Open Access
Add to BibSonomy
Abstract
Multicore fiber (MCF) has a larger mode-area (LMA) compared to traditional single-core fiber, making it easy to get a mode area of more than 3000 µm2 with an optimized MCF structure. Here, a fine-structured 19-core fiber based on chalcogenide glass was fabricated using a combined method involving extrusion, drilling, and rod-in-tube for the first time. The fiber has a minimum transmission loss of 1.8 dB/m at 6.7 µm. When the bending radius exceeds 6 cm, a low bending loss of about 0.6 dB appears, and the experimental data are in good agreement with the simulation results. In addition, the supermode characteristics of the 19-core fiber are analyzed from both perspectives of simulation and experiment, and these results are perfectly in good agreement. We believe it opens a new way to develop high-power and bend-resisting fiber with such kind of multicore structure.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Fiber-based lasers are capable of good beam quality, wavelength tunability, compact size, and low cost-effectiveness [1–3]. Therefore, they have a wide range of applications in medical [4], military countermeasures [5], industrial processing [6], and other fields. Nevertheless, non-linear effects in optical fibers can limit the laser power output ability. The use of fibers with LMA can effectively increase the threshold of non-linear effects [7]. However, increasing the mode area either through an increase in core size [8] or a reduction in the numerical aperture (NA) [9] of conventional step fibers can lead to multimode generation and poor bending performance, respectively. Consequently, there arises a need for innovative LMA fiber structures to circumvent these challenges and facilitate the augmentation of output power.
MCF has multiple cores, so the energy coupling [10] between the cores allows the MCF to obtain a LMA, which shows much help in increasing the laser output power and damage threshold. In 2021, a silica-based MCF based on the stacking method was fabricated with a mode area of 1432 µm2 at 976 nm and an output power of up to 25 W [11]. The wide transmission window and excellent optical properties of chalcogenide glasses [12] make them being one of the most important materials for mid-infrared optical devices and infrared optical fibers. Remarkable breakthroughs in simulations were attained by Barh et al. and Maji et al. in 2013 and 2015, respectively, wherein they achieved ultra LMA of 75000 µm2 and 100500 µm2 through investigations of chalcogenide-based MCF structures [13,14]. However, the aforementioned two fiber structures have 61 cores, thus leading to an intricate fabrication process. Therefore, these studies are limited to the preliminary simulation phase. The first 7-core chalcogenide fiber based on extrusion [15] was prepared by our group in 2019. Moreover, a 13-core chalcogenide fiber, materialized via a continuous two-stage extrusion method in 2021, achieved a mode area of 8000 µm2 [16]. However, the above two MCF structures based on the extrusion method are less stable and have severe core deformation.
In this paper, a mid-infrared 19-core fiber based on chalcogenide glass with a stable structure was successfully prepared by the combination of extrusion, drilling, and rod-in-tube method. The effects of core arrangement, core radius, and core spacing on the mode area of the fiber are discussed. After optimization, its mode area exceeds 3000 µm2. Moreover, the fabricated fiber has a minimum loss value of 1.8 dB/m at 6.7 µm. We have tested the energy distribution of the in-phase mode of the fiber at 1.55 µm, subjecting it to thorough analysis concerning its variation in response to different fiber diameters. In addition, we have also comprehensively evaluated the bending loss of fiber, substantiating the fiber's robust bending resistance by integrating theoretical simulation and experimental verification.
2. Fiber design and optimization
Figure 1(a) presents the cross-section of the designed 19-core fiber. The blue part of the figure presents the high refractive index cores, which are arranged in a hexagonal shape. Notably, the spatial gap between the first ring core and the central core is designated as parameter a, while the separation between the second ring core and the first ring core is denoted as b. We chose Ge9As23Se68 as the core material and Ge10As22Se68 as the cladding material because of their similar refractive index and glass transition temperature (Tg). We chose two bulks of glasses with similar refractive index to achieve a smaller NA. A smaller NA aids in reducing the fiber dispersion and minimizing the beam broadening effects. Moreover, the smaller the NA, the weaker the ability of the core to confine the beam, which is beneficial to the energy coupling between adjacent cores. Figure 1(b) presents the 2-12 µm refractive index measured by an infrared ellipsometer (IR-vase MARKII, J.A. Woollam Co.). The Sellmeier coefficient can be calculated by the Sellmeier equation based on the measured refractive indices [17]:
Fig. 1. (a) Fiber cross-section and refractive index distribution. (b) Refractive index distribution of core and cladding and the calculated NA.
