Beam cleaning effect in multimode fiber tapered amplifier with CW single-frequency seed

Multimode optical fibers have attracted a lot of interest over the past decade due to their great potential for fiber lasers with enhanced power, as well as new nonlinear optical phenomena manifesting with the use of high-energy pulsed multimode beams. One of these phenomena, spatial self-cleaning is a most intriguing nonlinear effect involving the transformation of a speckled beam into a smooth beam with a bell-shaped profile at the multimode fiber output which is observed above some threshold value of peak power for sub-nanosecond pulses. The effect is usually observed in passive multimode fibers with graded-index (GRIN) profile, but other types of fibers may exhibit similar behavior. For example, in active (Yb-doped) multimode GRIN fiber taper a highly-multimode pulsed output beam observed at low power experiences self-cleaning after amplification to peak power of ~20 kW with quality parameter M2 improved from >10 to ~4. Here we study amplification of a continuous-wave (CW) single-frequency (SF) radiation in similar multimode GRIN fiber taper and demonstrate beam cleaning effect at much lower output power with beam quality improvement to near- Gaussian diffraction-limited beam (M2<1.5). A detailed characterization of the output beam has been performed.