A fiber optic combiner is a type of fiber optic connection device. Through precise fiber fusion technology, it maximizes the coupling of optical energy output from the transmitting fiber into the receiving fiber, and minimizes the impact on the system caused by its insertion into the optical path. Fiber optic combiners are important components in fiber laser systems. Their quality not only directly determines the power level and beam quality of fiber lasers but also serves as a crucial guarantee for the safe and stable operation of lasers. The earliest multimode fiber combiner was proposed by IPG in the United States. Beam splitters/couplers redistribute the signal energy in one fiber into different fibers according to characteristics such as wavelength and polarization. The function of a coupler is to convert optical signals into electrical signals, enabling conduction between the fiber ports of two fiber end sections. So, what are the types of fiber optic combiners? What is the research progress at home and abroad in recent years? What are the technical difficulties in key processes during the manufacturing process? Classification of fiber optic combiners According to their functional use, fiber optic combiners can be divided into two main categories: power combiners and pump combiners. (1) Pump combiners mainly combine multiple paths of pump light into a single fiber for output, primarily used to increase pump power. (2) Power combiners combine multiple paths of single-mode laser into a single fiber for output, used to increase the output power of the laser. According to their structural composition, fiber optic combiners can be divided into two types: N×1 fiber combiners that do not contain a signal fiber, and (N+1)×1 fiber combiners that include a signal fiber. Unlike N×1 fiber combiners, the central fiber in (N+1)×1 fiber combiners is a signal fiber. During manufacturing, N fibers must be tightly and symmetrically arranged around the signal fiber, with the central signal fiber used for inputting signal light. N×1 combiners can be both power combiners and pump combiners, with their function depending on the type of N input fibers. If all N fibers are single-mode fibers or large-mode-area fibers, they can be directly connected to N lasers to increase the laser output power, thus functioning as power combiners. If all N fibers are multimode fibers, they are connected to N pump sources to increase the pump power of the laser, thus functioning as pump combiners. (N+1)×1 combiners are all pump combiners, mainly used in fiber amplification systems. The central single-mode fiber in such combiners is the signal fiber, used for transmitting signal light, while the surrounding N multimode fibers are pump fibers, used for transmitting pump light. These combiners are typically used in MOPA structures. Side-pump combiners have a central signal fiber with a single-mode or quasi-single-mode waveguide core for laser transmission, and six surrounding fibers as pump fibers for pump light transmission. The seven fibers are neatly arranged, fused, tapered, and then spliced with the output double-clad fiber. End-pump fiber combiners The difference between side-pump combiners and end-pump combiners is that the pump fibers of side-pump combiners are tapered and attached to the cladding of the signal fiber, while the signal fiber itself is not fused or tapered. Therefore, in principle, the signal transmission of side-pump combiners is superior to that of end-pump combiners. Research progress at home and abroad Due to the important role of fiber power combiners in achieving high-power fiber lasers, many domestic and foreign institutions have conducted relevant research. The main research progress is summarized as follows: The earliest report on fiber power combiners appeared in the patent for IPG's 20 kW high-power fiber laser system. In this system, fiber power combiners were mainly used to combine multiple single-mode fiber lasers, thereby increasing the pump light power of the laser oscillator, as shown in Figure 3. The patent pointed out that by using high-brightness single-mode fibers as input lasers, the pump laser output by the fiber power combiner can achieve few-mode output with a beam quality of M²<8, and ideally even M²<4. Compared with LD-pumped lasers, the pump laser achieved by power combiners has significantly improved power and brightness, providing pumping conditions for single-fiber lasers to achieve higher power. Based on this, IPG achieved 10.5 kW and 20 kW few-mode single-fiber laser outputs in 2009 and 2013, respectively. In 2010, Yariv Shamir and others from Tel Aviv University in Israel conducted theoretical and experimental research on fused-taper fiber power combiners. The