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Abstract
High-purity Ge-As-Se-Te glasses have been well prepared via an effective double-distillation method. These glasses exhibit robust characteristics, withstanding input power levels as high as 12 W (68 kW/cm2). Utilizing extrusion-based fabrication, a large-core chalcogenide step-index fiber has been produced with a core diameter of 200 µm and a low optical loss of 0.78 dB/m at 7.25 µm. The fiber mode field area exceeded 26522 µm2. The fiber exhibits excellent transmittance in the whole mid and far infrared region of 2-12 µm, and its loss has been also certificated to be 1.85 dB/m at 10.6 µm by a CO2 laser. Further, the fiber is capable of high-intensity laser delivery of 16.13 kW/cm2, even under a high temperature of 150°C. At last, a high transmission efficiency of 44.9% has been recorded in this fiber, and the output power density is as high as 4.01 kW/cm2. All these results show that the fiber has the potential to be used in far-infrared laser machining and medical operation.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
The CO2 laser is capable of emitting high-power radiation in the far-infrared region, specifically between wavelengths of 9 and 11 µm. This capability makes it widely applicable in fields such as laser cutting, welding, and medical surgery [1–4]. Traditional CO2 laser systems, however, tend to be bulky and lack the flexibility needed for more dynamic applications, necessitating the development of flexible fibers suitable for energy transfer. Three primary categories of fibers for CO2 laser transmission have been investigated: hollow-core, polycrystalline, and tellurium-based chalcogenide fiber. Hollow-core fiber offer a high laser damage threshold and low Fresnel reflection at their incident and outgoing surfaces [5]. But the hollow-core fibers are commonly used in the field of optical fiber sensors due to their sensitivity to bending [6], and its bending loss is usually higher than that of glass fibers. The average mechanical bending strength of sulfur-based optical fibers is in the range of 200 to 450 MPa [7]. Polycrystalline fiber, particularly those based on halide crystals, are proficient in transmitting mid-infrared laser across a broad spectrum ranging from 3-18 µm. For instance, Mimura et al. [8] prepared a single crystal CsBr fiber with a diameter of 1 mm and an optical loss between 0.3-0.4 dB/m. Impressively, when pumped with an input power of 55 W, the fiber achieved an output power of 47 W, corresponding to a transmission efficiency of approximately 70%. While silver halides fibers exhibit increased scattering losses due to periodic growth striations, sub-grain boundaries and repetitive bending, and their growth conditions are demanding [9]. In contrast, chalcogenide glasses offer a promising alternative [4]. These materials are particularly suited for fiber fabrication and efficient propagation of mid-infrared light. Their core-clad structure is easily prepared and exhibits fine beam quality. The high losses experienced by chalcogenide glass fibers beyond 10 µm are primarily attributed to intrinsic multi-phonon absorption [10]. Among the family of chalcogenides fibers, Se-based and Te-based glasses exhibit cutoffs at 12 µm and 18 µm, respectively. They also display long-wave cutoffs compatible with CO2 laser applications. But the pure net-work of Te is not a good glass former. The addition of selenium (Se) to the ternary Ge-As-Te system leads to a compromise between stable glass formation regions and long-wavelength transparency [11]. As a result, the As-Se-Te fibers are suitable for CO2 laser transmission. Existing research has demonstrated the robustness of chalcogenide fibers under high-power conditions. For example, Nishii et al. [12] prepared a chalcogenide fiber comprised of a Ge-As-Se-Te cladding and a Ge-Te-Se core. Under extreme cooling conditions, this fiber, measuring 1.5 meters in length, displayed a remarkable ability to handle a maximum power density of 2.2 kW/cm2 when pumped by a 10.6 µm continuous-wave CO2 laser. In 2004, Shiryaev et al. [7] fabricated a Te-As-Se fiber with noteworthy characteristics. The fiber, which had a core diameter of 900 µm and a length of 1 meter, boasted an impressively low loss of 0.07 dB/m at a wavelength of 7.3 µm. When transmitting a CO2 laser with a wavelength of 9.3 µm, the fiber was able to carry a power output of 1.68 W. Most recently, in 2022, Zou et al. [13] introduced a single-mode Se-based fiber with a length of 15 cm. The fiber exhibited significant efficiency, achieving a maximum output power of 0.42 W with an input power of 0.89 W. In summary, fibers fabricated from Se/Te-based chalcogenide glass exhibit adequate stability for both manufacturing and laser power transmission applications. However, their performance is still constrained by inherent impurities and structural defects that contribute to fiber loss.
