XCAN: a highly scalable femtosecond coherent amplification network

For the last decade, Coherent Beam Combining (CBC) has proven to be a successful method for increasing the output peak and average powers of femtosecond fiber amplifiers [1]. Within the framework of the XCAN project, we aim at the demonstration of the scalability of such laser systems by coherently combining 61 fiber amplifiers. Therefore, we use a highly scalable approach relying on a tiled-aperture geometry along with an interferometric phase measurement and control which can theoretically handle the coherent combination of up to 10 000 beams [2]. After very promising results obtained with 19 passive fibers [3], we present the first results obtained using seven active fibers. Figure 1: (a) Experimentally obtained far field. (b) Combined beam with corresponding M²-values. (c) Measured temporal profile of the combined beam. (d) Corresponding spectrum. The seven beams are arranged in a hexagonal array in the near field, collimated by a microlens array, and the combined beam is found in the far field. Based on a characterization of our experimental fiber array, the maximum efficiency is calculated to be η = 0.56. By filtering the central lobe, we obtain an experimental efficiency of η = 0.48 (fig. 1a,1b), which corresponds to 86 % of the theoretical value stated above. The measured M² of 1.15 on each axis proves a good quality of the combined beam. Finally, the beam is temporally compressed to 244 fs with a time-bandwidth product of 0.55, close to the simulated Fourier transform limit of 0.48 (fig. 1c,1d). The average power of the output beam is measured to be 71 W, corresponding to a compression efficiency of 0.86. The next steps will include the reduction of the pulse repetition rate and the resulting increase in peak power. Finally, the number of beams will be increased to 61 and the spatial and spectral characteristics of the combined beam will be studied.