In recent years, mid-infrared lasers have caught increasing attention because of their significant applications in a number of fields such as defense security, environment monitoring, and medical surgery. They mainly include solid state laser, gas laser, optical parametric oscillator, quantum cascade laser (QCL), and fiber laser. Among them, since QCL features small size, lightweight, and ultra-wide wavelength coverage (3-13 μm commercially available currently), it is considered a promising compact and practical mid-infrared laser. However, the output power of a single QCL is limited to 10 W level. Laser beam combining technology is considered an effective way to significantly improve the output power of QCL. In this technology, a higher power level is achieved by superposing the output power of multiple lasers. Compared with spatial beam-combining technologies such as spectral beam combining and coherent beam combining, optical fiber beam-combining technology has the advantages of compact structure and good robustness, and is the preferred technology for improving the output power of QCL. Thus, we aim to develop a compact mid-infrared combiner for the power enhancement of QCL.

As-S chalcogenide glass is employed to fabricate the optical fiber combiner because of its excellent thermal stability against crystallization and relatively high laser damage threshold. The chemical compositions of the core and cladding glass are As₄₀S₆₀ and As₃₈S₆₂ respectively. The fabrication of 7×1 fiber combiner includes the preparation of high-purity glass, optical fiber, and capillary tube, fiber bundle assembling and tapering, taper zone cutting, and fiber combiner armoring. The As₄₀S₆₀ and As₃₈S₆₂ chalcogenide glasses are prepared in low-OH quartz tubes by the vacuum melt-quenching method. The As-S optical fiber is fabricated by the rod-in-tube method, the cladding tube is by the rotational method, and the As₃₈S₆₂ capillary tube is by the combination of extrusion and thermal-drawing methods. The fiber bundle tapering is conducted on a self-made longitudinal tapering system. First, seven fibers with a length of about 50 cm are cut out, and one end of the fiber (about 8 cm long) is immersed in dimethylacetamide (DMAC) solvent to dissolve the surface polymer. The polymer-free ends of the seven fibers are then inserted into the As₃₈S₆₂ capillary tube with a length of about 12 cm, and the capillary tube is glued with the fiber bundle using a high-temperature adhesive. Subsequently, the fiber bundle is placed into the tube furnace of the longitudinal tapering system, the fiber-free end of the capillary tube is connected to a vacuum pump to maintain lower pressure inside the capillary tube, and the fiber bundle is tapered at about 270 ℃. Finally, the tapered region of the fiber bundle is cut and the obtained fiber combiner (without fusing output fiber) is armored.

The fabricated As₄₀S₆₀/As₃₈S₆₂ fiber has a core diameter of 200 μm and a cladding diameter of 250 μm. It shows good transmission performance in the 2-6.5 μm with a background loss of about 0.5 dB/m. The losses at 3 μm and 4.6 μm are (0.56±0.04) dB/m and (0.63±0.05) dB/m respectively. Based on the fabricated fiber, the 7×1 fiber combiner is designed. The numerical simulation shows that the appropriate taper reduction ratio Ris 2-4, and the length of the taper transition zone should be more than 500 μm. Following the design, 7×1 fiber combiners with R of 3 and 4 are fabricated (Fig. 8). The taper transition zone is about 2 cm long. The cross-sectional images of the output end of the fiber combiners show that the fiber monofilaments are arranged in a good regular hexagonal shape, and the fiber bundles do not undergo significant deformation after being tapered. The measurements show that the port transmission efficiency ηof the fiber combiner is 90.7%-92.5% at 3 μm and 87.2%-90.8% at 4.6 μm when R=3, and it is 88.1%-91.4% and 85.1%-87.5% at 3 μm and 4.6 μm respectively when R=4 (Table 1).

We develop a 7×1 chalcogenide glass fiber combiner and investigate its mid-infrared transmission properties. The fiber combiner is formed by fusing and tapering an As₄₀S₆₀/As₃₈S₆₂ fiber bundle. The core and cladding diameters of the individual fiber are 200 μm and 250 μm respectively, with the numerical aperture of 0.38-0.35 (@ 2-6 μm). The taper ratio R of the final fiber combiner is 3 or 4, and the length of the taper transition zone is about 2 cm. The results show that when R=3, the port transmission efficiency of the fabricated fiber combiner at 3 μm and 4.6 μm is more than 90% and 87% respectively, and when R=4, it is more than 88% and 85% respectively. There is no obvious crosstalk between the fiber monofilament at the output end of the fiber combiner. The results indicate that the fabricated fiber combiner is an efficient laser combining device and is promising in mid-infrared laser power enhancement and wide spectrum synthesis.