team systematically studied the output beam quality of fiber power combiners under adiabatic tapering conditions. Theoretical analysis results indicated that for 3 input fibers, the ideal beam quality of the combined laser is M²≈2.4, while for 7 input fibers, the ideal beam quality is M²≈3.5. In experiments, the team produced fiber power combiners without output fibers and with graded-index fibers as output fibers using the twisting method, respectively, and conducted laser combining experiments. The results obtained were relatively close to the theoretical values. In 2011, Noordegraaf and others from the Technical University of Denmark reported a 7×1 fiber power combiner made using the sleeve method. They used a low-refractive-index glass tube to fuse and taper 7 single-mode fibers, then spliced them with a multimode fiber with a core diameter of 100 μm. The maximum output power achieved was 2.54 kW, and the beam quality of the output laser measured at 600 W output power was M²≈6.5. In the same year, JDSU in the United States used a fiber power combiner to combine 7 lasers with a power of 600 W each, ultimately obtaining a laser with an output power of 4.2 kW. The input fibers used in the combiner had a core diameter of 20 μm and a numerical aperture of NA=0.08, while the output fibers had a core diameter of 50-100 μm. The experiment obtained a beam parameter product (BPP) of 2.5 mm·mrad for the output combined laser, corresponding to a beam quality of M²≈7.3. In 2014, the University of Jena in Germany produced 7×1 fiber power combiners using two schemes based on the sleeve method. Using output fibers with a core diameter of 50 μm, they achieved combined laser output greater than 5 kW, with measured beam qualities of M²≈6.5 and M²≈4.6, respectively. The first scheme designed a low-refractive-index glass tube outside the cladding of the input fibers to constrain the laser, and the combiner was structurally tapered twice: the first tapering combined the input laser into a multimode fiber with a core diameter of 100 μm, and the second tapering tapered the multimode fiber to couple the laser into an output fiber with a core diameter of 50 μm. The second scheme did not treat the input fibers with a low-refractive-index glass tube but directly inserted the input fibers into a low-refractive-index glass tube and performed only one tapering to combine the input laser into an output fiber with a core diameter of 50 μm. Comparing the results of the two schemes, it was found that the second scheme was significantly superior to the first in both coupling efficiency and beam quality. In recent years, relevant domestic institutions have also carried out extensive research on fiber power combiners, such as Wuhan Raycus, Tsinghua University, and National University of Defense Technology, all of which have reported patents or achievements. In 2012, Yan Dapeng and others from Wuhan Raycus used 4 fiber laser modules with an output power of 1100 W (20/400 μm) and a 4×1 fiber power combiner to combine lasers, achieving a 4 kW fiber laser output with a 50/400 μm output fiber. However, no information on beam quality was provided in the report. The National University of Defense Technology has conducted extensive research on fiber power combiners, established a research and development platform for high-power fiber devices, and successfully produced fiber power combiners and fiber end caps capable of withstanding high power. In 2015, they developed a 7×1 fiber power combiner with input fibers of 20/400 and an output fiber core diameter of 100 μm (structure shown in Figure 8), with an efficiency of over 98%, a power handling capacity of 6.08 kW, and a beam quality M²=10. After further increasing the output power of the 7 input fiber lasers, they achieved a combined fiber laser output of 12 kW with a 100 μm output fiber in 2016, and realized stable light output for a long time, taking an important step in the industrialization of domestic industrial high-power fiber lasers. Industrial multimode fiber lasers all use fiber power combiners to achieve high-power laser output, and they have low requirements for the beam quality of the combined fiber laser. Therefore, the core diameter of the output fiber is generally large, typically 100-300 μm, and some even 600-1000 μm. The larger the core diameter of the output fiber, the smaller the tapering ratio for fusion bundling, which relatively reduces the manufacturing difficulty of fiber power combiners. Currently, such fiber power combiners used in industry are relatively mature. To achieve high-power laser combining output while maintaining high beam quality, the core diameter of the output fiber must be reduced. Due to the larger tapering ratio, the