In this paper, we selected a set of stable glass components (Ge20As20Se15Te45/Ge20As20Se17Te43) in the Ge-As-Se-Te (GAST) glass system, aiming to fabricate an optical fiber tailored for high-efficiency transmission of CO2 lasers. The sources of impurities and structural defects in optical fibers are explored and uncovered in detail. A high-purity, defect-free bulk glass was prepared by a novel double-distillation process. Comprehensive analyses are conducted to clarify the damage mechanisms affecting optical fibers, guiding us in the optimization of fiber dimensions and fabrication techniques. Consequently, we achieve an optical fiber characterized by low loss and high flexibility, rendering it well-suited for CO2 laser applications.
2. Fiber fabrication and characterizations
2.1. Fiber host glass preparation
The chalcogenide glass was prepared via a common melt-quenching method. Firstly, the quartz ampoules were immersed in Aqua regia for 15 hours to remove the metal impurities on its surface, then washed with distilled water 5 times. Subsequently, the ampoules were vacuum-dried at 220°C for 15 hours to evaporate free water. The purification procedure for the glass is shown in Fig. 1. The 5N (99.999%) elemental Ge, As, Se, and Te were used as raw materials to fabricate glass and fiber. Although these raw elements possessed declared high purity, there are still some impurities contaminated, as the chalcogenide glasses are high sensitivity to impurities [14]. So, it is necessary to further purify these raw materials. Germanium, selenium, and tellurium were weighed in a clean environment, arsenic was weighed in a vacuum glove box because of its oxidizability, both four raw materials and reductant magnesium were then loaded in the static vacuum distillation setup. The setup was evacuated down to 1 × 10−4 Pa accompanied by 250°C for 2.5 hours. This process can remove the impurities of free water and arsenic oxide on the surface effectively. The ampules were placed in a two-zone furnace for vacuum distilling. During the 15 hours reaction, the glass precursor was distilled from the high-temperature region (ampule A: 800°C) to a low-temperature region (ampule B: 450°C) due to the huge difference in their respective saturated vapor pressure, other impurities were kept at the ampule A due to their ultra-low saturated vapor pressure. After distillation, ampule B was sealed and put into a muffle furnace, the precursor was homogenized at 750°C for 11 hours so that stable atomic bonds and glass network were formed and kept oxygen and hydrogen impurities away from the connecting with glass host. The closed container B with un-melted glass components was used as the initial stage of dynamic distillation, and the glass in container B was distilled into container C. By the way, it is no need to anneal the glass sample in step 2 because of subsequent dynamic vacuum distillation.
In the dynamic vacuum distillation process, the fresh ampoule C connected with ampoule B, and a vacuum pump was joined on the other side. All the system was operated under a continued vacuum of 1 × 10−4 Pa, the ampoule B was preheated at 250°C for 2.5 hours to remove the moisture in the raw material, then heated from 30°C to 750°C for 5 hours to separate high boiling point impurities from glass matrix. Finally, the glass matrix was recollected in ampoule C and the impurities were left in ampoule B. The high-purity glass sample was obtained by melting as described above and annealing below Tg (190.7°C).
2.2. Infrared transmission and impurities
Glass samples were cut into 15 mm discs and polished on both sides. The infrared transmission spectra of the glass were recorded by a Fourier transform infrared spectrometer (FTIR, Nicolet 380) in the range of 4000-400 cm−1. Figure 2 shows the transmission of purified Ge20As20Se15Te45 and Ge20As20Se17Te43 glasses with a thickness of ∼15.0 mm and unpurified Ge20As20Se20Te40 with a thickness of ∼2.0 mm. In near-infrared wavelength range, the transmission edge is due to the electron transitions from the valence band to the conduction band [15], while at longer wavelengths, the transmission is limited by the multi-phonon absorption of the glasses [16]. Ge-Te bonds exhibit lower tensile strength than Ge-Se bonds, resulting in a broader transparency window in comparison to Ge-As-Se glasses. Compared with unpurified Ge20As20Se20Te40, the purified glass with only a slight Se-H peak at 4.53 µm. Even with a sharp increase in thickness, the transmission remains unchanged, and only a few impurity absorption peaks are observed. The double-purified GAST glass shows high purity, and its Se-H and As-H impurity concentration are only 1.61 ppm and 0.07 ppm, respectively.
Fig. 2. Transmission spectra of purified Ge20As20Se15Te45 and Ge20As20Se17Te43 glasses and unpurified Ge20As20Se20Te40 glass.
