Transverse mode instabilities in high power fiber amplifiers: a linear stability analysis

Area of Science:

• Fiber optics
• Laser physics
• Nonlinear optics

Background:

• Transverse modal instability (TMI) limits power scaling in fiber amplifiers.
• Existing models like stimulated thermal Rayleigh scattering (STRS) do not fully capture TMI dynamics.

Purpose of the Study:

• To develop a theoretical framework for analyzing TMI in fiber amplifiers.
• To incorporate coupled effects of population inversion and thermal refractive index changes.
• To identify distinct TMI regimes and the underlying physical mechanisms.

Main Methods:

• Analysis of steady-state fundamental mode amplification stability under transverse perturbations.
• Incorporation of population inversion dynamics and thermally induced refractive index changes.
• Derivation of analytical expressions for TMI power threshold and characteristic frequency.

Main Results:

• Identified two TMI regimes: population-inversion-dominated (low power, small core) and thermally dominated (high power).
• Showed TMI onset is linked to a nonlinear phase-matching condition (weak-wave retardation).
• Derived compact analytical expressions for TMI threshold and frequency based on fiber parameters.

Conclusions:

• The framework provides a unified and intuitive description of TMI.
• Offers new insights into the power-scaling limitations of high-power fiber systems.
• Connects TMI onset to fundamental nonlinear optical phenomena.