Coherent combining of ultrashort fiber-amplified laser pulses

We report on a novel concept to scale the performance of ultra-fast lasers by means of coherent combination. Pulses from a single mode-locked laser are distributed to a number of spatially separated fiber amplifiers and coherently combined after amplification. The splitting and combination process is based on the polarization combining technique using polarization cubes. A Hänsch-Couillaud detector measures the polarization state of the combined beam. The error signal (deviation from linear polarization) is used to stabilize the optical path lengths in the different channels with a piezo mounted mirror. In a proof-of-principle experiment the combination of two femtosecond fiber-based amplifiers in a CPA systems is presented. A combining efficiency as high as 97% has been achieved. Additional measurements were carried out to investigate the stability of the system. The concept offers a unique scaling potential and can be applied to all ultrafast amplification schemes independent of the architecture of the gain medium.