Mode dynamics in the amplification of a CW single-frequency laser seed in a multimode fiber taper

Spatial beam self-cleaning is one of the most intriguing nonlinear effects in multimode optical fibers: it involves the transformation of a speckled intensity pattern into a smooth beam with a bell-shaped profile at the fiber output. This effect was initially observed above some threshold value of peak power for sub-nanosecond pulses in passive multimode (MM) fibers with graded-index (GRIN) profile; a similar behavior is also observed with other types of fibers or with femtosecond pulse durations. For example, it was reported that a highly-multimode pulsed beam propagating in active (Yb-doped) MM GRIN fiber tapers experiences self-cleaning after amplification to peak powers of ⁓20 kW, with a beam quality parameter improved from ⁓10 to ⁓4. Here we study the amplification of a continuous-wave (CW) single-frequency radiation in a similar MM GRIN fiber taper. Surprisingly, the beam cleaning is observed at much lower output powers. The beam quality is improved to near-Gaussian (<1.5) after amplification to ⁓20 W of CW power. We study the dynamics of the output transverse modes by a real-time mode decomposition technique. This reveals a relatively slow (1 Hz range) and unusual evolution in the first few seconds, followed by a relaxation into a stationary state, consisting of a bright spot on a low-power speckled pedestal.