Reduced-symmetry LMA rod-type fiber for enhanced higher-order mode delocalization

Area of Science:

• Optical Fiber Technology
• Laser Physics

Background:

• Large-mode-area (LMA) fibers are crucial for high-power fiber lasers and amplifiers.
• Modal instabilities, often driven by higher-order modes (HOMs), limit laser performance.
• Existing LMA fiber designs struggle to suppress HOMs effectively, especially LP11 modes.

Purpose of the Study:

• To present a novel micro-structured LMA asymmetric rod-type fiber design.
• To demonstrate enhanced preferential gain for the fundamental mode in active fibers.
• To suppress higher-order modes (HOMs) and improve modal instability thresholds.

Main Methods:

• Designing a fiber with reduced cladding symmetry using germanium-doped silica inclusions.
• Performing high-resolution spatially and spectrally (S²)-resolved mode analysis.
• Characterizing beam quality (M²) and effective mode area (Aeff).

Main Results:

• Achieved strong higher-order mode (HOM) delocalization.
• Confirmed HOM contributions below 1% and LP1m-like HOMs below the detection limit.
• Demonstrated single-mode operation with M²=1.3 and Aeff=2560 μm² at 1064 nm.

Conclusions:

• The proposed fiber design enables efficient single-mode operation in LMA fibers.
• This design offers a pathway to improve modal instability thresholds in high-power fiber lasers.
• The fiber's ability to suppress LP11 modes is key to enhancing laser performance and stability.