Table 1. Refractive coefficient of glasses
The fiber was simulated numerically with COMSOL software based on the finite element method, and a customized perfect-matching layer (PML) was implemented within the simulation process [18]. The PML is a technique used to absorb the reflected waves at the boundary of the simulation area, which plays a role in reducing the calculation error and improving the accuracy during the simulation. Considering that the MCF has multiple cores in the same cladding area, and single core is single-mode transmission, each core of the 19-core fiber is also single-mode transmission respectively when the core spacing is large enough. When the spacing between the cores becomes smaller, transverse coupling occurs among these adjacent cores. Therefore, each supermode can be obtained by combining a single transmission mode with different phases [19]. The 19-core fiber comprises a total of 19 supermodes. Figure 2 presents the mode energy distribution of the seven typical supermodes in a 19-core fiber. Among them, the mode in Fig. 2(a) has zero phase difference between adjacent cores, the largest propagation constant compared to other modes, and the mode energy has a quasi-Gaussian distribution. Therefore, it possesses the best beam quality among the 19-core fiber supermodes [20,21].
The effective mode area is given by the following Eq. (2) [22]:
Fig. 3. Variation of Aeff of fiber with wavelength under (a) different core numbers and (b) different core arrangements.
Figure 4(a) presents the variation of Aeff with increasing core radius. It can be observed that the Aeff of the fiber gradually increases with the core radius increasing. By referring to the normalized frequency V depicted on the left axis, it becomes evident that the fiber achieves single-mode transmission when r = 7.5 µm. Therefore, we choose r = 7.5 µm as the core radius size of the fiber. Figure 4(b) presents the variation of the Aeff of the fiber with the core spacing. Notably, the Aeff exhibits an increasing trend followed by a subsequent decline with the increasing in core space. This is because within a certain range of core spacing, the increase of spacing will gradually increase the mode area and finally maximize the effect of strong-coupling. However, when the core spacing is larger than 22 µm, the optical fiber mode will gradually lose its coupling effect and tend to transmit independently, that is, the mode area will decrease. There is a maximum value at the space of 22 µm. Therefore, we choose Λ=22 µm as the core spacing size of the fiber.
Fig. 4. (a) Variation of V and Aeff of fiber with core radius. (b) Variation of Aeff of fiber with core space.
3. Fiber fabrication and performance testing
The first step is to prepare the host glasses, which include a bulk of Ge9As23Se68 core glass and two bulks of Ge10As22Se68 cladding glass. Firstly, the raw materials of Ge, As and Se are filled into quartz ampoules, and the oxide impurities are removed by adding deoxidizer Mg metal, followed by a process of distillation and purification under vacuum. Next, the distilled materials in the ampoules were melted in a shaking furnace, ultimately leading to their quenching in water. The three prepared glasses were cut into 20 mm-thick glass blocks, polished on both sides and then tested for their properties. The transmission spectrum of the glass was measured by Fourier transform infrared spectrometer (FTIR), and the measurement results are shown in Fig. 5(a). Notably, a small amount of impurity peaks exist in the glass and the transmittance is above 50%. The reason for the difference of impurity peaks is that these external impurities could be introduced during the glass preparation process, as the raw material is easily contaminated in the air. In the end, all these subtle differences of raw-materials purity, air humidity and preparation procedures will cause a change in impurity absorption intensity. The glass transition temperature (Tg) was measured by differential scanning calorimetry (DSC), with the corresponding results presented in Fig. 5(b). Evidently, the difference between the Tg values of the two glasses is negligible, which is favorable for the extrusion of the glass and the drawing of the fiber at high temperatures.
The second step is the preform fabrication. The Ge9As23Se68 and Ge10As22Se68 glasses were extruded by the thin cladding extrusion method, and preform rod with a diameter of 3 mm was obtained as shown in Fig. 6(a). The second piece of Ge10As22Se68 glass was drilled using a computer numerical controlled (CNC) precision machine tool, and the obtained cladding empty tube is shown in Fig. 6(b). Finally, the thin rod is inserted into the cladding empty tube, and the final preform is shown in Fig. 6(c).
Fig. 6. (a) Fabrication of preformed rod by extrusion method. (b) Fabrication of cladding empty tube by drilling method. (c) Fabrication of optical fiber preform by rod-in-tube method.
