All-fiber 7 × 1 signal combiner for high power fiber lasers Hang Zhou, Zilun Chen,* Xuanfeng Zhou, Jing Hou, and Jinbao Chen College of Optoelectronic Science and Engineering, National University of Defense Technology, Changsha 410073, China *Corresponding author: [email protected] Received 21 January 2015; revised 3 March 2015; accepted 3 March 2015; posted 4 March 2015 (Doc. ID 232980); published 3 April 2015
We present an all-fiber 7 × 1 signal combiner for high power fiber lasers. Through theoretical analysis, the fabrication method is confirmed and the taper length of the fiber bundle is chosen to be 1 cm to ensure a high transmission efficiency of the combiner. Based on the theoretical results, an all-fiber 7 × 1 signal combiner with high transmission efficiency is fabricated. A capillary with low refractive index is fused around the bundle of signal fibers to make an additional cladding layer. Then the fiber bundle is tapered to match the core of the output fiber and then spliced with the output fiber. The combiner is tested with a 500 W fiber laser and a temperature increase of 13°C/kW without any active cooling is observed in the combiner. The power transmission efficiency is measured to be close to 99% for each input port and the beam quality M2 is around 10. © 2015 Optical Society of America OCIS codes: (140.3510) Lasers, fiber; (140.3298) Laser beam combining; (060.2340) Fiber optics components. http://dx.doi.org/10.1364/AO.54.003090
1. Introduction
Fiber lasers have numerous applications in many fields due to their advanced features, and their output power has been extended into the kW regime [1–5]. However, the output power of single fiber laser is limited by strong nonlinear effects and complicated thermal management. In order to achieve higher power output, beam combining has become a major approach. Compared with the coherent beam combining, incoherent beam combining is a significantly easier method for further scaling of the output power of a fiber laser [6,7]. There have been several practical methods to realize incoherent beam combining [8] and taper fiber bundle (TFB) technology has been proven to be superiority [9–12]. In 2011, Noordegraaf et al. [13] reported an all-fiber 7 × 1 signal combiner that supported up to 2.5 kW 1559-128X/15/113090-05$15.00/0 © 2015 Optical Society of America 3090
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combined output power and only a minor temperature increase was observed. At an intermediate power level of 600 W the beam parameter product (BPP) was measured to be 2.22 mm·mrad, corresponding to an M2 value of 6.5. Yan et al. [14] achieved a 4 kW continuous wave (CW) fiber laser with an allfiber 4 × 1 signal combiner, and a coupling efficiency of nearly 98%. In Ref. [15], Shamir et al. reported a 3 kW level incoherently combined laser with a measured power transmission efficiency of 98% using an all-fiber Y-branch combiner, and the M2 factor of ≤3.5 was measured at a 600 W output level. However, although a fiber signal combiner supporting kW laser output has been reported, experimental results have been given without a detailed fabrication process, which makes them impossible for guiding other researchers, and the highest power transmission efficiency of the reported N × 1 signal combiners is not over 98%. Power transmission efficiency is a key factor for the combiner. If the transmission efficiency of the combiner is low, the power
loss would be very high when a high power fiber laser is transmitted, which would lead to a serious heat load and fiber damage. In this paper, we investigate the fabrication of a 7 × 1 signal combiner in detail theoretically and experimentally. In theory, two fabrication methods are analyzed and compared and an optimum combiner structure with high power and high transmission efficiency is confirmed. Based on theoretical analysis, a successful fabrication of a 7 × 1 signal combiner is done and the detailed fabrication process is demonstrated. Each port of the combiner is tested by a 500 W fiber laser and only a minor increase in device temperature is observed. The power transmission efficiency and beam quality are measured. The power transmission efficiency is close to 99% and the M2 value is around 10. 2. Theoretical Analysis
Generally, the fabrication process of a N × 1 combiner is composed of three steps: tapering, cleaving, and splicing [10,11], and tapering is the most important step. In recent years, mainly two manufacturing methods of TFB are practically used [16–18]. One is tapering after twisting the fiber bundle and then the tapered bundle is spliced with the output fiber. The other one is inserting the fiber bundle into a capillary and then the capillary is tapered and spliced with the output fiber. These two methods are schematically shown in Fig. 1. In both methods, the cores and claddings of all of the fibers decrease according to the same taper ratio; thus, the fiber pitch (the distance between the cores of two adjacent fibers) decreases along the taper. h (equal to R − r) is the change of the fiber pitch from the initial end face to the final end face of the taper, as shown in Fig. 2. Generally, the output fibers of high power fiber lasers are double-cladding fibers (DCF) with 20 μm and 400 μm as the core and inner cladding diameters. Ideally, the input fibers of the power combiner should be the same as the output fibers of the fiber lasers to be combined. However, the core-cladding diameter ratio of 20/400 μm DCF is very large, which leads to large distances among fiber cores of the fiber
Fig. 1. Two main methods for manufacturing TFB. (a) twist and (b) no twist.
