Low-threshold mode instability in Yb3+-doped few-mode fiber amplifiers

Area of Science:

• Optics and Photonics
• Materials Science

Background:

• Few-mode fiber amplifiers are crucial for high-power laser systems.
• Nonlinear effects in optical fibers can limit amplifier performance.

Purpose of the Study:

• Investigate spatio-temporal instability in Ytterbium-doped few-mode fiber amplifiers.
• Analyze the nonlinear power transformation of the fundamental mode.

Main Methods:

• Experimental measurements of instability in Yb(3+)-doped PM fiber amplifiers.
• Theoretical analysis and numerical simulations of mode transformation.
• Investigated the role of population and electronic index gratings.

Main Results:

• Spatio-temporal instability observed in 8.5 μm core diameter Yb(3+)-doped few-mode PM fiber amplifiers (2-30 W pump power).
• Nonlinear power transformation of LP(01) fundamental mode to higher-order modes confirmed.
• Self-consistent growth of higher-order modes and traveling electronic index gratings observed.
• Instability threshold increases with signal frequency bandwidth.

Conclusions:

• Nonlinear mode transformation is a key factor in Yb(3+)-doped few-mode fiber amplifier instability.
• Understanding these instabilities is vital for optimizing high-power fiber laser design.
• Signal bandwidth management can potentially mitigate instability issues.