Thulium-doped fiber lasers (TDFLs) have undergone rapid development in recent years owing to their widespread use in medical treatment, laser radar, laser remote sensing, and military applications. The highest power achieved by TDFLs is in the kilowatt level; however, thermal effects limit their further increase in power. To overcome this challenge, large-mode-area fibers have been proposed. Recently, our group proposed a novel type of Tm³⁺-doped heterogeneous helical cladding fiber with an inner isolation ring (HHCRF) for large-mode-area operation, and the corresponding single mode core diameter is 60 μm. To maintain the single-mode waveguide characteristics of the HHCRF under high-power operating conditions, the effect of temperature on the refractive index should be considered when designing fiber parameters such that the designed fiber parameters are adaptable to high temperatures. Therefore, we herein present relevant design methods that are conducive to promoting the further development of related studies.

In this study, we investigate the three-dimensional laser power and temperature distributions of a novel heterogeneous helical cladding large-mode field fiber under dual-end pumping conditions. We obtain the three-dimensional distribution of refractive-index changes in the fiber and calculate the temperature adaptability of the optical fiber. The specific steps are as follows: First, without considering temperature variables, the fundamental-mode transmission conditions of the fiber were obtained using the COMSOL software. The initial structural parameters of the fiber were obtained, and a theoretical model of the fiber laser amplifier was established to obtain the distribution characteristics of the optical field and temperature field inside the fiber. Subsequently, the thermally induced refractive index change was calculated. Based on the new refractive-index distribution of the optical fibers, the single-mode transmission characteristics of the optical fibers were verified. If the single-mode transmission condition cannot be satisfied at this time, then the initial structural parameters of the fiber are changed and the procedure is repeated from the first step shown in Fig. 3. If the single-mode transmission condition is satisfied, then the fundamental and higher-order mode losses of the fiber under this condition can be obtained. However, the results obtained from the first round of calculations are unstable. This step must be repeated until the mode loss is relatively small compared with the mode loss obtained from the previous round of calculations. This result is assumed to reflect a stable operating state, and suitable fiber structure parameters can be obtained under this high-power operating condition.

In the calculations, bidirectional pumping was adopted, where a pump power of 100 W in the forward and backward directions and a seed optical power of 10 W were utilized. Water was used for cooling. When the fiber length L is set to 4 m, the signal-light output power is similar to the maximum value; thus, the length fiber was set to 4 m. The obtained signal-light output power is 75.02 W. The three-dimensional distribution of temperature inside the optical fiber and the thermally induced refractive index changes are shown in Figs. 7 and 8, respectively. The highest temperature T inside the optical fiber is 172 ℃, and the thermally induced refractive index change Δn is 0.001305. The axial loss distributions of the optical fibers under stable operating conditions are shown in Fig. 9. After five rounds of calculations, as shown in the flowchart in Fig. 3, the fiber appeared to be in a stable operating state. The rate of change in the loss values of the fundamental and higher-order modes in each round compared with that in the previous round is shown in Fig. 11. In the fifth round, the rate of change in the basic mode loss is -6.26959^-5, whereas that in the high-order mode loss is -3.45006^-5, thus indicating that the fiber has entered a stable operating state. At this point, the fundamental-mode loss is 0.159 dB/m and the higher-order mode loss is 6.550 dB/m. The fiber possesses single-mode transmission capability.

This study analyzes the effect of thermally induced refractive-index changes on single-mode transmission characteristics under high-power operation conditions based on a new type of thulium-doped heterogeneous helical cladding (HHC) large-mode area and single-mode fiber. A theoretical solution model was established to provide a solution for investigating the temperature adaptability of large-mode-field single-mode fibers. Using this method, optimized design parameters for a new type of HHC large-mode-field fiber were obtained: the cladding refractive index n₃ is 1.4380, the core refractive index n₀ is 1.4388, the central angle θ′ is 12°, and the isolation ring thickness d is 4 μm. This ensures that the HHC fiber can maintain single-mode operation even at a pump power of 100?300 W. The results of this study provide a theoretical reference for the design of large-mode-field single-mode optical fibers.