Fig. 2. Change of the fiber pitch of the TFB. R is the fiber pitch of the initial end face of the TFB, r is that of the final end face of the TFB, and h is the change of the fiber pitch.
bundle. Thus, the tapering ratio will be a large value and the NA of the input fiber will become very high. In this case, the output fiber cannot receive all of the output light from the input fibers, resulting in the decrease of the transmission efficiency. On the other hand, a large taper ratio will also make the packaging difficult due to the increased dimensions of the device. In order to solve this problem, a kind of DCF with 20/130 μm as the core/inner cladding diameters, respectively, is adopted to be the input fiber of the combiner. This fiber has the same core size as the output fiber of the lasers to be combined, thus the splice loss is very low and a small taper ratio can be chosen to maintain a high transmission efficiency and compact device size. Therefore, we chose a 20/130 μm fiber as the input fiber of our combiner in the following simulation and experimental work in this paper. When the seven signal fibers are arrayed closely in parallel, in the region where the coatings are stripped, the fibers cannot cling to each other, thus the fibers cannot be tapered into one fiber bundle. While twisting the fibers is a good method to solve this problem, the twisting angle is not a certain value but the fibers should cling closely to each other for a long enough distance to form the taper region. The twisting angle will affect the laser field propagation. The output fiber of the signal combiner has a core of 100 μm and the diameter of the fiber bundle is 390 μm; thus, the change of the fiber pitch is about 80 μm. When the mode of the laser propagating in the signal fiber is LP01 , LP11 , and LP02 , the change of the laser field in the signal fiber is simulated using the beam propagation method (BPM) and the results are shown in Fig. 3. From the figure it can be found that the increase of the twisting angle deteriorates the laser field acutely. Consequently, a fiber combiner manufactured through twisting the fiber bundle degrades the laser beam quality. Since there is no requirement of beam quality for a pump combiner, twisting is the most usual fabrication method for a pump combiner. However, a signal combiner requires good beam quality, thus the twisting method is not suitable. There is another method to press the fibers to cling to each other. That is inserting the seven signal fibers into a fluorine-doped low-index capillary in parallel. This capillary will be fused with the fiber bundle in a 10 April 2015 / Vol. 54, No. 11 / APPLIED OPTICS
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Fig. 3. Fields of no twist and twist 1, 2, 3 times, respectively, for three incident eigenmodes: LP01 (top), LP11 (middle), and LP02 (bottom). Fig. 5. Efficiencies of three modes versus the taper length.
single entity by a filament. The single entity can be regarded as a special fiber that has seven cores and a cladding formed by the low-index capillary tube. Here we define an equivalent diameter for this entity, as shown in Fig. 4. Then, tapering this single entity to have an equivalent diameter matched up with the core diameter of 100 μm of the output fiber NA 0.20. The single entity is cleaved at a specific location and spliced with the output fiber and then a combiner is obtained. Transmission efficiency is another important parameter of a signal combiner, which stands for the capability of the combiner to combine the powers of several fiber lasers into one fiber. It is defined as the ratio of the output power to the total input power. For a combiner, whether the laser brightness is conserved from the N input fibers to the output fiber is one decisive factor for the transmission efficiency. The brightness conservation can be described by the ratio of integrated brightness of the total input lasers to the integrated brightness of the output laser, as in the following equation [18–21]: D2out NA2out ≥ N × D2in NA2in ;
In addition, transmission efficiency is also affected by the taper length. Using the BPM, three modes (LP0;1 , LP0;2 , and LP1;1 ) propagating from the signal fibers to the output fiber are simulated and the transmission efficiencies are calculated to explore the relationship between the transmission efficiency and the taper length. The results are presented in Fig. 5. From Fig. 5, the transmission efficiency of the low-order mode is apparently higher than that of the high-order mode. On the other hand, as the taper lengthens the efficiencies of each mode first increase dramatically from a low value to a high value and then the increase trend becomes flattened when the taper length is over 8 mm. The low-order mode has no loss in this case. In the latter theoretical and experimental investigations a 1 cm long taper is chosen. After confirming the structure of signal combiner, the laser fields of the combiner at different locations are calculated as shown in Fig. 6.
