A mode-division multiplexing system based on few-mode fibers (FMFs) can significantly enhance information capacity. With the increasing demand for expanded channels and extended transmission distances, fiber fusion splicing between FMFs and devices has become increasingly important. Unlike ordinary single-mode fibers, FMFs have larger core sizes and can support multiple modes. When fusion splicing is not properly performed, various problems, including mode coupling and mode dispersion, may occur. Mode coupling transfers the mode energy carrying the signal to other modes within the fiber, decreasing the purity of the original mode, increasing interchannel crosstalk, and greatly affecting the communication quality of the system. To analyze quantitatively specific parameters during the fusion splicing of FMFs, we utilized off-axis digital holography to measure the purity of multiple high-order modes in the fibers. We optimized the arc time and relative arc power during fusion between the FMFs based on mode purity measurements.

This study evaluated the fusion quality between FMFs with fusion loss and mode purity variation. We used different arc times and relative arc powers to splice the two FMFs. We also employed a mode-selective coupler to generate high-order modes. We built an off-axis digital holographic interference system to measure the mode-purity variation after each fusion. The light source used in this study was a Santec tunable laser (MLS-2100) with a working wavelength of 1550 nm. The erbium-doped fiber amplifier (EDFA) is a self-made erbium ytterbium co-doped double-clad fiber amplifier, where a working power of 20 mW was used in the experiment. Both fiber port couplers use a PAF2-2B produced by Thorlabs, and the charge coupled device (CCD) uses an LD-SW6401715-UC-G short-wave infrared camera produced by the Liding Optoelectronic Technology Company. The fusion splicer used was the 80C, produced by Fujikura Company.

For the two types of FMFs with the same or approximately the same core radius within the error range, the effects of optimized arc time and relative arc power on the mode purity can be ignored. Suppose the standard arc power is I_s. For the same two FMFs, we find that when the relative arc power is I_s+25 bit and the arc time is 3100 ms, the purity variation of the high-order mode, excluding LP₀₂, is the least. Figure 5 and Table 1 show the results. We also conducted comparative experiments using the fusion splicer auto-fusion program, where the results are shown in Figure 6. Following fusion splicing, the purity decreases by 12.3127 percentage points, indicating that the automatic fusion program cannot effectively maintain the purity of the transmission mode inside the fiber during fusion splicing between FMFs. For the two types of FMFs with approximately the same radius, we find that the best fusion parameters are a relative arc power of I_s+32 bit and an arc time of 3000 ms, as shown in Figure 7 and Table 3. When significant differences are observed in the cores of the few-mode optical fibers, optimizing the arc time and intensity is insufficient to eliminate the effects of mode field mismatch. Particularly, when ring-core fibers are used to fuse with specific FMFs, the purity of the azimuthal modes is improved over that before fusion, with a maximum increase of 2.9137 percentage points.

This study evaluated the fusion quality of four types of FMFs under different fusion parameters based on mode purity measurements. We optimized the fusion parameters for the different FMFs with the same core radius. When the core radii of the FMFs are the same or approximately the same, using the optimized arc time and relative arc power can reduce the influence of the splicing on the mode purity. When appropriate fusion parameters are used, splicing ring-core fibers and FMFs can increase the purity of high-order modes, which may be due to the ring-core fibers filtering out the radial modes of the transmitted optical field in the FMFs. This method of optimizing FMF fusion parameters based on the proposed mode purity measurements is also applicable to the connection of FMF breakpoints, mode-generating devices, and transmission fibers in mode-division multiplexing systems under different fusion equipment.