Direct experimental observation of stimulated thermal Rayleigh scattering with polarization modes in a fiber amplifier

Modal interference can lead to intensity modulations in optical fibers, which can produce refractive index gratings under the influence of quantum defect heating in a fiber laser. These gratings are perfectly phased matched for mode couplings, which can lead to transverse mode instabilities at high average powers in fiber lasers. A detailed understanding of this process is critical for further power scaling of fiber lasers. We have directly observed and characterized this quantum-defect-assisted mode coupling for the first time using polarization modes in a PM fiber amplifier, providing solid experimental evidence for this key mechanism for transverse mode instability in fiber lasers.