manufacturing difficulty of the fiber combiner increases. For fiber power combiners with a reduced output fiber core diameter, based on improved manufacturing technology, the research team developed a 7×1 fiber power combiner with input fibers of 20/400 and an output fiber core diameter of 50 μm in 2016. The experiment achieved an output power of 6.26 kW, a combining efficiency greater than 98%, and high beam quality laser combining with M²=4.3. Based on the fiber power combiners developed in the laboratory, high-power combining of 1018 nm fiber lasers for co-band pumping was also achieved. Using 50 μm and 100 μm output fibers, laser combining greater than 2000 W was realized, providing a pump source with extremely high brightness for achieving single-fiber high-power fiber lasers. In addition to fiber laser combining, supercontinuum light source combining was also achieved. Based on 3×1 and 7×1 power combiners, 200 W and 700 W supercontinuum combining were realized, breaking the limit of achieving high-power supercontinuum light sources with a single fiber. In addition, fiber end caps are high-power devices designed for processing the output end face of high-power fiber lasers and amplifiers. They reduce the optical power density at the output end by expanding the beam of the output fiber, protecting the fiber end face from damage. At the same time, the output surface of the glass cone rod is coated with an anti-reflection film to avoid backlight affecting the laser or amplifier, ultimately achieving safe output of high-power fiber lasers. Currently, the laboratory has established an experimental platform for manufacturing fiber end caps, capable of achieving high-strength, low-loss fusion between any glass cone rod and fiber. The end caps used in fiber power combiner testing experiments are independently developed by the laboratory and have shown good performance when carrying 10,000 watts or more. Key process technology difficulties The basic structure of a power combiner mainly includes three parts: input fibers, fused-taper fiber bundle, and output fiber. Basic structure of power combiner First, to ensure that the fused-taper fiber bundle can be well spliced with the output fiber after fusion tapering, the cross-section of the fiber bundle must be circular, and the pump fibers must be tightly arranged in a certain geometric pattern, usually a regular hexagonal pattern. During manufacturing, the input fibers are first bundled, then the bundled input fiber bundle is fused and tapered to form a fused-taper fiber bundle. The waist of the fused-taper fiber bundle is then cut and spliced with the output fiber. Finally, a suitable packaging and heat dissipation structure is designed to ensure the combiner can work stably for a long time. Metal copper or aluminum with high thermal conductivity is usually used as the packaging and heat dissipation shell, and if necessary, a water-cooling structure is designed on the metal package. Fiber lasers use fusion splicing to connect fiber devices. To achieve higher power indicators for lasers, high-quality fiber fusion is very important. During fiber fusion, losses are inevitably generated, which will continuously accumulate light and heat during laser operation, potentially leading to degradation of beam quality or damage to optical devices. Raycus Laser has adopted a unique fusion joint thermal management technology, overcome the technical challenge of high-power thermal balance, and through sufficient thermal management simulation optimization and innovative water-cooling design, can ensure the long-term stable operation of the laser. The manufacturing process of fiber power combiners mainly includes four steps: bundling, fusing, and tapering of multiple fibers; cutting of the bundled and tapered fibers; splicing with the output fiber; and packaging of the combiner. Each step is crucial for completing a high-efficiency fiber power combiner capable of carrying high power. The main process difficulties are: (1) Tight arrangement of multiple fibers with high core duty ratio The tight arrangement of multiple fibers is a prerequisite for bundling, fusing, and tapering. Currently, there are two main methods for tightly arranging multiple fibers: the twisting method and the sleeve method. The twisting method is also the mainstream method for manufacturing end-pump couplers, mainly using a 7-hole or 19-hole tube for spatial positioning of fibers, then twisting to achieve tight arrangement. The sleeve method uses a glass tube as a constraint fixture for multiple fibers to achieve regular arrangement. In addition, since the most commonly used kilowatt-level fiber lasers on the market currently have output fibers mainly of 20/400 type, with a