Fig. 1. Schematic diagram for explaining the relationships between output mode beating and mode coupling in the cascade fiber link. (a) Mode components generated by the mode crosstalk of fiber splicing, which shows the different origins of high-order modes at the output end. (b) Mode beating frequency for the discrete excitation of the LP₀₁ and LP₁₁ modes in monolithic fiber. (c) Mode beating frequencies for the cases of the cascade fiber link.

Fig. 2. Experimental setup for mode coupling diagnosis in the high-power fiber delivery system. (a) Schematical diagram of the cascade fiber link. (b) Mode diagnosis module for the real-time measurement of beam quality and inner mode coupling.

Fig. 3. Characterization of the high-power fiber delivery system. (a) Measured beam quality M² at the highest transmission power. (b) Theoretical calculations of the output passive fibers for the normalized frequency, effective refractive index and bending loss at different wavelengths. (c) Mode diagnosis for tracing the mode coupling origins in the fiber delivery system. (d) Theoretical calculations of mode beating frequencies for the cases of monolithic fiber and cascade fiber, respectively.

Fig. 4. Customized manipulations of the different origins of mode coupling. (a), (b) Mode diagnosis results with tight bending applied on the output pigtail fiber of the MFA or the output passive fiber. (c), (d) Effects of rearrangement or vertical stress on mode coupling origins.

Fig. 5. Determination of critical bending condition of the output passive fiber. (a) Mode diagnosis results with different bending radii for the output passive fiber. (b) Power records and MPI evolutions at different bending radii.

Fig. 6. Output properties of the high-power delivery system. (a) Beam quality factor M² measured at the output space. (b) Mode diagnosis results with the critical bending of 10 coils applied on the output passive fiber. (c) Power evolutions of the output laser before and after mode diagnosis.