Transverse Mode Instability in Raman Fiber Amplifiers

Hybrid ytterbium-Raman fiber amplifiers have reached multi-kW levels in recent years, showing great promise for kW powers at new wavelengths. Many applications need diffraction-limited output which is limited by transverse mode instability (TMI). In this work, we extended our 3D amplifier model for studying TMI in ytterbium fiber amplifiers to include Raman gain and used this new model to study TMI in two Raman fiber amplifiers: a hybrid ytterbium-Raman fiber amplifier and a passive Raman fiber amplifier. Our previous study shows that gain saturation effect in ytterbium fiber amplifiers plays a significant role in suppressing TMI in kW ytterbium fiber amplifiers. This new study shows that in a Raman fiber amplifier in the absence of similar connection between local Raman gain and local Stokes wave, TMI is not much affected by seed power, fiber length or core diameter in the absence of mode-dependent loss. Mode-dependent loss is a key factor for increasing the TMI threshold in this case.