Mode instabilities in Yb:YAG crystalline fiber amplifiers

Area of Science:

• Laser Physics
• Materials Science
• Optical Engineering

Background:

• Mode instabilities (MI) limit the power scaling of fiber amplifiers.
• Yb:YAG (Ytterbium-doped Yttrium Aluminum Garnet) is a promising gain medium for high-power lasers.
• Understanding MI thresholds is crucial for designing robust high-power fiber laser systems.

Purpose of the Study:

• To numerically simulate and determine the mode instability (MI) threshold in Yb:YAG crystalline fiber amplifiers.
• To compare the MI threshold of Yb:YAG crystalline fibers with Yb-doped silica-glass fiber amplifiers.
• To investigate the underlying physical mechanisms contributing to the MI threshold in Yb:YAG crystalline fibers.

Main Methods:

• A full numerical model was employed, combining the finite-difference beam-propagation method with rate equations for signal field propagation.
• The time-dependent heat equation was solved using the alternating-direction-implicit method.
• An iterative method was used to achieve steady-state conditions for the Yb:YAG amplifier, accounting for temperature-dependent laser performance.

Main Results:

• Simulated MI thresholds in Yb:YAG crystalline fiber amplifiers were found to be at least 28 times higher than those in Yb-doped silica-glass fiber amplifiers.
• The MI threshold for Yb:YAG crystalline fibers can reach up to tens of kilowatts.
• Higher thermal conductivity, lower thermo-optic coefficient, and strong gain saturation were identified as key factors for the elevated MI threshold.

Conclusions:

• Yb:YAG crystalline fiber amplifiers exhibit significantly higher mode instability thresholds compared to conventional silica-glass fiber amplifiers.
• The superior thermal properties and strong gain saturation of Yb:YAG contribute to its enhanced resistance to mode instabilities.
• These findings suggest Yb:YAG crystalline fibers are a promising platform for developing ultra-high-power fiber laser systems.