Coherent beam combination using non-separable phase-controlled Dammann grating

We present an improved scheme for coherent beam combination using a type of non-separable phase-controlled Dammann grating (NPDG), which is capable of generating multiple equal-intensity diffraction orders with good uniformity and high efficiency in the far field. Compared with conventional separable two-dimensional (2D) Dammann gratings, the efficiency of such gratings is significantly improved around 10% due to the non-separable structure. Such improvement in the diffraction efficiency is of high significance for coherent laser combination. Furthermore, the phases of those diffraction orders for this type of gratings are only two-valued in the far field, which leads to a great simplification of phase matching during the procedure of coherent beam combination. As an example, a 5×5 NPDG with efficiency of 73.6% is theoretically designed and a NPDG sample with efficiency as high as 66.8% is experimentally fabricated by lithography and wet-etching techniques. Also, we implemented further a proof-of-principle experiment for coherent beam combination based on the 5×5 NPDG sample, and the results show that the combining efficiency is as high as 65.7%.