cladding diameter of 400 μm, directly bundling 7 20/400 fibers will result in a very small core duty ratio. If input fibers are to be coupled into the output fiber, the tapering ratio of the bundled fibers will be very large, increasing the difficulty of tapering. How to increase the core duty ratio of large-diameter cladding fibers, i.e., how to reduce the cladding diameter, is a key process for realizing fiber power combiners. Currently, the commonly used method is strong acid etching, and ensuring the smoothness of the fiber surface after strong acid etching and controlling the taper are the first processes that need to be broken through. (2) Ultra-low loss, arbitrary diameter tapering technology for bundled fibers After multiple fibers are tightly arranged, the bundled fibers are fused and tapered. Currently, fusion tapering equipment is relatively mature, and heat sources are diverse, including hydrogen-oxygen flames, electrodes, graphite wires, and the newly introduced carbon dioxide lasers. Since the core diameter of the output fiber is generally small, large-ratio tapering of the bundled fibers is required during tapering, which requires optimizing tapering parameters to achieve ultra-low loss, arbitrary diameter fusion tapering of bundled fibers. After tapering, cutting the bundled and tapered fibers is also a key technology, requiring extensive research to select suitable cutting knives and strictly optimize parameters to achieve high-standard cutting of bundled and tapered fibers. (3) Low-loss splicing technology between cut bundled tapered fibers and output fibers After the bundled tapered fibers are cut, splicing with the output fiber is the most critical step in manufacturing the combiner. The level of splicing loss directly determines the efficiency and power handling capacity of the fiber power combiner. It is necessary to optimize splicing parameters or reduce the cladding diameter of the output fiber through the aforementioned etching technology to achieve ultra-high-quality splicing between the two. (4) Efficient cladding light stripping technology for output fibers After the combiner is manufactured, some light in the output fiber is more or less coupled into the cladding. When the cladding light enters the output end cap, it will diverge to the edge of the end cap and be converted into heat, causing the end cap temperature to rise sharply. Therefore, effective cladding light stripping of the output fiber of the combiner is required. In summary, after the fiber power combiner is manufactured, it needs to be packaged for heat dissipation protection. Although the efficiency of power combiners is generally over 98%, when carrying high power, part of the lost light is converted into heat, causing the combiner to heat up. Therefore, how to achieve efficient cooling and packaging of the combiner to turn it into a mature device is also a relatively critical process technology. Summary From the development history of fiber power combiners, their manufacturing methods are mainly divided into two categories: the twisting method and the sleeve method. Early fiber power combiners were mainly manufactured based on the twisting method, but the reported output powers were not very high. In recent years, fiber power combiners generally adopt the sleeve method combined with low-refractive-index glass tubes. From the reported results, fiber power combiners manufactured based on the sleeve method have greater advantages in high-power handling capacity. From the perspective of beam quality, to improve the beam quality of the output laser of fiber power combiners, the number of modes in the output fiber must be reduced. This goal can be achieved in two ways: one is to reduce the number of modes supported by the output fiber itself, i.e., reduce the core diameter or numerical aperture of the output fiber, but this may reduce the transmission efficiency of the combiner and weaken its power handling capacity. The other is to selectively excite the output modes in the output fiber. On the one hand, active phase control can be used to select modes, i.e., coherent combining is adopted to improve the beam quality of the output laser. On the other hand, mode selection can be achieved by optimizing the structure of the combiner, mainly including reducing the number of modes excited by input fibers and improving the mode field matching between input and output fibers. In summary, fiber power combiners are core components for achieving high-power fiber lasers, but there are still many key technologies to be further broken through to achieve high-power, high-beam-quality combined laser output based on fiber power combiners. Sources: Infrared and Laser Engineering, Fiber Laser, tiantian007t