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Abstract
We demonstrate an all-fiber 7 × 1 signal combiner with an output core diameter of 50 μm for high power incoherent beam combining of seven self-made Yb-doped single-mode fiber lasers around a wavelength of 1080 nm and output power of 2 kW. 14.1 kW combined output power is achieved with a total transmission efficiency of higher than 98.5% and a beam quality of M2 = 5.37, which is close to the theoretical results based on finite-difference beam propagation technique. To the best of our knowledge, this is the highest output power ever reported for all-fiber structure beam combining generation, which indicates the feasibility and potential of >10 kW high brightness incoherent beam combining based on an all-fiber signal combiner.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
High power Yb-doped fiber laser (YDFL) has been widely adopted in many industrial application (especially material processing), defense technology, scientific research and so on, owing to the advantages of high efficiency, beam quality and flexible operation [1–4]. The significant progress of fiber laser is known to be mainly contributed by the fast development of rare-earth ion-doped fibers and laser diodes (LDs) [5]. However, the further scaling of the output power is typically restricted by nonlinear effects like inelastic scattering, thermal effect, limited pump brightness as well as mode instability [2, 3, 6–8]. A promising way for the purpose of further scaling the output power is laser beam combining technology including coherent beam combining (CBC) and incoherent beam combining. Coherent combining can achieve high combining efficiency and excellent beam quality while it requires phase-stabilized laser beams and precise phase control for all individual beam [9–11]. Compared with coherent combining, spectral beam combining (SBC) is a promising concept of incoherent combing for lower requirement for phase control meanwhile maintaining beam quality [12–14]. However, narrow-linewidth laser sources are critical for wavelength-selective by this method, thus leading to the undesirable limitation factor of stimulated Brillouin scattering (SBS). Additionally, CBC and SBC system are mostly free space structure with bulk optical components, which makes it more preferred for many application to realize incoherent combining with an all-fiber structure because of the compactness and stability, especially when the beam quality does not to be diffraction limited.
Recently, fiber combiners by using tapered fiber bundles (TFB) technology [15, 16] have shown great potential in achieving all-fiber high power incoherent beam combining [17–19]. In 2011, an all-fiber 7 × 1 signal combiner for incoherent beam combining with 2.5 kW output was reported, with the core diameter of the output fiber is 100 μm and the measured beam quality is M2 = 6.5 at a power level of 600 W [20]. In 2012, a 4 kW incoherent beam combing of fiber lasers was achieved with an all fiber 4x1 signal combiner [21]. In 2014, fiber lasers with a total average power of 5.1 kW were combined by a 7 × 1 signal combiner with 50 μm output core and M2 was measured to be 6.8 with five ports [22]. An impressive result of a fiber combiner in terms of beam quality was reported in 2015, which have successfully combined 5.7 kW into an output fiber with a 50 μm core diameter, leading to a M2 of 4.6 [23]. Beam quality of the signal combiners for high power incoherent beam combining has been theoretically and experimentally investigated by our group [24–27], reaching a 6.26 kW output power with high beam quality of M2 ≈4.3 [25], which demonstrates the feasibility of either high power or high-brightness incoherent beam combining based on all-fiber signal combiners. Since SBC have been reported to realize 10 kW level output power [13, 14], it is worthy of experimental exploration and verification for the technology optimization, the performance and the potential of incoherent beam combination based on an all-fiber signal combiner, with respect to the beam quality, thermal state and stability under a power level of >10 kW.
In this paper, we demonstrate incoherent beam combination of seven all fiber laser sources around 1080 nm by a self-made all-fiber 7 × 1 signal combiner with an output core diameter of 50 μm. First, the structure of the combiner is introduced and the beam quality of the signal combiner is theoretically analyzed. Then, the combiner is designed and fabricated exploiting the 7 × 1 format, where seven input fibers are combined into a 50 μm core output fiber. The device has been tested using seven single-mode fiber lasers with 2 kW CW power. A total power of 14.1 kW could be transmitted with loss of less than 1.5% and the beam quality of M2 = 5.37 at 14 kW output power is measured.