Finally, the fiber was fabricated from the preform in Fig. 6(c). The size and fitting degree of the fiber was controlled by adjusting the haul-off speed, feeding speed, and negative pressure value of the fiber-drawing tower (SGC, customized, UK). Figure 6(d) presents the cross-section of the fiber under the optical microscope (Keyence, VHX-1000). Since the refractive index of the core and the cladding glass are similar, the profile of each core cannot be clearly seen in the diagram. From the inset of Fig. 7(a), it can be seen that the cores of MCF are arranged in a perfect hexagon. After rescaling the fiber structure in a same proportion, it can be concluded that the deformation of the core spacing is about 2%, and the deformation of the core radius is about 12%.
Fig. 7. (a) Transmission loss of the fabricated fiber; green round: loss at 1.55 µm; blue round: loss at 2.94 µm; black round: loss at 8.96 µm; insert: energy distribution image. (b) Fiber transmittance at 1.55 µm.
The fiber loss was measured using a traditional cut-back method with an original fiber length of 1.2 m and a cutting length of 30 cm for each segment. This measurement utilized an FTIR black-body light source. The spectrum of Fig. 7(a) presents that the average loss of the fiber is about 4 dB/m, and the minimum loss at 6.7 µm is 1.8 dB/m. The fiber losses were also measured by lasers of 1.55 µm, 2.94 µm, and 8.96 µm, and are recorded as 3.8 dB/m, 2.9 dB/m, and 8 dB/m, respectively, which are marked with green, blue, and black circles in Fig. 7(a). There are some impurity absorption peaks in the loss spectrum, including Se-H absorption peaks at 3.53 µm, 4.12 µm, and 4.57 µm, the H2O absorption peak at 6.31 µm and Ge-O absorption peak at 7.9 µm. The remaining absorption peaks at the long wave are plastic absorption peaks caused by the protective polymer around the fiber. The fiber is prepared by the rod-in-tube method, so the scattering loss is high. In the future, the overall loss of the fiber will be reduced by further purification of the glass and reduction of the interface defects. The inset of Fig. 7(a) presents the fiber’s energy distribution measured by the FTIR spectrometer (Nicolet 5700). It can be observed that the fiber is arranged in a perfect hexagon.
In addition, we used a 1.55 µm laser to test the fiber’s transmission efficiency, with the corresponding result is shown in Fig. 7. The length of the fiber used is 55 cm. The fitting result presents that the transmission efficiency of the 19-core fiber at this wavelength is about 44%. Due to the actual fiber absorption loss, the facet Fresnel reflection loss, coupling loss and other factors in the test process, the overall power transmission efficiency of the fiber is about 44%. Although this data of transmission efficiency has certain advantages over other chalcogenide based all-solid microstructured fibers [23,24], it is also expected to improve for the aim of better laser transmission in the future.
After coupling a single-mode LD laser of 1.55 µm to the fiber, the near-field energy distribution of the fiber was measured by a near-infrared fiber field analyzer (Xenics, XEN-000298). Figure 8 presents the two-dimensional energy distribution of the fiber in the same phase mode. The first row presents the simulated two-dimensional energy distributions of fibers with diameters of 108 µm, 128 µm, 146 µm, and 180 µm, respectively. In contrast, the second row presents the recorded two-dimensional energy distributions of the fibers, and the inset presents the picture of the near-field energy distribution. As the output diameter becomes larger, the fiber’s mode area expands, thus resulting in weakened coupling strength between adjacent cores and a flattened mode field distribution. It also proves that the test results of the fiber energy distribution are consistent with the simulation results.
Fig. 8. Two-dimensional energy distribution of simulated (row 1) and tested (row 2) for the same phase mode with different output diameters: (a, e) 108 µm; (b, f)128 µm; (c, g)146 µm; (d, h)180 µm.