(1)
where NAin and NAout are the NA of the input and output fibers; Din and Dout are the core diameters of the input and output fibers, respectively; and N is the number of input fibers. According to the fiber parameters used in this paper, Eq. (1) can be satisfied with 1002 × 0.202 ≥ 7 × 202 × 0.082 , so the transmission efficiency can approach 100%, ideally.
Fig. 4. Cross section of the signal combiner at the splice point. 3092
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Fig. 6. Laser field of the combiner at (a) the initial location; and (b) the final end face of the tapered bundle. (c) The output laser field of the combiner; and (d) the far field of the output laser from the combiner.
Table 1.
Port No. 1 2 3 4 5 6 7
Fig. 7. (a)–(c) Cross sections of the original bundle, an arbitrary location in the transition region, and the final end face of the tapered bundle. (d) Image of the multimode output fiber facet; and (e) is the side view of the splice between the tapered bundle and the output fiber. The left part presents the output fiber and the right side is the tapered bundle.
The amplitude and phase at the output end face of the signal combiner are calculated, and then the beam quality can be obtained [22,23]. The calculated results are M2x 11.4 and M2y 11.7. 3. Experiment
Based on the calculated results, we fabricate a 7 × 1 signal combiner using a Vytran 3400 system. Seven fibers with core diameter of 20 μm and inner-cladding diameter of 130 μm (core and cladding NA of 0.08 and 0.46, respectively) are inserted into a capillary. The inner and outer diameters of this capillary are 800 μm and 1500 μm, respectively. These fibers and the capillary comprise a bundle. Then this bundle is tapered until all of the fiber cores can be covered by the core of the chosen output fiber of the combiner. In the experiment, the output fiber has a core of 100 μm. According to the inner/outer diameter ratio of the capillary, the diameter of the bundle is tapered to be 260 μm. The cross sections of the original bundle, an arbitrary location in the transition region, and the final end face of the tapered bundle are shown in Fig. 7(a)–7(c). The cross section of the output fiber of the combiner is shown in Fig. 7(d) and the side view of the splice between the tapered bundle and the output fiber is shown in Fig. 7(e) in which the left part presents the output fiber and the right side is the tapered bundle. Thus, a signal combiner is obtained.
Transmission Efficiencies of Each Port of the Combiner
Efficiency (%) 98.8 98.9 98.8 99.4 99.2 99.1 98.9
Then, the transmission efficiency of the combiner is tested by a 500 W CW fiber laser at 1064 nm with an output fiber of 20 μm and 400 μm as its core and inner cladding diameters, respectively. For measuring the transmission efficiency, the fiber laser is in turn spliced to each one of the signal ports of the combiner. The efficiencies are measured and shown in Table 1. All of the ports have approximately the same high efficiency, with an average value close to 99%. During the measurement, without adding any active cooling, only a minor increase of temperature is observed in the combiner. The temperature of the combiner is monitored simultaneously by a thermal imager. An observation result is saved and shown in Fig. 8 and in the figure the left bright spot (around 20 cm at the ruler) is the splice point of the fiber bundle and the output fiber, and the right bright one is the position where the coating of the output fiber is stripped. When the output power increases to 500 W, the temperature of the combiner rises from an environmental temperature of 23.9°C–30.5°C without any active cooling. Accordingly, the temperature rise can be calculated to be 13°C/kW. This value can be decreased when active cooling is used. Limited by the number and output power of the fiber laser in our lab, only an experiment with a 500 W fiber-laser coupling into one input port is performed. According to our experience, a power combiner can continuously operate safely under 60°C. Based on this consideration an estimate can be made that the fabricated power combiner can support >3000 W power output. The beam quality of the output laser from the combiner is measured by launching seven fiber lasers at 1064 nm with 2 W output power into the seven input
Fig. 8. (a) Experimentally measured power of the combiner; and (b) the temperature distribution of the combiner monitored by a thermal imager when a 500 W laser is coupled. 10 April 2015 / Vol. 54, No. 11 / APPLIED OPTICS
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Fig. 9. Beam quality M2 measurement result of the combiner by launching seven fiber lasers at 1064 nm with 2 W output power into the seven input ports.
