurity bands obviously. Figure 5(b) displays the refractive index (n) changes with wavelength for Ge15As25Se40Te20 glass. The decrease in n with the increase in wavelength showing the normal dispersion behavior of the glass may be correlated with the decrease in absorption coefficient. This glass has n of 2.94–2.79 in the 1.4–12 µm wavelength range. By contrast, the n in the wavelength range of 1.4–12 µm for As2Se3 glass is 2.81-2.77 [34]. The n of Ge15As25Se40Te20 glass is evidently larger than that of As2Se3 glass due to the presence of Te atoms, which can be explained by the fact that the increase of the average mass of the atoms making up the glass can contribute to the increase of n [17]. In addition, the composition of Ge15As25Se40Te20 can also be considered as (GeAs)40(SeTe)60 which is in the same stoichiometry between metallic elements and chalcogenide ones that As2Se3. Compared to As2Se3, a part of As is substituted by Ge which contribute to lower the n [35]. On the opposite, substituting Se by Te in Ge-As-Se glass is known to increase the n [36]. In conclusion, the situation for Ge15As25Se40Te20 glass is a compromise between an index lowering effect and an increasing one which finally result in a higher n than As2Se3.
Fig. 5. (a) IR absorption spectrum of bulk Ge15As25Se40Te20 glass (illustration: IR transmission spectrum of the glass in the wavelength range of 2.5-8.5 µm). (b) Refractive index (n) varies with wavelength for Ge15As25Se40Te20 glass.
Figure 6 covering the wavelength range of 2.5–12 µm exhibits the attenuation spectrum of the as-drawn fibers from Ge15As25Se40Te20 glass. Some absorption bands still exist at 2.85, 3.53, 4.12, 4.57, 5.02 and 7.8 µm due to the vibration of OH, Se-H, Se-H, Se-H, As-H and Ge-O, respectively. However, some strange behaviors of the impurities loss change between the bulk glass and fiber can be found, such as the peak loss position changing from 4.57 µm to 5.02 µm and the different change ratios between Se-H bands, the possible reasons for which are as follows. First, the fiber length applied in measuring fiber loss was long enough to highlight all those weak impurity absorption bands. Second, the entrance of impurities from PES coating into the glass preform during heating may contribute to increase of fiber loss at certain wavelengths. Third, compared to the glass disk used in the FTIR measurement, the fiber has much more surface area influence in the loss measurement. For example, the absorption of the PES itself may result in a large increase in fiber loss at a particular wavelength due to the interaction of fiber evanescent waves with PES. In addition, the absorption bands of the atmosphere (e.g. H2O and CO2) in the infrared region may overlap with the specific impurity absorption bands of glass preform because of the broadening effect on these atmospheric absorption bands [37]. At last, the impurities in the furnace of the fiber drawing tower may also adhere to the surface of the fiber during fiber drawing process, eventually resulting in increase of fiber loss at certain wavelengths. This fiber exhibits a minimum optical loss of 0.6 dB/m at 6.05 µm, and the increase of optical losses in the 10–12 µm region is due to intrinsic multi-phonon absorption ascribed to the As-Se and As-Se-As bonds [38].
3.2 Thermal properties of the glass
The DSC thermogram of Ge15As25Se40Te20 glass is shown in Fig. 7 with the illustration of corresponding thermal expansion curve. Only one Tg can be found in the DSC curve, indicating the absence of phase separation. As shown in the thermogram, the Tg is 188 °C and crystallization below 450 °C is absent. The difference between the Tg and the crystallization temperature, which corresponds to the stability criteria of glass against crystallization, is >262 °C. This finding indicates this glass a good candidate for fiber drawing. In comparison, the Tg obtained from the thermal expansion curve is 172 °C, approximately 16 °C lower than the value obtained from the DSC curve due to the additional compressive stress and slow heating rate in the thermal expansion test [39]. The softening point of the glass is 240 °C, and the thermal expansion coefficient is 36.7 × 10−6 °C−1.
Fig. 7. DSC thermogram of Ge15As25Se40Te20 glass (illustration: thermal expansion curve of the glass).
