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Abstract
At present, increasing the output power of a single fiber laser is a problem worthy of attention, and a fiber signal combiner which can carry high power and high beam quality is a solution to this problem. In this paper, a high beam quality 4×1 fiber signal combiner was developed. Firstly, the simulation model of a 4×1 fiber signal combiner was established, and the factors affecting the beam quality and the transmission efficiency of combiner were simulated. Secondly, based on corrosion technology, a high beam quality 4×1 fiber signal combiner with an output fiber of 50/400 µm (NA=0.12) was fabricated by using the taper-fused fiber bundles technology according to simulation results. Finally, four Yb-doped fiber lasers with a central wavelength of 1080 nm were used to test the output power and beam quality of combiner. The test result showed that the total output power of the signal combiner was 12.03 kW, the overall transmission efficiency was more than 96%, and the M2 factor was measured to be 4.03.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
In recent years, with the progress of manufacturing technology of double-cladding fiber and pumping technology, fiber lasers have achieved rapid development [1,2], and have been widely used in industrial processing, national defense, biological medical and other fields [3–5]. The output power and beam quality of fiber lasers have been continuously improved. However, due to the influence of thermal damage, nonlinear effect, pump brightness and mode instability, it becomes more and more difficult to improve the output power of a single fiber laser [6,7]. Beam combining technology is an effective way to break through the bottleneck of power improvement of a single fiber laser. The beam combining technology based on fiber signal combiner has the advantages of compact structure, high reliability, low cost, good stability, which is widely used in the high-power all-fiber laser system [8].
In 2018, a 4×1 fiber signal combiner with input fiber core diameter of 20 µm (NA=0.08) and output fiber of 105/125 µm (NA=0.15) was fabricated by In Seok Choi. With 624.5 W signal power injected to the signal combiner, the maximum output power was 612 W, corresponding to the efficiency of 98%. The beam quality factor, M2 was measured to be approximately 14.6 [9]. In the same year, a 7×1 fiber signal combiner using input fiber core diameter of 20 µm and output fiber of 50/70/360 µm (NA=0.22) was reported by our research group. The test result showed that the output power of combiner was 14 kW and the beam quality was M2=5.37 [10]. Although this fiber signal combiner is capable of carrying high output power, the beam quality is not good enough. In 2020, based on the corrosion technology, a 3×1 fiber signal combiner with input fiber of 20/400 µm (NA = 0.08) and output fiber with core diameter of 50 µm (NA=0.22) was fabricated by our research group. When the total input power of three lasers was 6140 W, the output power was 6015 W and the corresponding transmission efficiency was 98%. M2 was measured by a beam quality analyzer as 3.6 [11].
At present, the fiber signal combiner that can carry both high power and high beam quality is still a problem worthy of further exploration. In this paper, in order to maintain high beam quality and further improve the synthetic output power, we fabricated a 4×1 fiber signal combiner using input fiber of 20 µm core diameter and output fiber (50/400 µm, NA=0.12) based on corrosion technology. During the fabrication process, input fibers were etched with hydrofluoric acid to increase the core duty ratio of taper-fused fiber bundles (TFB). After the production, four Yb-doped fiber lasers were used to test the main performance parameters of the combiner. Through the test, the maximum output power of combiner was 12.03 kW with the transmission efficiency of 96% and the value of M2 is about 4.03.
2. Theoretical analysis and simulations
There are many factors that affect beam quality and transmission efficiency of fiber signal combiner, such as the number of input fibers, the core diameter of output fiber, and the core duty ratio of TFB, and so on.
As shown in the Fig. 1, the core duty ratio of TFB can be defined as the ratio of the core diameter of tapered fiber to the distance between two adjacent fibers, expressed as P = d/Δ. Generally speaking, a larger core duty ratio of TFB means that the input optical field is more concentrated, and the center spot of the output optical field accounts for a larger proportion. Therefore, the analysis shows that increasing the core duty ratio of TFB can improve the beam quality of fiber signal combiner. To increase the core duty ratio of TFB, the distance between two adjacent fibers can be reduced by making the cladding as thin as possible. The output fiber of laser used to our target fiber signal combiner has a core and cladding of 20 µm and 400 µm. If the 20/400 µm fiber is directly used as input fiber of combiner to be tapered, it will not only lead to the small core duty ratio of TFB, which will worsen beam quality of combiner, but also lead to the need of a longer cone region for combiner to meet the conditions of adiabatic tapering, which is difficult in operation.