2.3. Fiber fabrication
The core glass Ge20As20Se15Te45 and the cladding glass Ge20As20Se17Te43 were prepared with outer diameters of 26 mm and 46 mm respectively, their heights are both 15 mm. The preform was prepared by a modified peeling extrusion [17]. Under the force (∼107 KN) pushing, the core glass and the stepped mold were slowly inserted into the cladding glass at a temperature of about 330°C. Finally, the core and cladding glasses with high viscosity were slowly co-extruded out at a rate of nearly 3 mm/min synchronously under the force of about 107 KN. The resulting preform possessed a diameter of 9 mm and an approximate length of 20 cm. An extruded Ge20As20Se15Te45 preform is provided in Fig. 3. Subsequently, the fiber was fabricated using a fiber drawing tower, here, argon was continuously filled in the heating chamber to reduce the risk of oxidation of preform, and then the chamber was gradually heated to 380°C for fiber drawing. The fibers with a core diameter of 400 µm are obtained at a constant traction rate of approximately 5 m/min, while those with a core diameter of 200 µm are obtained at around 20 m/min.
3. Result and discussion
3.1. Fiber parameters
The refractive index of the bulk glasses was measured with an IR ellipsometer (IR-VASE MARK II, J. A. Woollam Co.), and the index values were used to calculate the numerical aperture (NA) of the fiber. The refractive indices of the two glasses are plotted in Fig. 4(a). The core and cladding glasses have refractive indices of 3.16-3.08 and 3.11-3.04 in the spectral region of 2.5-14 µm, respectively. The observed index difference between these two glasses is about 0.05. The NA and normalized frequency V number at 2.5-14 µm are given in Fig. 4(b). Specifically, at a wavelength of 10.6 µm and a fiber core diameter of 200 µm, the NA and V values are approximately 0.54 and 32, respectively. According to the empirical formula of N≈V2/2, where N is the number of the supporting modes, indicates that the fiber supports more than 500 modes.
Fig. 4. (a) Measured refractive index curves of core and cladding glasses; (b) Calculated fiber NA and normalized frequency V.
In the aspect of the fiber characteristics simulation, the fundamental mode dispersion curve of the optical fiber in the range of 9-11 µm is simulated by commercial RSOFT [18] software. The zero-dispersion wavelength (ZDW) is located at 10.31 µm, as shown in Fig. 5(a). According to the calculated effective refractive index and dispersion distribution, the effective mode field area can be calculated by the following formula [19]:
The effective mode area and LP01 mode intensity distribution of the fiber are simulated by COMSOL [20] software, as shown in Fig. 5(b). The calculated effective mode field area is 26520 µm2 at 10.6 µm. Given the sufficiently large dimensions of the fiber core, which enables the support of multiple transmission modes, it follows that the transmission power threshold experiences a quantifiable increase. Moreover, the enlarged core and significant mode field area collectively serve to diminish the energy density propagating through the fiber. This feature is advantageous in mitigating the risk of nonlinear optical damage within the fiber.
Fig. 5. Simulation results of the Ge-As-Se-Te fiber: (a) Dispersion; (b) Effective mode area of the fundamental mode.
Fiber loss was measured by the standard cut-back method via an FTIR spectrometer (Nicolet 5700) equipped with a liquid nitrogen-cooled HgCdTe detector [21,22].The full fiber length for measurement was ∼2 m because of its brittleness and the typical cut-back length was ∼0.5 m. The fiber attenuation at 10.6 µm was also measured by a CO2 fiber laser, and the recorded loss fits well with that by the FTIR method. We prepared three types of fibers using three different methods and subsequently compared these fiber losses, as illustrated in Fig. 6. The static distillation core/cladding structure fiber, double-distillation single index fiber, and double-distillation core/cladding structure fiber are represented by a, b, and c, respectively. As evident from Fig. 6, the fiber corresponding to the black line exhibits a minor absorption peak in the 4-6 µm wavelength range, attributed to a significant concentration of residual deoxidizer particles in the glass. This results in an optical loss ranging from 3.67 to 5.28 dB/m within the 6-11 µm spectrum, achieving a minimum value of 3.67 dB/m at a wavelength of 9.26 µm. In contrast, the fiber represented by the blue line incurs a loss between 2.51 and 4.01 dB/m in the same spectral region, with the minimum loss registered at 1.12 dB/m at 8.56 µm and a value of 2.87 dB/m at 10.6 µm. The fiber represented by the red line shows a loss varying between 0.78 and 2.53 dB/m, with its minimum being 0.78 dB/m at 7.25 µm and a loss of 1.91 dB/m at 10.6 µm. Simultaneously, a loss of 1.85 dB/m was recorded when measured using a CO2 continuous-wave laser. Despite both the blue-dotted and red lines undergoing double-distillation, notable discrepancies exist in their respective optical losses. These can be attributed to fiber defects and airborne particulate contamination present on the surface of the single-index fiber.