3.3 Bending strength analysis of ChG fiber
For brittle materials, due to the randomness of strength fracture, the criterion for the strength cannot be determined by the average intensity alone. Moreover, understanding the reliability and dispersion of the strength from a statistical viewpoint is necessary [40]. The fiber strength is dominated by the maximum size defect inside or outside the fiber, which meets the Weibull distribution function and is expressed as
where F is the cumulative probability of failure, σ is the bending strength of the fiber, m is the shape factor, and η is the scale parameter. Suppose σ0 = ηm, Eq. (2) can also be expressed as Subsequently, substituting the measured D value into Eq. (1) can obtain the bending strength. The cumulative probability of failure was calculated by ranking strength in ascending order, where F (σi) = (i − 0.3) / (N + 0.4), where N is the number of samples, and i is the ith data.The Weibull distribution of mechanical bending strength for uncoated as-drawn fibers is given in Fig. 8. An average bending strength (at probability of failure of 48.5%) of approximately 180.67 MPa for 400 µm fiber was achieved. The straight-line behavior of the data indicates the presence of only one mechanism of fiber failure. As shown in Fig. 8, the values of abscissa vary from 5.15 to 5.24 and the Weibull slope based on linear fit result is 49.18, both illustrating the relatively concentrated distribution of the fiber strength. This finding also indicates the homogeneity of the defect size and the flaw population. Furthermore, the minimum bending radius for 400 µm fiber is less than 15 mm. Overall, these findings are indications of high quality of the fiber, which can be demonstrated by the breaking of all fibers at the middle section during bending tests compared with that of ruleless fracture of those low-quality fibers.
3.4 CO2 laser transmission in ChG fiber
The dependence on the input and output CO2 laser power through a 1 m long and 400 µm diameter Ge15As25Se40Te20 fiber is shown in Fig. 9. The optical loss of this fiber at 10.6 µm is approximately 5 dB/m. The output power increases almost linearly with the input power. When a laser power of 6.16 W was launched into the fiber, a maximum transmitted power of 1.37 W was achieved without any damage in the fiber, and the corresponding laser power density at the fiber input and output ends was 4.9 and 1.09 kW/cm2. This finding suggests the possibility of delivering a power of approximately 8.56 W with a 1000 µm diameter fiber. However, the fiber input end was burnt when the input power reached 6.5 W (5.17 kW/cm2). This finding can be attributed to the preparation of imperfect fiber endfaces, which can lead to local dramatic heating and eventually burn out the fiber. Therefore, precision polishing of fiber facets is necessary for high-intensity power delivery. Before reaching the damage threshold of the fiber, the fiber output power is approximately 23% of the input laser power, which is 8% lower than that of theoretical transmission efficiency. The main reason for this difference is the Fresnel losses caused by high n of the Ge15As25Se40Te20 fiber. For this fiber, a normal reflection loss at each fiber end is calculated at approximately 22% from the n of 2.8 at 10.6 µm, leading to relatively large total Fresnel loss of around 36% due to multiple reflections. However, the radiation loss occurrence at the bending part of the fiber will be smaller than that of other IR fibers due to the high n. Moreover, the transmission efficiency can be improved by applying anti-reflection (AR) coatings on the fiber endfaces if the AR coatings of transmittance and high damage threshold at 10.6 µm are studied [41]. Also, the maximum output power to be expected from this fiber is about 1.72 W if a potential AR coating is applied.
Fig. 9. Dependence on the input and output CO2 laser power through the coated fiber (length = 1 m, diameter = 400 µm).
4. Conclusions
After taking combined chemical and physical methods of purification, although some hydrogen impurities that can be further removed using high-purity quartz tube still existed, low-loss Ge15As25Se40Te20 fibers were successfully drawn with the minimum optical loss of 0.6 dB/m at 6.05 µm. The average mechanical bending strength of the 400 µm fiber is approximately 180.67 MPa with a Weibull slope of 49.18, which are both indications of the excellent fiber quality and the MIR sensing potential. The as-drawn fibers were also used for delivering 10.6 µm CO2 laser power with fiber length and diameter of 1 m and 400 µm, respectively. A maximum output power of 1.37 W (1.09 kW/cm2) was obtained with the corresponding input power of 6.16 W (4.9 kW/cm2), and approximately 23% of the input power can be detected at the fiber output end. The transmission efficiency can be further improved by reducing the fiber loss, polishing the fiber endfaces, and adopting AR coatings of high damage threshold and transmittance at 10.6 µm on both fiber endfaces. Overall, if the difficulties of optical loss and AR coatings are solved, then this fiber has potential use in applications involving high-intensity MIR laser transmission.
Funding
National Natural Science Foundation of China (NSFC) (61377099, 61435009); Ningbo University (K. C. Wong Magna Fund).
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