Design and Preparation of Mid-infrared 7x1 Sulfide Fiber Combiner

As a core component of a fiber laser system, the fiber combiner not only directly determines the pump and output power of fiber lasers, but also serves as an important guarantee for the safe operation of the all-fiber laser system in a high-power environment. At the same time, the fiber combiner is simple and stable in structure, and not easy to be interfered with by the outside environment. It can realize the expansion of power and spectrum without a lot of optical components and free space optical devices. The fiber combiner that can transmit lights in the mid-infrared band has attracted much attention because of its wide application in national defense, military, scientific research and business. In addition, due to its significant design and preparation difficulties, only a few institutions currently master the relevant technology. As a very important part of mid-infrared photonic devices, it has become a research hotspot in this field all over the world. In this paper, in theory, the optical field distribution of multimode fiber combiner is analyzed and the power loss of several main modes in the fiber core is calculated. The mechanism of loss in the tapering process of fiber combiner is elucidated. The basic criteria of adiabatic tapering and brightness conservation are analyzed in detail, which lays a foundation for the design of high transmission efficiency and good beam quality combiners. Experimentally, starting with the preparation process and key technologies of mid-infrared sulfide fiber combiner device, the 7x1 sulfide fiber combiner device with high performance has been successfully developed after solving the key problems of the ordered arrangement, melting tapering, end cutting, homogeneity fusion and end angle polishing. The key performance indexes are tested and analyzed. In terms of testing and characterization, the transmission efficiency, beam quality, structural stability and power damage threshold of the 7x1 mid-infrared sulfide fiber combiner are tested by using mid-infrared light source, detector and beam quality analyzer. The average transmission efficiency of nearly 80%(@ 4.778 mu m)is obtained for different ports of the 7x1 sulfide fiber combiners. When the diameter of the output fiber core is about 350 mu m, the best M-x/y(2) value of the 7x1 sulfide fiber combiner is 19.63/22.48. The tensile tension at the fusion point is more than 300 g, which is better than similar structures. When the input laser power (@ 1.976 mu m) exceeds 10 W, the maximum output power of the 7x1 sulfide fiber combiner can reach 4.32 W. This paper provides some ideas for the processing of mid-infrared sulfide fiber and the fabrication of fiber combiner devices. On the one hand, mid-infrared fiber as the main body of mid-infrared fiber combiner, its performance largely determines the final performance of the fiber combiner. In recent years, mid-infrared single-mode fiber, multi-mode fiber and mid-infrared doped fiber with low loss have been prepared. However, compared with commercial quartz fiber, the preparation technology of high performance mid-infrared fiber is still immature, which is the main reason limiting the development of mid-infrared fiber combiners and other mid-infrared fiber devices. On the other hand, how to optimize the soft glass fiber processing platform and eliminate the influence of preparation process on device performance as much as possible is also a problem to be solved in the future.