ports. An M2 − 200 s laser beam profiler made by Spiricon Company is utilized. The measurement result is presented in Fig. 9 and the M2 values are 9.9 for the x-axis and 10.3 for the y-axis, which are both smaller than the simulation values. This can be interpreted that in the experiment the TFB has a smaller diameter than the 100 μm used in the numerical simulation. The current M2 factor is relative high mainly because the output fiber core is large and many optical modes can propagate in the output fiber. Consequently, a smaller-core fiber can lead to a better beam quality. In the future research work, output fibers with smaller cores (such as 50 μm) will be chosen to improve the beam quality of the combiner. 4. Conclusions
We investigate the fabrication of a 7 × 1 signal combiner theoretically and experimentally in this paper. Two fabrication methods are analyzed and compared in theory and an optimum combiner structure with high transmission efficiency is confirmed. Then a successful fabrication of a 7 × 1 signal combiner is done and the detailed fabrication process is demonstrated. Each port of the combiner is tested by a 500 W fiber laser and only a moderate temperature increase, 13°C/kW, without any active cooling steps is observed. The power transmission efficiency is close to 99% and the beam quality M2 is around 10. This work was supported by the National Natural Science Foundation of China (No. 61370045) and the Hunan Provincial Natural Science Foundation of China (No. 12JJ4061). References 1. J. Limpert, A. Liem, H. Zellmer, and A. Tünnermann, “500 W continuous-wave fiber laser with excellent beam quality,” Electron. Lett. 39, 645–647 (2003). 3094
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2. Y. Jeong, J. K. Sahu, D. N. Payne, and J. Nilsson, “Ytterbiumdoped large-core fiber laser with 1.36 kW continuous-wave output power,” Opt. Express 12, 6088–6092 (2004). 3. D. Gapontsev, “6 kW CW single mode ytterbium fiber laser in all-fiber format,” in Proceedings of Solid State and Diode Laser Technology Review, Albuquerque, Mexico, 2008. 4. D. J. Richardson, J. Nilsson, and W. A. Clarkson, “High power fiber lasers: current status and future perspectives,” J. Opt. Soc. Am. B 27, B63–B92 (2010). 5. Y. Ma, X. Wang, J. Leng, H. Xiao, X. Dong, J. Zhu, W. Du, P. Zhou, X. Xu, L. Si, Z. Liu, and Y. Zhao, “Coherent beam combination of 1.08 kW fiber amplifier array using single frequency dithering technique,” Opt. Lett. 36, 951–953 (2011). 6. H. Russell and W. B. Roh, “Incoherent beam combining using stimulated Brillouin scattering in multimode fibers,” Opt. Express 8, 246–254 (2001). 7. T. H. Loftus, A. Liu, P. R. Hoffman, A. M. Thomas, M. Norsen, R. Royse, and E. Honea, “522 W average power, spectrally beam-combined fiber laser with near diffraction limited beam quality,” Opt. Lett. 32, 349–351 (2007). 8. A. B. Grudinin, D. N. Payne, P. W. Turner, L. J. A. Nilsson, M. N. Zervas, M. Ibsen, and M. K. Durkin, “Multi-fibre arrangements for high power fibre lasers and amplifiers,” U.S. patent 6,826,335 (30 November 2004). 9. A. Kosterin, V. Temyanko, M. Fallahi, and M. Mansuripur, “Tapered fiber bundles for combining high-power diode lasers,” Appl. Opt. 43, 3893–3900 (2004). 10. A. Wetter, M. Faucher, M. Lovelady, and F. Séguin, “Tapered fused-bundle splitter capable of 1 kW CW operation,” Proc. SPIE 6453, 64530I (2007). 11. F. Séguin, A. Wetter, L. Martineau, M. Faucher, C. Delisle, and S. Caplette, “Tapered fused bundle coupler package for reliable high optical power dissipation,” Proc. SPIE 6102, 61021N (2006). 