3.2 Laser damage threshold of glass host
The transmission of the CO2 laser through the host glass and fiber was examined using an experimental setup illustrated in Fig. 7. The laser damage threshold and energy transfer efficiency of both the host glass and fiber were systematically assessed for a side-by-side comparison. A SYNRAD 48-2 CO2 laser, with a maximum achievable output power of 25 W, served as the laser source. The laser beam, possessing an initial diameter of 3.5 mm, was precisely focused using a ZnSe lens with a focal length of 50 mm, resulting in a beam diameter of 150 µm at the focus. During laser transmission, the input and output ends of the fiber were kept in the air at room temperature, without any other special cooling. The incident power (Pin) and the output power (Pout) were measured by a Coherent 203 power meter.
In terms of CO2 laser energy transferring, we initially evaluated the laser-induced damage threshold of the purified Ge20As20Se15Te45 glass rod. This rod had dimensions of 9 mm in diameter and 11 mm in length. Upon increasing the laser power incrementally up to 12 W, we observed that the glass fractured, corresponding to a power density of 68 kW/cm2. According to the established theories of laser absorption and thermal damage following irradiation by pulse lasers with durations exceeding 10−13 s, it is reasonable to think that the damage is caused by laser heat accumulation [23]. Figures 8(b) and (c) reveal distinct thermal damage features on the incident surface of the glass, including notable thermally-induced fractures and ablation pits. In contrast, the exit surface exhibited only minor visible thermal wounds. This thermal damage pattern is indicative of localized heat accumulation on the incident surface. When this heat accumulation surpasses the glass’s melting point, ablation pits form on the surface. Concurrently, due to the Gaussian distribution of CO2 laser energy, the intensity at the beam center significantly exceeds that at the periphery, thereby overwhelming the material’s thermal tolerance and leading to damage characterized by penetration from the beam center. This form of damage is characterized by a broad wound area, relatively shallow depth, and a hemispherical shape [24]. In contrast, damage resulting from penetrating thermal effects is marked by its strong penetration but smaller damage diameter.
Fig. 8. (a) Schematic diagram of glass damage mechanism; (b) Laser damage on the front surface; (c) Laser damage on the back surface.
3.3 Laser power transferring by the fiber
The CO2 laser transmission results of Ge-As-Se-Te fibers prepared by different methods are shown in Fig. 9. Among them, a, b, and c represent static distillation large core-cladding fiber, double-distillation single refractive index fiber, and double-distillation large core-cladding fiber, with core diameters of 200, 200, and 400 µm, respectively. Based on the Fresnel Reflection principle, the highest transmission efficiency of the fiber is calculated theoretically. It is assumed that the end face of the fiber is cut vertically by the fiber cleaver, and the incident light is perpendicular to the end face of the fiber. The refractive index of the core glass at 10.6 µm is ∼3.10. According to the formula [25] of R = [(n−1)/(n + 1)]2, the Fresnel reflection of single end face of the fiber is calculated to be ∼26%. Considering that the laser undergoes two end-face reflections and experiences multiple internal reflections, and neglecting losses from these multiple reflections, the theoretical transmission efficiency is estimated at 48%. This value is indicated by a solid black line in Fig. 9. The fiber damage threshold is recorded as the power density at which localized fiber damage occurs. Comparative performance analysis with previous work [4] is also conducted. The transmission efficiency and damage threshold of different fibers are shown in Table 1. The transmission efficiency of fiber c is 44.9%, the maximum laser power density is 16.13 kW/cm2, and the output power density is 4.01 kW/cm2. Fiber b attains the highest incident intensity at 29.49 kW/cm2, marking it as having the largest damage threshold among the three fibers. However, the transmission efficiency of the fiber begins to decrease when the incident power of the fiber reaches 3.6 W. Such a power saturation behavior just before the damage is consistent with the previous observation in As-S and Ge-As-S MMFs with 1 mm core diameter [26]. The significant decrease in transmission efficiency before the fiber failure implies that the heat accumulates around the area where the fiber section will fail and the heat ought to be converted from the reduced fraction of the transmitted power. These parameters are excellent compared to the reported Ge-As-Se-Te fibers. In contrast, the damage threshold and transmission efficiency of the unrefined fiber a register only at 10.92 kW/cm2 and 30.6%, respectively. The comparative analysis reveals that fibers a and b exhibit excellent transmission properties with minimal heterogeneous particle impurities. Our double-distillation process significantly elevates the matrix glass’s purity, and the large-core Ge-As-Se-Te fiber fabricated using an optimized extrusion method offers enhanced energy transmission efficiency.
