Passive coherent combination of two ultrafast rod type fiber chirped pulse amplifiers

Using passive coherent beam combining of two ultrafast fiber amplifiers, we demonstrate the generation of high temporal quality 300 fs and 650 μJ pulses corresponding to 60 W of average power at a repetition rate of 92 kHz. Furthermore, at 2 MHz of repetition rate record coherent combining average powers of 135 W before and 105 W after compression are measured. A combining efficiency higher than 90% is maintained over the whole range of output powers and repetition rates investigated demonstrating the efficiency and robustness of the passive combining technique. The measured pulse-to-pulse relative power fluctuation at high energy is 2%, indicating that the system is essentially immune to environmental phase noise. We believe the passive combining method to be an attractive approach for compact multi-GW peak power femtosecond fiber-based sources.