Abstract

In order to make a source of ultrashort broadband pulses at high average power with fiber technology, the most studied design is based on chirped pulse amplification (CPA) with a chain of fiber amplifiers of optimized characteristics. Several alternative approaches have been investigated, that aim at performance scaling of ultrafast lasers either by time or spatial division [1,2] each requiring a final combining stage. More recently it was suggested to split the initial pulse spectrum in few spectral bands which are subsequently amplified in separate fiber amplifiers before being coherently recombined at the output [3,4]. This is the discrete version of the concept of spatially dispersed amplification initially studied by Christov for dye amplifiers [5]. The advantage of the design is that spectral narrowing from the gain can be avoided so that amplification of ultrabroadband pulses can be readily achieved and that nonlinear effects can be mitigated. It is even possible to imagine the use of different fiber amplifiers to cover a wider band. We propose here a new version of the concept which is based on the use of a multicore fiber (MCF) for a more compact and more robust implementation. We have numerically and experimentally investigated this new scheme for spatially dispersed stretcher-free amplification of ultrashort laser pulses. In view of a basic proof of concept demonstration, a 190 fs laser pulse at 1032 nm has been split in five spectral components which were separately transmitted in different cores of a passive MCF before being recombined coherently. The MCF was a polarization maintaining 19 cores microstructured fiber, fabricated by stack and draw technology at IRCICA. Only the central linear array of 5 cores was used in the experiments. Coherent control of the combined fields was performed by a deformable mirror (DM), inserted before the dispersive device and the fiber input. The synthesized pulse at the system output had duration very close to that of the laser input, once the phase of the different spectral bands has been adjusted by the servo-controlled DM. The figure above compares the autocorrelation trace of the initial laser pulse (dashed line) with those of the synthesized pulse after recombination. Two cases are shown (i) before and (ii) after adjustment of the spectral phase for pulse recovery. It was demonstrated that the multicore fiber is extremely robust with respect to environmental perturbations so that stable operation was maintained for several hours with the servo loop opened. The difference in group velocity between the fields of different carrier wavelengths has been compensated by opto-geometric effects through a proper bending of the MCF. With a splitting of the laser field in only five spectral components, a nearly 25x fold increase in the power threshold for the onset of nonlinear effects in a stretcher free amplifier should be obtained, giving rise to improved energetic performances. It was shown also that the scheme offers the capability of tailoring the transmitted (amplified) pulse by adjustment of the spectral phases. Twin pulses and top hat pulses were experimentally synthesized.

© 2013 IEEE