Spatial Beam Self-Cleaning in Tapered Yb-Doped GRIN Multimode Fiber With Decelerating Nonlinearity

Abstract

We experimentally demonstrate spatial beam self-cleaning in an Yb-doped gradedindex multimode fiber taper, both in passive and active configurations. The input laser beam at 1064 nm was injected for propagation from the small to the large core side of the taper, with laser diode pumping in a counterdirectional configuration. The Kerr effect permits to obtain high-beam quality amplification with no accompanying frequency conversions. As a result, our nonlinear taper amplifier may provide an important building block for multimode fiber lasers and amplifiers.

Spatial beam nonlinear control with multimode GRIN fiber amplifiers

“…The corresponding experimental setup is described and presented in Ref. 11 At first, we characterised our tapered amplifier by studying the amplified spontaneous emission (ASE) noise, and the amplification of the 1064 nm signal as a function of the pump LD power. Fig.…”

“…5 Subsequent experiments have shown that the output transverse spatial distribution at high powers may be controlled, by varying the fiber coupling conditions of the input laser. 8 Nonlinear pulse propagation and beam reshaping in standard or tapered GRIN MMFs [9][10][11] have been investigated in both the normal and in the anomalous dispersion regime, 12,13 using pulses with duration ranging from sub-nanoseconds to femtoseconds. [14][15][16] Active MMFs are key components for a future generation of fiber lasers, which may enable to scale-up by orders of magnitude the peak power of mode-locked laser sources.…”

“…First, we used a T-YD GRIN-MMF with a core diameter varying exponentially along the longitudinal direction. 10,11 By propagating pulses at 1064 nm from the largest end of the T-YD MMF, amplification permits a combination of beam self-cleaning and SCG, spanning from the visible to the mid-infrared. In order to use T-YD MMFs in a fiber laser, frequency conversion phenomena should be avoided: therefore, the amplified laser signal must necessarily propagate from the smaller to the larger core diameter.…”

Nonlinear beam cleanup in Yb-doped GRIN multimode fiber taper

“…On the other hand, the available gain in fiber lasers, provided by active rare-earth doping [1,2], is limited by the presence of Raman and Brillouin scattering, and transverse mode instabilities (TMI) [3,4]. Using a tapered Yb-doped (YD) MMFs, with a varying fiber core size along its axis, is a solution to mitigate TMI, hence to obtain high output quality beams by filtering high-order modes [5].…”

“…In this work, we experimentally demonstrate spatial KBSC in a tapered YD MMF, where the laser pulse propagates from the smallest to the widest core. The experimental setup is described [5]. Briefly, the injected laser pulses at 1064 nm, propagating from the small (47 µm) to the large (96 µm) diameter of a 5 m tapered YD GRIN MMF.…”