Table 1. Comparison of main parameters of three different Ge-As-Se-Te fibers at 10.6 µm with those in Ref. [4].
From the perspective of fiber loss, the increase in loss directly leads to a decrease of transmission efficiency. Impurities in Ge-As-Se-Te glass are mainly divided into soluble and insoluble impurities. Soluble impurities are involved in the formation of a glass network. This changes the original molecular network structure to a certain extent, thus affecting its physical properties and reducing the damage threshold of the material [27,28]. In the chemical static distillation of glass raw materials, on the one hand, the high vapor pressure caused by high temperature could carry the MgO2 dust and mix them into the pure glass matrix, on the other hand, the glass matrix itself will react with the quartz ampoule. These factors collectively contribute to the formation of numerous heterogeneous impurity particles within the glass. Such particles are prone to ionization upon laser irradiation, leading to localized temperature spikes in the fiber. Therefore, minimizing impurities in Ge-As-Se-Te fiber can significantly enhance its laser damage threshold. Additionally, a major bottleneck in Ge-As-Se-Te fiber transmission efficiency is its high refractive index. To mitigate this issue, the fiber end faces can be coated with an antireflection film to improve both transmission efficiency and damage threshold.
The heat distribution of the fiber under laser irradiation was measured by an infrared thermal camera, as shown in Fig. 10. The data indicate that heat primarily accumulates at both fiber ends. This may be attributed to contamination on the optical fiber end-face. The influence of heat conduction on fiber laser transmission is mainly the thermal lens effect of the fiber [29,30]. The thermal lensing effect is similar to that of self-focusing, which is an energy aggregation phenomenon caused by the change in the local refractive index of the material when irradiated by a laser [29]. Firstly, the refractive index of Ge20As20Se15Te45 glass was measured. The change function equation was obtained by mathematical fitting, and the dn/dT was calculated according to the obtained function. Then, the dn/dT at each temperature, the thermal conductivity of the glass (Kc = 1.95 × 10−3 W/m/K), the wavelength λ of 10.6 µm, the normalized frequency V = 32, the radius of the fiber core ra = 200 µm, n0 = 3.097 and other data are brought into for consideration with the thermal volume density calculation formula [30]:
A relationship curve between the thermal volume density and temperature is obtained, as shown in Fig. 11. According to these data, in the fiber with a core diameter of 200 µm, the thermal volume density gradually increases with rising temperature. Specifically, at a fiber temperature is 45°C, Ph is 8.25 W/m. When the fiber temperature rises to 80°C, Ph rises to 11.65 W/m. The results indicate that the threshold for the occurrence of the thermal lens effect in the fiber is significantly higher than that of laser transmitted within a given unit length. The temperature within the fiber gradually increases with the accumulation of laser energy when the laser is incident on the core. The threshold of its thermal lens effect is also increasing, which is beneficial to avoid the thermal lens effect in the fiber. However, Ph will increase sharply with the increase of temperature. Therefore, another cooling device is needed to reduce the fiber temperature in a reasonable range, which can reduce heat accumulation and avoid the thermal lens effect. In addition, increasing the fiber length can also effectively avoid the influence of the thermal lens effect.
Fig. 11. Relationship between temperature and refractive index of the fiber with thermal lens effect.
4. Conclusion
In summary, high-purity Ge-As-Se-Te glasses were synthesized utilizing an innovative double-distillation method, which demonstrated marked advantages in reducing impurities and improving material integrity. Subsequently, a Ge-As-Se-Te optical fiber was fabricated via an extrusion process, achieving a minimum loss of 0.78 dB/m at a wavelength of 7.25 µm and a low-loss parameter of 1.85 dB/m at 10.6 µm. When subjected to continuous-wave CO2 laser irradiation, the fiber exhibited a remarkable energy transmission efficiency of 44.9%. These metrics underscore the fiber’s considerable potential for applications requiring high-flexibility CO2 laser output. As such, the developed fiber offers a promising avenue for advancements in far-infrared laser processing and medical applications. Future work could explore the integration of this fiber type into complex laser systems and investigate its long-term stability and reliability.
Funding
National Natural Science Foundation of China, (U22A2085, 62205163); Natural Science Foundation of Zhejiang Province, ( LY20F050010, LQ21F050005); Open Fund of State Key Laboratory of Infrared Physics, (SITP-NLIST-YB-2022-11; Ten-Thousands Talents Program of Zhejiang Province; Leading and top-notch personnel training project of Ningbo; Outstanding talent training program of Jiaxing; K. C. Wong Magna Fund in Ningbo University.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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