Modeling of transient modal instability in fiber amplifiers

Area of Science:

• Optics and Photonics
• Laser Physics
• Thermal Engineering

Background:

• Fiber amplifiers are crucial for high-power laser systems.
• Transient modal instability can limit amplifier performance and reliability.
• Understanding the underlying physics is essential for designing stable amplifiers.

Purpose of the Study:

• To develop and present a comprehensive model for transient modal instability in fiber amplifiers.
• To investigate the relationship between output power and the onset and severity of modal instability.
• To characterize the key features of this instability.

Main Methods:

• Combined optical beam propagation method with local rate equation solutions for laser gain.
• Incorporated refractive index perturbations due to the thermo-optic effect.
• Utilized a time-dependent thermal solver with a quantum defect heating source term.

Main Results:

• The model successfully predicts transient modal instability at specific output power levels (241, 270, 287 W).
• Instability is characterized by power coupling to un-seeded modes and distinct stable/unstable regions within the fiber.
• Rapid intensity variations along the fiber and increased instability severity with higher power were observed.

Conclusions:

• The developed model provides a robust framework for analyzing transient modal instability in fiber amplifiers.
• Output power is a critical parameter influencing the onset and progression of modal instability.
• The findings are vital for optimizing the design and operation of high-power fiber amplifiers to ensure stability.