We use the simulation software to build a 3D simulation model of 4×1 fiber signal combiner (as shown in the Fig. 2), which uses the beam propagation method (BPM) to calculate the mode field distribution at each position of combiner, and then we use the code written by our research group to calculate the M2 factor. The value of M2 is calculated as M2 =(Mx2+ My2)/2. The cone region’s length of the simulation model is set to 15 mm and the waist length is set to 5 mm. In the calculation process, when the length of output fiber in the simulation model is changed from 11 mm to 15 mm, we find that the beam quality of combiner fluctuates periodically along with the length of output fiber in a small range, so the average value of beam quality at different distances of beam propagating into output fiber is adopted.
We get beam quality factor and transmission efficiency of combiner with different cladding diameter of input fiber under the condition of the fundamental mode input, as shown in the Fig. 3. As can be seen from Fig. 3, as the cladding diameter of input fiber gradually decreases from 240 µm to 120 µm, the beam quality of combiner becomes better and transmission efficiency increases. However, when the core duty ratio of TFB increases to a certain extent, the beam quality of combiner deteriorates from the optimal condition, and transmission efficiency drops sharply. As can be seen from the simulation results, when the cladding diameter of input fiber is 140 µm, the beam quality factor is minimized as M2=3.33; when the cladding diameter of the input fiber is 120 µm, the transmission efficiency reaches the maximum of 98.28%.
Fig. 3. Beam quality and transmission efficiency of combiner with different cladding diameter of input fiber
In practice, the output mode of the laser is not only the fundamental mode, but also some high-order modes. We obtain the M2 factor corresponding to the different proportions of LP01 and LP11 by calculation, as shown in the Table 1.
Table 1. The M2 factor corresponding to the different proportions of LP01 and LP11
In order to explore the influence of higher-order modes in input beam on performance parameters of output beam, we make a simulation calculation when the cladding diameter of the input fiber is 120 µm, and the result is represented in Fig. 4. As can be seen from Fig. 4, the beam quality of input beam has an insignificant influence on the beam quality and transmission efficiency of output beam. This simulation indicates that the beam quality and transmission efficiency of combiner’s output beam can be improved as long as the input laser with good beam quality is utilized.
Fig. 4. The influence of the input beam quality on the output beam quality and transmission efficiency of combiner
In addition to the core duty ratio of TFB, the core diameter of output fiber also has an effect on output beam quality of fiber signal combiner. Therefore, we simulate the beam quality and transmission efficiency of combiner corresponding to the diameter of output fiber core in the range of 30-60 µm when the cladding diameter of the input fiber is 120 µm, as shown in Fig. 5. It can be seen from Fig. 5 that the transmission efficiency of combiner increases with the increase of the core diameter of output fiber, but the beam quality deteriorates accordingly. In this case, the core diameter of output fiber of 50 µm is chose to achieve both good beam quality and high transmission efficiency for the signal combiner.
Fig. 5. Beam quality and transmission efficiency of combiner with different core diameter of output fiber
Due to the inevitable inaccuracy of the fabrication process, the diameter of TFB cannot be precisely controlled in 50 µm. Therefore, the influence of TFB’s diameter on the beam quality and transmission efficiency of combiner is calculated under the condition that the cladding diameter of the input fiber is 120 µm, as shown in Fig. 6. It is evidence in Fig. 6 that when the diameter of TFB is greater than 45 µm, the diameter of TFB has little effect on the beam quality and transmission efficiency of the fiber signal combiner.
All the above simulation discussions are calculated under the condition that the center of TFB and output fiber are aligned. However, in practice, the center of TFB and output fiber may not be completely aligned in the splicing process due to the small deviation caused in the tapering. L is defined as the deviation distance between the center of TFB and output fiber. Therefore, the influence of the deviation distance on the beam quality and transmission efficiency of combiner is simulated and calculated when the diameter of TFB is 45 µm and the diameter of input fiber is 120µm, and the results are showed in Fig. 7. As can be seen from Fig. 7, when the center of TFB and output fiber deviate by more than 6 µm, the transmission efficiency and beam quality of combiner decrease significantly. Combined with the specific size parameters of combiner and the above simulation results, it can be seen that when the two centers deviate by more than 6 µm, the light has been transmitted into the cladding of the output fiber, leading to a degraded beam quality and a low transmission efficiency. Subsequently, the light transmitting in the fiber cladding can cause serious heating of the coating layer of the output fiber, which affects the overall performance of combiner.