2. Numerical analysis
As shown in Fig. 1(a), the structure of the signal combiner is based on seven commercially available double-clad fibers with a 20 μm core and an NA of 0.06, which are inserted into a low-index fluoride-doped capillary (NA = 0.22) whose inner diameter guarantee the centrosymmetric arrangement of seven fibers and then adiabatically tapered to be spliced with a multi-mode delivery fiber. An image of the input and output end-facet tapered fiber bundle is shown in Figs. 1(b) and 1(c). As illustrated in Fig. 1(c), the end-facet has a shape of petal due to the collapse of the air holes between the input fibers and the capillary. The core to core distance was estimated to be (R is the cladding diameter of the original signal fiber) [25], according to the assumption that the cladding area of the fiber bundle before and after collapse process keep unchanged. Since the mode field will partially propagate out of the core along the taper section, the circumcircle of all the cores of fiber bundle should be smaller than the 50 μm core diameter of the output fiber.
Fig. 1 (a) Schematic of the signal combiner; (b) Input and (c) output cross-section of the fiber bundle. The longitudinal position A and B correspond to (b) and (c) respectively.
We investigate the 7 × 1 all-fiber signal combiner based on advanced finite-difference beam propagation technique. The simplified calculation model was designed and discussed in detail in [24, 25]. In the simulation, transversal and longitudinal step length are set to be 0.4 and 0.6 μm. The lengths of the taper region and taper waist are 15 mm and 10 mm, while the length of the output fiber is set to 10 mm. The taper ratio ( = Λ/Λ0, Λ and Λ0 is the core-to-core distance of each input fiber before and after taper respectively) is chosen to be 10.6%, which makes a cladding diameter of the tapered fiber bundle to be 45 μm. Since single-mode fiber laser are utilized to realize high power coherent combining, seven LP01 modes with λ = 1080 nm are launched into every input port respectively, with the identical power ratio of 1/7. Figure 2 indicates the calculated field intensity profile at different longitudinal positon, including the input, the end of fiber bundle, the output as well as the far field intensity profile. The beam quality is another critical parameter to describe the performance of a signal combiner. In this work, M2 is taken as the characteristics parameter to analyze the incoherent combination performance of the fiber combiner, which can be calculated from the field intensity profile at the end-facet of the combiner. The calculated results are M2x = 4.69 and M2y = 4.65.
Fig. 2 Calculated transverse field intensity profile at different longitudinal positon, including the input (top left, corresponding to A in Fig. 1(a)), the end of fiber bundle (top right, corresponding to B in Fig. 1(a)), the output of the delivery fiber (bottom left, corresponding to C in Fig. 1(a)) as well as the far field intensity profile (bottom right).
3. Experimental results
There are four steps to fabricate a combiner: (1) Make a fiber bundle. Seven input fibers with a core of 20 μm are placed centrosymmetrically within a low-index fluorine-doped glass capillary (NA = 0.22, Dinner/outer = 758/1500 μm). The input fibers have inner-cladding diameter of 400 μm, which can be decreased by hydrofluoric acid to ensure an appropriate size to be inserted into the glass capillary. (2) Fiber bundle tapering. The fiber bundle is tapered until all the fiber cores can be covered by the core of the output fiber. A multi-mode fiber with a core of 50 μm is chosen as the output fiber in the experiment. Thus, the diameter of the fiber cladding area is tapered to be around 50 μm with a taper length of 1.5 cm, which is sufficient to satisfy adiabatic condition. (3) Cleaving. The tapered fiber bundle is then cleaved by ultrasonic cutter at the taper waist, whose diameters are measured with 47μm for the fiber cladding area, as shown in the final end face of the tapered bundle in Fig. 3(left). (4) Splicing. The input fiber bundle is spliced with the delivery output fiber (d = 50/70/360 μm, NA = 0.22) by a commercial splicer whose heating source is a CO2 laser. The side view of the splice point between the tapered fiber bundle and the output fiber is shown in Fig. 3 (right), where the right part presents the output fiber and the left is the tapered bundle.
Fig. 3 Cross section of the end-facet of the tapered fiber bundle (left) and the side view of the splice between the tapered bundle and the output fiber (right).
As illustrated in Fig. 4, the high power beam tests and beam combining have been accomplished by seven self-made 2 kW-class continuous-wave single-mode fiber lasers at a wavelength around 1080 nm and an M2 value of 1.2-1.4, with an output fiber of 20 μm and 400 μm as its core and inner cladding diameters, respectively. For the purpose of minimize end-facet back reflections, the combiner is terminated with a self-made quartz block head (QBH). Since the most limiting factor for power scaling in the signal fiber combiner is the growth of the working temperature and heating within the taper section and the splice, with a view to maintain a stability of the combiner especially under a power level up to 10 kW class, the fiber bundle and the splice with the output fiber is embedded in low refractive index adhesive, which can minimize the heating phenomenon as much as possible through a heat-sink device with an active water-cooling.
