Coherently combined fiber laser system delivering 120 µJ femtosecond pulses

We report on the coherent combination of two chirped pulsed fiber lasers. The beams coming from two 100 µm core diameter ytterbium-doped rod-type fibers were coupled in a Mach-Zehnder-type interferometer by means of a polarization beam splitter. Active stabilization of the interferometer was achieved by using a piezo-mounted mirror driven by a Hänsch-Couillaud polarization detection system. Pulses with 120 µJ energy and a compressed duration of 800 fs were obtained. This, compared with the 66 µJ coming from each single amplifier, results in a combining efficiency as high as 91%.