12. H. Yu, D. A. V. Kliner, K. H. Liao, J. Segall, M. H. Muendel, J. J. Morehead, J. Shen, M. Kutsuris, J. Luu, J. Franke, K. Nguyen, D. Woods, F. Vance, D. L. Vecht, D. Meng, R. Duesterberg, L. Xu, J. Skidmore, M. Peters, N. Guerin, J. Guo, J. Cheng, J. Du, B. Johnson, D. Yin, A. Hsieh, P. Cheng, A. Demir, J. Cai, R. Gurram, K. Lee, R. Raju, D. Zou, R. Srinivasan, M. Saini, L. Zavala, V. Rossin, E. P. Zucker, H. Ishiguro, and H. Sako, “1.2-kW single-mode fiber laser based on 100-W highbrightness pump diodes,” Proc. SPIE 8237, 82370G (2012). 13. D. Noordegraaf, M. D. Maack, P. M. W. Skovgaard, J. Johansen, F. Becker, S. Belke, M. Blomqvist, and J. Laegsgaard, “All-fiber 7 × 1 signal combiner for incoherent laser beam combining,” Proc. SPIE 7914, 79142L (2011). 14. D. Yan, C. Li, L. Li, and X. Liu, “4 kW continue wavelength fiber laser with all fiber 4 × 1 signal combiner,” in Proceedings of Laser and Terahertz Science and Technology (Optical Society of America, 2012), paper MTh3A. 15. Y. Shamir, R. Zuitlin, Y. Sintov, and M. Shtaif, “3 kW-level incoherent and coherent mode combining via all-fiber fused Y-couplers,” in Proceedings of Frontiers in Optics (Optical Society of America, 2012), paper FW6C. 16. M. Garneau, F. Gonthier, and N. Vachon, “Multimode fiber outer cladding coupler for multi-clad fibers,” U.S. patent 7,933,479 (26 April 2011). 17. A. Salokatve, “Optical fiber combiner and method of manufacturing thereof,” U.S. patent 2009/0154881 A1 (18 June 2009). 18. D. J. DiGiovanni and A. J. Stentz, “Tapered fiber bundles for coupling light into and out of cladding-pumped fiber devices,” U.S. patent 5,864,644 (26 January 1999). 19. B. Sévigny, P. Poirier, and M. Faucher, “Pump combiner loss as a function of input numerical aperture power distance,” Proc. SPIE 7195, 719523 (2009). 20. T. A. Birks and Y. W. Li, “The shape of fiber tapers,” J. Lightwave Technol. 10, 432–438 (1992). 21. J. Love and W. Henry, “Quantifying loss minimisation in single-mode fibre tapers,” Electron. Lett. 22, 912–914 (1986). 22. A. Siegman, “How to (maybe) measure laser beam quality,” in OSA Trends Opt. Photonics Series (Optical Society of America, 1998), paper MQ1. 23. A. Siegman, “Defining, measuring, and optimizing laser beam quality,” Proc. SPIE 1868, 2–12 (1993).
We present an all-fiber 7 × 1 signal combiner for high power fiber lasers. Through theoretical analysis, the fabrication method is confirmed and the taper length of the fiber bundle is chosen to be 1 cm to ensure a high transmission efficiency of the combiner. Based on the theoretical results, an all-fiber 7 × 1 signal combiner with high transmission efficiency is fabricated. A capillary with low refractive index is fused around the bundle of signal fibers to make an additional cladding layer. Then the fiber bundle is tapered to match the core of the output fiber and then spliced with the output fiber. The combiner is tested with a 500 W fiber laser and a temperature increase of 13°C/kW without any active cooling is observed in the combiner. The power transmission efficiency is measured to be close to 99% for each input port and the beam quality M2 is around 10. © 2015 Optical Society of America OCIS codes: (140.3510) Lasers, fiber; (140.3298) Laser beam combining; (060.2340) Fiber optics components. http://dx.doi.org/10.1364/AO.54.003090
1. Introduction
Fiber lasers have numerous applications in many fields due to their advanced features, and their output power has been extended into the kW regime [1–5]. However, the output power of single fiber laser is limited by strong nonlinear effects and complicated thermal management. In order to achieve higher power output, beam combining has become a major approach. Compared with the coherent beam combining, incoherent beam combining is a significantly easier method for further scaling of the output power of a fiber laser [6,7]. There have been several practical methods to realize incoherent beam combining [8] and taper fiber bundle (TFB) technology has been proven to be superiority [9–12]. In 2011, Noordegraaf et al. [13] reported an all-fiber 7 × 1 signal combiner that supported up to 2.5 kW 1559-128X/15/113090-05$15.00/0 © 2015 Optical Society of America 3090