Fig. 7. Beam quality and transmission efficiency of combiner with different deviation distance between the center of TFB and output fiber
3. Fabrication
The fiber of 50/400 µm (NA=0.12) is selected as the output fiber of the target 4×1 fiber signal combiner and the core diameter of input fiber is 20 µm. According to the result of simulation, the cladding diameter of the input fiber needs to be 120 µm to achieve high beam quality of the target combiner. Etching the fiber with hydrofluoric acid only reduces the thickness of the cladding and does not affect the size and structure of fiber core. Therefore, as shown in the Fig. 8, the 20/400 µm input fiber is etched to 120 µm with hydrofluoric acid.
We choose the tubing method to fabricate the fiber signal combiner. Fluorine-doped glass tube with inner and outer diameters of 800/1500 µm is adopted because compared with other ordinary glass tubes, this kind of glass tube has greater toughness and is not easy to break, which is conducive to reducing the difficulty of operation, and it is so thick that it is good for cleave.
First, the glass tube is washed in ethyl alcohol and then ethyl alcohol in the tube is dried. Next, the glass tube is first tapered using Vytran GPX-3400. The theoretical value of the diameter of the circumferential circle composed of four fibers with a cladding diameter of 120 µm is 290 µm. Considering the slight error of the tapering process, the outer diameter of the glass tube is tapered to about 555µm and then four etched input fibers are inserted into the tube to form a fiber bundle. Then, after the fiber bundle is tapered to form the TFB with the cone region’s length of 15 mm and the waist length of 5 mm, it is cleaved at the waist of the TFB. As shown in the Fig. 9, the diameter of the circumferential circle composed of the input fibers in the glass tube at the cleaving place is slightly less than 50 µm.
An area about 2 cm in length at one end of the output fiber is etched to 125 µm by hydrofluoric acid to match the diameter of the TFB. As shown in Fig. 10, an area about 2 cm in length behind the etched thinning area is stripped in order to make the cladding surface of the output fiber rough enough to effectively strip out the cladding light in the output fiber [11]. The cleaved TFB is spliced with the prepared output fiber to fabricate the desired fiber signal combiner. Figure 11(a) shows the image after alignment of TFB and output fiber, while Fig. 11(b) shows the image of the splicing point.
After the fiber signal combiner is made, it is fixed in a metal block with a groove, above which is a metal cover plate. The circulating water at 25°C flows over the side of the metal block to cool the combiner while it is being used. The physical image of the metal packaging is shown in Fig. 12.
4. Experiment results
As shown in Fig. 13, four Yb-doped fiber lasers with a maximum output power of more than 3 kW are used to test the characteristics of power handling ability and beam quality of the combiner.
The central wavelength of the laser is 1080 nm, and the core diameter of its output fiber is 20 µm. As shown in Fig. 14, the beam quality factor M2 measured by Primes LQM+200 system is about 1.27 when the fiber laser is directly fused to 20/400 µm end cap. It is noted that the algorithm that uses the radius of the circle containing 86% of the beam energy to define the beam width is adopted in the measurement of beam quality.
The test result shows that when the total input power is 12.5 kW, the total output power is 12.03 kW (as shown in Fig. 15), corresponding to the efficiency of 96%. The curve drawn in Fig. 16 shows the relationship between the total input power and the total output power of the combiner. At this point, the thermal imager detects that the temperature of the fiber signal combiner is 43°C (ambient temperature is 25°C), so the temperature rise of the fiber signal combiner is 1.5°C/kW. Figure 17 shows the beam quality of combiner is measured as M2=4.03 when the total output power is 12.03 kW. There is a certain gap between the measured beam quality and the simulation result M2=3.33, so is the transmission efficiency. The main reasons for the gap between the experiment and simulation are as follows: firstly, the simulation results are obtained under the ideal condition of only the fundamental mode input but lasers used in the experiment contains a small amount of high-order modes, which will cause certain degradation of the beam quality; secondly, in the manufacturing process of fiber signal combiner, tapering, cleaving and splicing will inevitably introduce a small amount of loss, and at the same time lead to the degradation of the beam quality. Therefore, the production process of fiber signal combiner can still be optimized to reduce the loss and improve carrying power and output beam quality of combiner.
5. Summary
We report a 4×1 fiber signal combiner fabricated by etching the input fibers. The combiner can carry an output power of 12.03 kW with the transmission efficiency of 96%, and the beam quality is measured to be M2=4.03. It can be concluded from the result that it is feasible to improve the beam quality of fiber signal combiner by increasing the core duty ratio of TFB base on the etched input fiber technology.
Funding
Outstanding Youth Science Fund Project of Hunan Province Natural Science Foundation (2019JJ20023); National Natural Science Foundation of China (11974427).
Acknowledgments
The authors would like to thank Xie Yie, Sun Yangmei, Liu Peng, and Jiang Lianzhou for the sincere help and support on the fabrication and test.
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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