The transmission efficiency are measured and shown in Table 1. All of the ports have approximately a high efficiency larger than 98.5%. The transmission loss in the combiner can be resulted from the light leakage that is not guided in the fiber core due to the unwanted and inevitable deformation or bends of the seven signal fibers during the arrangement in the capillary before the tapering as well as the inadequate alignment of the fiber bundle and the output fiber before splicing. It is then absorbed by contaminations and the embedding material around the splice of the fiber bundle and the output delivery fiber [23]. It is very likely that this minor transmission loss will lead to heating within the component, especially in high power up to 10 kW level. Figure 5 indicates the heat performance of the combiner during the high power test. The temperature of the combiner is monitored by a thermal imager. As shown in Fig. 5(a), the temperature of the combiner rises with the increase of transmitted power. As the thermal imager was hold by hand during the testing and the distance between the component and the imager was not fixed precisely, it can be observed that the thermal coefficient fluctuates at different output power level, but with an average coefficient of approximately 2°C/kW at >10 kW level, reaching to 66.1°C when 14 kW output power is obtained. Therefore, further power scaling seems feasible but limited by the available laser power for every input port at that time. Figure 5(b) shows the infrared thermal image of the combiner and the temperature distribution along the longitudinal direction at an output power of 14 kW as well as the visible image recorded by the thermal imager. The hottest region locates at the taper section of the fiber bundle (close to the splice) and the splice with the output fiber, which may be caused by the imperfection of the splicing. The working temperature remain stable in a period of 10 minutes of testing time.
Table 1. Signal transmission efficiency of each port of the combiner
Fig. 5 Heat performance of the combiner: (a) The highest working temperature on the combiner versus transmitted power. (b) Thermal image of the combiner, showing a maximum temperature of 66.1°C at an output power of 14.1 kW. The top subgraph illustrates the temperature distribution along the longitudinal direction and the bottom shows the visible image taken by the thermal imager.
The beam quality evolution with the increase of the output power of the beam combing is measured by PRIMES LQM system, whose testing schematic is shown in Fig. 6(a). The combined laser beam is split by a highly reflectivity mirror (HR1 with a reflectivity of 99.95% @ 45°). The reflected beam, which contains ∼99.95% of the incident power, is sent to a power meter and the transmitted beam is used for beam quality measurement. For the aim of eliminating the influence of the residual cladding light from the QBH, a beam splitter mirror which can transmit the combined beam and reflect the pump light, is located behind the HR1. HR2 has the same reflectivity with HR1. The value of M2 at the 14 kW output power is 5.37 (shown in the subgraph of Fig. 6(b)), which is slightly higher but approaching the simulated result. It can be attributed to the tiny amount of high-order modes of the 2 kW fiber lasers via 20/400 μm fiber output and the taper waist diameter deviation of the theoretical model and the actual fabrication parameter.
Fig. 6 (a) Experimental configuration of beam quality analysis. HR, high reflectivity mirror (b) the results of measured M2 at an output power of 14 kW (subgraphs show the intensity profile at the waist).
4. Conclusion
In conclusion, we have successfully combined 14.1 kW into an output fiber with 50 μm core diameter based on TFB signal combiner with a total transmission efficiency higher than 98.5%. The high beam quality M2 of 5.37 is measured at the highest output power, which is close to the theoretical result of ∼4.6. The hottest working temperature of the combiner growth are very low (2°C/kW at >10 kW level) and can be improved by optimizing the splicing process as well as the fabrication of tapered fiber bundle. Therefore, further scaling of the power handling appears to be feasible and all-fiber structure signal combiner has been proved to be a promising approach for high-brightness incoherent beam combination under a power level of >10 kW, after the 3D-structure beam combination methods as mentioned earlier have been successively reported to realize 10 kW level output power recently. Compared with those beam combination systems with various bulk optical components, for many application especially when a diffraction limited beam quality is not required, it is more preferred to realized high power high-brightness beam combination based on an all-fiber combiner when it comes to the compactness, stability and low-cost of this technique.
Funding
National Key R&D Program of China (2017YFF0104600); Natural Science Foundation of China (NSFC) (61370045)
Acknowledgments
The authors would like to thank Sun Yanmei, Xie Yie, Liu Peng, Chen Junlv, He Jiawei, Chen Jingchun, Liu Jinwei for the sincere help and support on the high power test.
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