Scalable Coherent Combining of Kilowatt Fiber Amplifiers Into a 2.4-kW Beam

We demonstrate coherent beam combining of multiple Yb-doped non polarization-maintaining fiber amplifiers operating at individual power levels above 1 kW. A 2.4-kW, M² = 1.2 output beam was generated by combining three amplifiers with a diffractive optical element (DOE). A single low-power sample of the combined output beam provided error signals for active phase and polarization locking of all three fibers. The beam combining efficiency of 80% was nearly constant with amplifier power, indicating the absence of significant thermal or nonlinear effects. Modeling anchored by precise measurements of the beam characteristics indicates potential for this architecture to scale to higher fiber counts, higher combining efficiency, and higher power.