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combined output power and only a minor temperature increase was observed. At an intermediate power level of 600 W the beam parameter product (BPP) was measured to be 2.22 mm·mrad, corresponding to an M2 value of 6.5. Yan et al. [14] achieved a 4 kW continuous wave (CW) fiber laser with an allfiber 4 × 1 signal combiner, and a coupling efficiency of nearly 98%. In Ref. [15], Shamir et al. reported a 3 kW level incoherently combined laser with a measured power transmission efficiency of 98% using an all-fiber Y-branch combiner, and the M2 factor of ≤3.5 was measured at a 600 W output level. However, although a fiber signal combiner supporting kW laser output has been reported, experimental results have been given without a detailed fabrication process, which makes them impossible for guiding other researchers, and the highest power transmission efficiency of the reported N × 1 signal combiners is not over 98%. Power transmission efficiency is a key factor for the combiner. If the transmission efficiency of the combiner is low, the power
loss would be very high when a high power fiber laser is transmitted, which would lead to a serious heat load and fiber damage. In this paper, we investigate the fabrication of a 7 × 1 signal combiner in detail theoretically and experimentally. In theory, two fabrication methods are analyzed and compared and an optimum combiner structure with high power and high transmission efficiency is confirmed. Based on theoretical analysis, a successful fabrication of a 7 × 1 signal combiner is done and the detailed fabrication process is demonstrated. Each port of the combiner is tested by a 500 W fiber laser and only a minor increase in device temperature is observed. The power transmission efficiency and beam quality are measured. The power transmission efficiency is close to 99% and the M2 value is around 10. 2. Theoretical Analysis
Generally, the fabrication process of a N × 1 combiner is composed of three steps: tapering, cleaving, and splicing [10,11], and tapering is the most important step. In recent years, mainly two manufacturing methods of TFB are practically used [16–18]. One is tapering after twisting the fiber bundle and then the tapered bundle is spliced with the output fiber. The other one is inserting the fiber bundle into a capillary and then the capillary is tapered and spliced with the output fiber. These two methods are schematically shown in Fig. 1. In both methods, the cores and claddings of all of the fibers decrease according to the same taper ratio; thus, the fiber pitch (the distance between the cores of two adjacent fibers) decreases along the taper. h (equal to R − r) is the change of the fiber pitch from the initial end face to the final end face of the taper, as shown in Fig. 2. Generally, the output fibers of high power fiber lasers are double-cladding fibers (DCF) with 20 μm and 400 μm as the core and inner cladding diameters. Ideally, the input fibers of the power combiner should be the same as the output fibers of the fiber lasers to be combined. However, the core-cladding diameter ratio of 20/400 μm DCF is very large, which leads to large distances among fiber cores of the fiber
Fig. 1. Two main methods for manufacturing TFB. (a) twist and (b) no twist.
Fig. 2. Change of the fiber pitch of the TFB. R is the fiber pitch of the initial end face of the TFB, r is that of the final end face of the TFB, and h is the change of the fiber pitch.