4. Bending characteristics analysis
For LMA fibers, bending loss is a key parameter [22]. Figure 9(a) compares the bending loss of a single-core fiber and a 19-core fiber at 1.55 µm. When the bending radius is less than 7.5 cm, the bending loss of single-core fiber increases exponentially with the decrease of the bending radius. This is because with the decrease of the bending radius, the energy of the mode in the fiber core is leaked and leads to fiber loss increasing. Furthermore, when the bending radius is less than 4.5 cm, the mode is completely leaked into the cladding. The bending loss of 19-core fiber fluctuates little with the decrease in bending radius. From the illustration, it can be observed that with the decrease of the bending radius, the mode of the optical fiber is deformed, but it is still bound in the core. Therefore, compared with single-core fiber, 19-core fiber has better bending performance. Figure 9(b) presents the simulation and test results of the bending loss of 19-core fiber at 1.55 µm. The simulation results present that the bending loss of the fiber increases exponentially with the decrease of the bending radius. We bent a 1.5-meter-long fiber into circles of different diameters and then tested the output power. The illustration presents the energy distribution of the fiber when the bending radius is 4 cm and 13 cm, respectively. The experimental results present that the bending loss of the fiber increases exponentially with the decrease of the bending radius. When the bending radius is greater than 6 cm, the loss fluctuates less with the change in bending radius, and the average loss is about 0.6 dB. The changing trend of the loss of the experimental results and the simulation results is the same, indicating that the 19-core fiber has good bending resistance. However, in the actual fabrication process of optical fiber, due to the material loss produced by glass fabrication and the scattering loss produced by optical fiber fabrication, the actual test results of bending loss are still greater than that of simulation.
Fig. 9. (a) Comparison of bending loss between step fiber and 19-core fiber. (b) The bending loss of the 19-core fiber was obtained by experiment and simulation at 1.55 µm.
When the fiber is bent in the positive direction along the x-axis, the equivalent refractive index distribution of the fiber cross-section is calculated as follows [25]:
where nR and n are the equivalent refractive index of the fiber after bending and the initial refractive index of the straight fiber, respectively, x is the coordinate along the positive direction of the x-axis, R is the bending radius, and Reff is the bending radius after introducing the elastic correction factor, and Reff = 1.28R.5. Conclusion
In summary, a 19-core chalcogenide-based optical fiber was prepared for the first time based on a combined technique of extrusion, drilling, and rod-in-tube after a well-designed fiber structure. The simulation shows that a huge mode area above 3000 µm2 can be achieved in the mid-infrared band. The experimental results demonstrate that the lowest loss of the fiber is 1.8 dB/m. The change of the in-phase mode of 1.55 µm exhibits a direct correlation with the fiber's diameter, and this trend closely aligns with the simulation results. Furthermore, a simulation-driven exploration of the bending priority between the 19-core fiber and its single-core counterpart was conducted, subsequently validated through empirical assessment of bending loss at radii smaller than 6 cm, yielding a mere 0.6 dB of loss. With further refinement in glass purification and fiber optimization, the potential for employing this fiber in high-power laser transmission and applications necessitating small-radius bending is promising.
Funding
National Natural Science Foundation of China (U22A2085, 62205163, 61935006); K. C. Wong Magna Fund in Ningbo University; Outstanding talent training program of Jiaxing; Leading and top-notch personnel training project of Ningbo; Zhejiang Provincial Ten Thousand Plan for Young Top Talents[China] ; Natural Science Foundation of Zhejiang Province of China (LGC19F050002, LQ21F050005, LY20F050010).
Disclosures
The authors declare that there are no conflicts of interest related to this article.
Data availability
Data underlying the results presented in this paper may be available from the corresponding author upon reasonable request.
References
1. D. J. Richardson, J. Nilsson, and W. A. Clarkson, “High power fiber lasers: current status and future perspectives,” J. Opt. Soc. Am. B 27(11), B63–B92 (2010). [CrossRef]
2. P. Wang, W. Clarkson, D. Shen, et al., “Novel concepts for high-power fibre lasers,” in Solid State Lasers and Amplifiers II, (SPIE, 2006), 144–155.
3. C. Jauregui, J. Limpert, and A. Tünnermann, “High-power fibre lasers,” Nat. Photonics 7(11), 861–867 (2013). [CrossRef]
4. M. C. Pierce, S. D. Jackson, M. R. Dickinson, et al., “Laser-tissue interaction with a continuous wave 3-µm fibre laser: Preliminary studies with soft tissue,” Lasers Surg. Med. 26(5), 491–495 (2000). [CrossRef]
5. A. Sijan, “Development of military lasers for optical countermeasures in the mid-IR,” in Technologies for Optical Countermeasures VI, (SPIE, 2009), 32–45.
