Abstract

High power fiber lasers (HPFLs) have shown record single-moded diffraction-limited output powers in the range of 20kW [1]. Output power limits are known to be set by the launched pump power, nonlinear effects (such as stimulated Raman (SRS) or Brillouin (SBS) scattering), optical damage, glass rapture or melting, and thermal lensing (TL) [2]. More recently, transverse mode instability (TMI) is identified as an additional severely power limiting effect [3] and a simple threshold formula was derived [4]. Although TL and TMI are both of thermal origin and show the same functional dependence on (λ₀/d₀)², where λ₀ is the signal wavelength and d₀ the core diameter [2,4], TMI can be detrimental for the output beam quality, while TL results in shrinkage of the initial (“cold”) mode field diameter (MFD) ω₀ [2] and therefore limits the output power scalability indirectly (e.g. by reducing the nonlinear thresholds). The TL power threshold has been set arbitrarily to the level that results in ω_TL≈0.7-0.8ω₀ [2], and therefore its impact on power scalability is somewhat artificial.

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