bundle. Thus, the tapering ratio will be a large value and the NA of the input fiber will become very high. In this case, the output fiber cannot receive all of the output light from the input fibers, resulting in the decrease of the transmission efficiency. On the other hand, a large taper ratio will also make the packaging difficult due to the increased dimensions of the device. In order to solve this problem, a kind of DCF with 20/130 μm as the core/inner cladding diameters, respectively, is adopted to be the input fiber of the combiner. This fiber has the same core size as the output fiber of the lasers to be combined, thus the splice loss is very low and a small taper ratio can be chosen to maintain a high transmission efficiency and compact device size. Therefore, we chose a 20/130 μm fiber as the input fiber of our combiner in the following simulation and experimental work in this paper. When the seven signal fibers are arrayed closely in parallel, in the region where the coatings are stripped, the fibers cannot cling to each other, thus the fibers cannot be tapered into one fiber bundle. While twisting the fibers is a good method to solve this problem, the twisting angle is not a certain value but the fibers should cling closely to each other for a long enough distance to form the taper region. The twisting angle will affect the laser field propagation. The output fiber of the signal combiner has a core of 100 μm and the diameter of the fiber bundle is 390 μm; thus, the change of the fiber pitch is about 80 μm. When the mode of the laser propagating in the signal fiber is LP01 , LP11 , and LP02 , the change of the laser field in the signal fiber is simulated using the beam propagation method (BPM) and the results are shown in Fig. 3. From the figure it can be found that the increase of the twisting angle deteriorates the laser field acutely. Consequently, a fiber combiner manufactured through twisting the fiber bundle degrades the laser beam quality. Since there is no requirement of beam quality for a pump combiner, twisting is the most usual fabrication method for a pump combiner. However, a signal combiner requires good beam quality, thus the twisting method is not suitable. There is another method to press the fibers to cling to each other. That is inserting the seven signal fibers into a fluorine-doped low-index capillary in parallel. This capillary will be fused with the fiber bundle in a 10 April 2015 / Vol. 54, No. 11 / APPLIED OPTICS
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Fig. 3. Fields of no twist and twist 1, 2, 3 times, respectively, for three incident eigenmodes: LP01 (top), LP11 (middle), and LP02 (bottom). Fig. 5. Efficiencies of three modes versus the taper length.
single entity by a filament. The single entity can be regarded as a special fiber that has seven cores and a cladding formed by the low-index capillary tube. Here we define an equivalent diameter for this entity, as shown in Fig. 4. Then, tapering this single entity to have an equivalent diameter matched up with the core diameter of 100 μm of the output fiber NA 0.20. The single entity is cleaved at a specific location and spliced with the output fiber and then a combiner is obtained. Transmission efficiency is another important parameter of a signal combiner, which stands for the capability of the combiner to combine the powers of several fiber lasers into one fiber. It is defined as the ratio of the output power to the total input power. For a combiner, whether the laser brightness is conserved from the N input fibers to the output fiber is one decisive factor for the transmission efficiency. The brightness conservation can be described by the ratio of integrated brightness of the total input lasers to the integrated brightness of the output laser, as in the following equation [18–21]: D2out NA2out ≥ N × D2in NA2in ;
In addition, transmission efficiency is also affected by the taper length. Using the BPM, three modes (LP0;1 , LP0;2 , and LP1;1 ) propagating from the signal fibers to the output fiber are simulated and the transmission efficiencies are calculated to explore the relationship between the transmission efficiency and the taper length. The results are presented in Fig. 5. From Fig. 5, the transmission efficiency of the low-order mode is apparently higher than that of the high-order mode. On the other hand, as the taper lengthens the efficiencies of each mode first increase dramatically from a low value to a high value and then the increase trend becomes flattened when the taper length is over 8 mm. The low-order mode has no loss in this case. In the latter theoretical and experimental investigations a 1 cm long taper is chosen. After confirming the structure of signal combiner, the laser fields of the combiner at different locations are calculated as shown in Fig. 6.
(1)
where NAin and NAout are the NA of the input and output fibers; Din and Dout are the core diameters of the input and output fibers, respectively; and N is the number of input fibers. According to the fiber parameters used in this paper, Eq. (1) can be satisfied with 1002 × 0.202 ≥ 7 × 202 × 0.082 , so the transmission efficiency can approach 100%, ideally.
Fig. 4. Cross section of the signal combiner at the splice point. 3092
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Fig. 6. Laser field of the combiner at (a) the initial location; and (b) the final end face of the tapered bundle. (c) The output laser field of the combiner; and (d) the far field of the output laser from the combiner.
Table 1.
Port No. 1 2 3 4 5 6 7
Fig. 7. (a)–(c) Cross sections of the original bundle, an arbitrary location in the transition region, and the final end face of the tapered bundle. (d) Image of the multimode output fiber facet; and (e) is the side view of the splice between the tapered bundle and the output fiber. The left part presents the output fiber and the right side is the tapered bundle.