6. K. Nagai and K. Shimizu, “Using a high-power fibre laser to cut concrete,” Appl. Sci. 11(10), 4414 (2021). [CrossRef]
7. N. Broderick, H. Offerhaus, D. Richardson, et al., “Large mode area fibers for high power applications,” Opt. Fiber Technol. 5(2), 185–196 (1999). [CrossRef]
8. M.-J. Li, X. Chen, A. Liu, et al., “Limit of effective area for single-mode operation in step-index large mode area laser fibers,” J. Lightwave Technol. 27(15), 3010–3016 (2009). [CrossRef]
9. S. Saitoh, K. Saitoh, M. Kashiwagi, et al., “Design optimization of large-mode-area all-solid photonic bandgap fibers for high-power laser applications,” J. Lightwave Technol. 32(3), 440–449 (2014). [CrossRef]
10. C. Xia, N. Bai, I. Ozdur, et al., “Supermodes for optical transmission,” Opt. Express 19(17), 16653–16664 (2011). [CrossRef]
11. H. Li, J. Zang, S. Raghuraman, et al., “Large-mode-area multicore Yb-doped fiber for an efficient high power 976 nm laser,” Opt. Express 29(14), 21992–22000 (2021). [CrossRef]
12. X. Liang, M. Zhong, T. Xu, et al., “Mid-Infrared Single-Mode Ge-As-S Fiber for High Power Laser Delivery,” J. Lightwave Technol. 40(7), 2151–2156 (2022). [CrossRef]
13. A. Barh, S. Ghosh, R. Varshney, et al., “Ultra-large mode area microstructured core chalcogenide fiber design for mid-IR beam delivery,” Opt. Commun. 311, 129–133 (2013). [CrossRef]
14. P. S. Maji and P. Roy Chaudhuri, “Design analysis of large mode area fiber based on chalcogenide glass with microstructured core,” Opt. Quantum Electron. 47(8), 3009–3021 (2015). [CrossRef]
15. X. Wang, K. Jiao, N. Si, et al., “Extruded seven-core tellurium chalcogenide fiber for mid-infrared,” Opt. Mater. Express 9(9), 3863–3870 (2019). [CrossRef]
16. J. Xiao, J. Wang, T. Xu, et al., “Large mode-area chalcogenide multicore fiber prepared by continuous two-stage extrusion,” Opt. Mater. Express 11(3), 791–800 (2021). [CrossRef]
17. J. Wang, Z. Feng, J. Wang, et al., “Dispersion tuning and supercontinuum generating in novel W-typed chalcogenide fiber,” Infrared Phys. Technol. 111, 103538 (2020). [CrossRef]
18. J.-P. Berenger, “Three-dimensional perfectly matched layer for the absorption of electromagnetic waves,” J. Comput. Phys. 127(2), 363–379 (1996). [CrossRef]
19. C. Xia, N. Bai, R. Amezcua-Correa, et al., “Supermodes in strongly-coupled multi-core fibers,” in 2013 Optical Fiber Communication Conference and Exposition and the National Fiber Optic Engineers Conference (OFC/NFOEC), (IEEE, 2013), 1–3.
20. Y. Zheng, J. Yao, L. Zhang, et al., “Supermode analysis in multi-core photonic crystal fiber laser,” in High-Power Lasers and Applications V, (SPIE, 2010), 259–264.
21. J. Ji, S. Raghuraman, X. Huang, et al., “115 W fiber laser with an all solid-structure and a large-mode-area multicore fiber,” Opt. Lett. 43(14), 3369–3372 (2018). [CrossRef]
22. X. Wang, S. Lou, and W. Lu, “Bend-resistant large-mode-area photonic crystal fiber with a triangular-core,” Appl. Opt. 52(18), 4323–4328 (2013). [CrossRef]
23. Q. Peng, J. Sheng, K. Yang, et al., “Single-Mode Segmented Cladding Chalcogenide Glass Fiber With Ultra-Large Mode Area,” J. Lightwave Technol. 41(17), 5722–5728 (2023). [CrossRef]
24. X. Feng, H. Ren, F. Xu, et al., “Few-moded ultralarge mode area chalcogenide photonic crystal fiber for mid-infrared high power applications,” Opt. Express 28(11), 16658–16672 (2020). [CrossRef]
25. D. M. Shyroki, “Exact equivalent-profile formulation for bent optical waveguides,” arXiv, arXiv:physics/0605002 (2006). [CrossRef]
