Spectral Beam Combining using a designed concave grating based on two stigmatic points

The aberrations of a concave grating have a serious influence on the beam quality. To improve beam quality in spectral beam combining (SBC) using a concave grating, an aberration-reduced approach which is called two stigmatic points is adopted. By means of the method, the aberration-reduced design steps were presented, and the mount parameters of the designed grating were obtained. The simulation results show that the low-aberration wavelength range can reach about 40 nm from the wavelength of 1530∼1570 nm, which is significantly larger than that of the grating with a stigmatic point. Further, the simulation results of SBC with 5-element indicate that, in both horizontal and vertical directions, the spot radii of the combined beam are almost equal, and the values of M_x² and M_y² are 1.36 and 1.38. Based on the ordered grating, an SBC experimental system was established. The diffraction experiment shows that, for the fiber laser with a wavelength of 1561.48 nm, the measured M_x² and M_y² are 1.44 and 1.46, respectively. Further, the SBC experiment shows that, for the combined beam of 2-element, the measured M_x² and M_y² are 1.52 and 1.54, respectively.