The amplitude and phase at the output end face of the signal combiner are calculated, and then the beam quality can be obtained [22,23]. The calculated results are M2x 11.4 and M2y 11.7. 3. Experiment
Based on the calculated results, we fabricate a 7 × 1 signal combiner using a Vytran 3400 system. Seven fibers with core diameter of 20 μm and inner-cladding diameter of 130 μm (core and cladding NA of 0.08 and 0.46, respectively) are inserted into a capillary. The inner and outer diameters of this capillary are 800 μm and 1500 μm, respectively. These fibers and the capillary comprise a bundle. Then this bundle is tapered until all of the fiber cores can be covered by the core of the chosen output fiber of the combiner. In the experiment, the output fiber has a core of 100 μm. According to the inner/outer diameter ratio of the capillary, the diameter of the bundle is tapered to be 260 μm. The cross sections of the original bundle, an arbitrary location in the transition region, and the final end face of the tapered bundle are shown in Fig. 7(a)–7(c). The cross section of the output fiber of the combiner is shown in Fig. 7(d) and the side view of the splice between the tapered bundle and the output fiber is shown in Fig. 7(e) in which the left part presents the output fiber and the right side is the tapered bundle. Thus, a signal combiner is obtained.
Transmission Efficiencies of Each Port of the Combiner
Efficiency (%) 98.8 98.9 98.8 99.4 99.2 99.1 98.9
Then, the transmission efficiency of the combiner is tested by a 500 W CW fiber laser at 1064 nm with an output fiber of 20 μm and 400 μm as its core and inner cladding diameters, respectively. For measuring the transmission efficiency, the fiber laser is in turn spliced to each one of the signal ports of the combiner. The efficiencies are measured and shown in Table 1. All of the ports have approximately the same high efficiency, with an average value close to 99%. During the measurement, without adding any active cooling, only a minor increase of temperature is observed in the combiner. The temperature of the combiner is monitored simultaneously by a thermal imager. An observation result is saved and shown in Fig. 8 and in the figure the left bright spot (around 20 cm at the ruler) is the splice point of the fiber bundle and the output fiber, and the right bright one is the position where the coating of the output fiber is stripped. When the output power increases to 500 W, the temperature of the combiner rises from an environmental temperature of 23.9°C–30.5°C without any active cooling. Accordingly, the temperature rise can be calculated to be 13°C/kW. This value can be decreased when active cooling is used. Limited by the number and output power of the fiber laser in our lab, only an experiment with a 500 W fiber-laser coupling into one input port is performed. According to our experience, a power combiner can continuously operate safely under 60°C. Based on this consideration an estimate can be made that the fabricated power combiner can support >3000 W power output. The beam quality of the output laser from the combiner is measured by launching seven fiber lasers at 1064 nm with 2 W output power into the seven input
Fig. 8. (a) Experimentally measured power of the combiner; and (b) the temperature distribution of the combiner monitored by a thermal imager when a 500 W laser is coupled. 10 April 2015 / Vol. 54, No. 11 / APPLIED OPTICS
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Fig. 9. Beam quality M2 measurement result of the combiner by launching seven fiber lasers at 1064 nm with 2 W output power into the seven input ports.
ports. An M2 − 200 s laser beam profiler made by Spiricon Company is utilized. The measurement result is presented in Fig. 9 and the M2 values are 9.9 for the x-axis and 10.3 for the y-axis, which are both smaller than the simulation values. This can be interpreted that in the experiment the TFB has a smaller diameter than the 100 μm used in the numerical simulation. The current M2 factor is relative high mainly because the output fiber core is large and many optical modes can propagate in the output fiber. Consequently, a smaller-core fiber can lead to a better beam quality. In the future research work, output fibers with smaller cores (such as 50 μm) will be chosen to improve the beam quality of the combiner. 4. Conclusions
We investigate the fabrication of a 7 × 1 signal combiner theoretically and experimentally in this paper. Two fabrication methods are analyzed and compared in theory and an optimum combiner structure with high transmission efficiency is confirmed. Then a successful fabrication of a 7 × 1 signal combiner is done and the detailed fabrication process is demonstrated. Each port of the combiner is tested by a 500 W fiber laser and only a moderate temperature increase, 13°C/kW, without any active cooling steps is observed. The power transmission efficiency is close to 99% and the beam quality M2 is around 10. This work was supported by the National Natural Science Foundation of China (No. 61370045) and the Hunan Provincial Natural Science Foundation of China (No. 12JJ4061). References 1. J. Limpert, A. Liem, H. Zellmer, and A. Tünnermann, “500 W continuous-wave fiber laser with excellent beam quality,” Electron. Lett. 39, 645–647 (2003). 3094
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