Femtosecond beam adaptive shaping and space-time coupling at the output of an ytterbium doped multimode fiber

Summary form only given. Controlling the output beam of a multimode fiber (MMF) is quite a hot topic currently because it offers several attractive perspectives. Among the various technics used to shape the output beam of a multimode fiber, the adaptive approaches rely on a measurement (objective function) which feeds an algorithm for further command of a spatial modulator for optimization. This approach gives several advantages: it does not need knowledge of the transmission matrix of the fiber, it is resilient to fiber perturbations and energy conservation is not needed. Beam focusing and profiling have been achieved already through a standard MMF [1], and more recently in an amplifying Yb-doped MMF [2] but using CW laser light. For some applications regarding amplification or nonlinear imaging one needs at the same time a control on both the spatial shape and the temporal shape.In this work we have studied the space-time couplings associated with the adaptive profiling (through wavefront control) of the sole spatial shape of a femtosecond beam amplified in an Yb-doped MMF. For the experiments we have used 200 fs pulses at 1030nm which wavefront was shaped by a deformable mirror. The laser pulses were then injected into a double clad Yb-MMF with a 90 μm core diameter (208 LP modes, 2 polarizations). The fiber piece was a 1.3m long loop and pumped at 980 nm by laser diodes in a backward configuration. On the fiber exit side, the optimization parameter was the average power collected in a targeted area localized in the near field. The black doted circles on the Fig.1 (a) and (b) represent the hard aperture which served to define the target area. The power in the target was maximized using a three steps phase shifting algorithm [3] which gave in one round the phase setting required for each actuator of the deformable mirror.