Fig. 1. Schematic illustration of a thermally expanded core fiber.

Fig. 2. Refractive index profile by the thermal diffusion of the dopant. Initial LMAF’s parameters are a = 7.5 μm, ncl= 1.4495, and Δn =0.0022.

Fig. 3. Enlargement of the LP01 MFDs at λ = 1.06 μm by the thermal diffusion of the dopant for SMFs and LMAFs.

Fig. 4. MFDs after the TEC treatment versus the NAs of the initial fibers.

Fig. 5. MFD after the TEC treatment versus the core diameter of the initial fibers.

Fig. 6. Distribution of LP01 mode field in the MFA.

Fig. 7. Transmission efficiency of the MFA versus the length of the heating region.

Fig. 8. Micrographs of the TEC fiber cross-section. (a) Initial SMF 6/125; (b) TEC SMF 6/125; (c) initial LMAF 15/130; (d) TEC LMAF 15/130.

Fig. 9. Schematic illustration measuring the performance of the MFA.

Fig. 10. Transmission loss of the MFA with TEC treatment.