Coherent combination of mid-IR laser beams using a Sagnac-type interferometer.

We report a high-efficiency coherent beam combination (CBC) system operating in the mid-infrared (2.8 µm) spectral region, implemented using two fluoride fiber amplifiers with a configuration of Sagnac-type interferometer. The system exhibits good performance at both continuous-wave (CW) and picosecond-pulse operation conditions. For the CW CBC, the system exhibits a high combination efficiency of >90%, delivering a maximum output power of 2.8 W with a beam quality factor (M) of <1.2. When using a pulsed laser as the seed light, two beams of 1-ps pulses can be amplified and simultaneously temporally-compressed to ∼170 fs in the two fiber amplifiers. Then high-efficiency (∼84%) coherent combination of amplified ultrafast laser beams can be achieved in this CBC system, giving rise to a recombined output power of ∼1 W. These results highlight the application potential of this Sagnac-type CBC set-up for power scaling of both CW and ultrafast fiber lasers at mid-IR wavelengths.