Transverse Mode Instability in High-Power Yb-Doped Double-Clad Fiber Amplifiers: A Three-Layer Optical–Thermal Analysis Based on Stimulated Thermal Rayleigh Scattering

Overview

Transverse mode instability in high-power ytterbium-doped double-clad fiber amplifiers is investigated through a coupled optical-thermal model that accounts for stimulated thermal Rayleigh scattering. The study examines a forward-pumped continuous-wave fiber amplifier configuration operating at 976 nm pump and 1064 nm signal wavelengths over a 12-meter optimal fiber length. The research addresses limitations in prior analytical approaches by explicitly resolving the three radial regions of double-clad fiber geometry rather than applying single-clad approximations.

Methods and approach

The investigation employs a three-layer optical-thermal coupled model that resolves the core, inner cladding, and outer cladding regions of double-clad fiber. Modal field computations utilize the weakly guiding approximation in the core and semi-weakly guiding approximation at cladding interfaces to accurately determine higher-order mode penetration into the inner cladding. The nonlinear stimulated thermal Rayleigh scattering coupling coefficient is evaluated with explicit incorporation of gain saturation effects and thermal eigenmodes characteristic of the multi-layer geometry. Transverse eigenvalues U01 and Umn relevant to TMI are computed from the rigorous modal analysis.

Key Findings

The three-layer formulation reveals that the inner cladding structure modifies both optical and thermal mode distributions compared to single-clad approximations. The optical-thermal overlap between the fundamental LP01 mode and higher-order modes is altered by the multi-layer geometry, which consequently modifies the effective strength of the stimulated thermal Rayleigh scattering mechanism. The predicted TMI threshold is directly influenced by these structural modifications, demonstrating that neglecting inner cladding effects leads to incomplete characterization of thermo-optic dynamics in double-clad fiber amplifiers.

Implications

The coupled optical-thermal model provides a quantitative framework for analyzing thermo-optic instability mechanisms in ytterbium-doped double-clad fiber amplifiers with physical consistency. The findings indicate that accurate prediction of TMI thresholds requires explicit treatment of multi-layer cladding geometry rather than simplified single-clad models. This analysis supports the design and optimization of high-power fiber laser systems with enhanced modal stability by enabling more precise evaluation of the interplay between optical mode confinement and thermal distribution in realistic fiber geometries.