Mid-infrared fiber lasers have important applications in fields such as gas sensing, medical diagnostics, and environmental monitoring. However, the output power of a single optical fiber is limited, necessitating the use of beam combining technology for power scaling. Conventional multimode fiber combiners inevitably excite higher-order modes during the combining process, leading to the degradation of beam quality (reduced brightness), which limits their use in high-brightness application scenarios. Although fluoride and chalcogenide glasses possess mid-infrared transmission capabilities, they suffer from issues such as poor thermal stability, low mechanical strength, and deliquescence. Tellurite glass, featuring both a broad infrared transmission window and excellent thermochemical stability, is a promising alternative material.

       To address these issues, this work proposes and fabricates a 3×1 double-tapered multimode combiner based on tellurite fiber, with its structure illustrated in Fig. 1. The device utilizes a secondary adiabatic taper structure to filter out higher-order modes by gradually reducing the normalized frequency (V) , thereby recovering the beam quality. Based on Beam Propagation Method (BPM) simulations, the taper parameters (taper ratio of 1.5, taper length of 15 mm) were optimized, and an all-fiber device was fabricated using tellurite glass with good thermal stability. Differential scanning calorimetry (DSC) analysis of the glass shows that its thermal stability parameter ∆T exceeds 130°C, providing a broad processing window for the tapering process. Experimental results indicate that under single-port excitation at 1960 nm, the combiner achieves a transmission efficiency of 86.1%, corresponding to a total insertion loss of approximately 0.66 dB. Loss composition analysis reveals that the splice point loss (~0.30 dB) is the primary contributing factor, whereas the additional loss introduced by the double-tapered structure is only about 0.19 dB. Compared to the conventional single-taper structure, the double-tapered structure improves the beam quality M2 from 15.9/13.9 to 11.6/10.2 (an improvement of ~26.8%), yielding a brightness recovery ratio (BRR) of ~1.8. Figure 2 summarizes four key metrics: beam quality evolution, transmission efficiency, loss composition, and brightness recovery ratio, comprehensively demonstrating the brightness recovery effect of the double-tapered structure and verifying the effectiveness of the secondary taper as a spatial mode filter. In the multi-wavelength combining demonstration in the 2-3 µm band, three lasers at 1960, 2120, and 2770 nm were employed as light sources. The transmission efficiencies at 1960 nm and 2120 nm are 86.1% and 85.5%, respectively, while the efficiency at 2770 nm is 66.2%, primarily limited by residual OH absorption. With a total input power of approximately 4.5 W, the combiner delivers a total output power of ~3.6 W, achieving a combining efficiency of 80.1%. Thermal characteristic testing demonstrates that at the maximum output power, the highest surface temperature of the device is only 57.8°C, and the temperature increases linearly with power without any thermal runaway. Thermograms show that the hot spot is concentrated at the splice location, as shown in Fig. 3. This work verifies the feasibility of realizing brightness recovery in soft glass fibers using a double-tapered structure, providing a compact and robust beam combining solution for high-brightness mid-infrared all-fiber laser systems.

Fig. 1. Schematic of the double-tapered tellurite fiber combiner. Top: 3D longitudinal view. Bottom: Corresponding cross-sections at the Bundle, TFB, ST-FC, and DT-FC.

Fig. 2. Systematic analysis of the DT-FC performance. (a) Evolution of beam quality and mode profiles throughout the fabrication process. (b) Transmission efficiency. (c) Loss contribution analysis. (d) BRR of the DT-FC relative to the ST-FC.

Fig. 3. Power scaling and thermal performance of the tellurite DT-FC. (a) Output versus input power for individual wavelengths and combined operation. (b) Maximum device temperature versus total output power. Inset: Thermal image at maximum load (~3.6 W).