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Observation of visible light flashes in high power near infrared narrow linewidth polarization maintained fiber laser caused by Stimulated Brillouin Scattering triggered cascaded Raman Scattering

We report a surprising observation of visible light flashes in the fusion splice point between the seed stage to the power amplifier in a 500 W narrow-linewidth, polarisation-maintaining Ytterbium-doped fiber laser. The flashes were observed to have a strong correlation with the backward pulses generated due to the onset of Stimulated Brillouin Scattering (SBS) in the laser. We believe these visible flashes appear as a result of second harmonic generation of higher order Raman Stokes components of the 1064nm laser signal. The Raman components are generated by the back-propagating, high peak power SBS pulses at threshold. The Raman Stokes in the core of the fiber generates their respective second harmonic counterparts in the cladding through a Cherenkov-type phase matching process.