Theoretical analysis of combining efficiency and beam quality degradation in a misaligned diffractive coherent combining system.

We theoretically investigate the combining efficiency and combined beam quality degradation induced by beam array misalignment in a coherent combining system based on diffractive optical elements. Theoretical model is established based on the Fresnel diffraction. We consider pointing aberration, positioning error and beam size deviation in array emitters as typical misalignments, and discuss their influences on beam combining by this model. The statistical analysis results and the accurate fitting curves of the degradation have been given based on the repetitive simulations with normal distributed random misalignments. According to the results, the combining efficiency is affected greatly by the pointing aberration and position error of the laser array, while the combined beam quality is just affected by the pointing aberration generally. Based on calculation with a series of typical parameters, the standard deviations of the laser array's pointing aberration and position error are required to less than 15µrad and 1µm respectively to maintain an excellent combining efficiency. If we only concentrate on the beam quality, the pointing aberration need to be less than 70µrad.

Investigation of combining-efficiency loss induced by a diffractive optical element in a single-aperture coherent beam combining system.

Filled-aperture geometries can be obtained using a diffractive optical element (DOE) in the coherent beam combining (CBC) architecture. Minimizing the beam deviation is crucial to maintain single-aperture output and reduce the combining-efficiency losses. In this study, we developed a theoretical model for investigating the combining-efficiency losses with beam deviation in a DOE-based CBC architecture. The beam deviations induced by the DOE-mount-tilt error, emitter-incident angular error, and DOE-groove-tilt error are discussed theoretically in detail and verified experimentally. The combining-efficiency losses caused by the three error sources are calculated. Meanwhile, the combining-efficiency losses affected by the beam size and the DOE period are analyzed. For an 11-channel CBC architecture with a DOE period of 50 µm and a beam size of 30 mm, the maximum combining-efficiency losses caused by the three error sources were 3.2%, 1.87%, and 36.41%, respectively, whereas those in case of a DOE period of 20 µm and a beam size of 10 mm were 14.34%, 8.58%, and 25.29%, respectively. We found that the combining-efficiency loss is most sensitive to the DOE-groove-tilt error.

Simulation and experimental study of laser-induced thermal deformation of spectral beam combination grating.

The multilayer dielectric (MLD) grating is a critical device for combining multiple laser beams into a single beam in a spectral beam combining (SBC) system. We established a theoretical thermal deformation model of the laser-irradiated MLD grating. Thermal deformation on the surface of the grating is simulated according to a series of parameters including the laser irradiation time, laser power density, and substrate size. To verify the model, we exposed a 960 l/mm, 50×50×1.5 mm3 grating to a laser power density of 3.61 kW/cm2 and observed the temperature change. We used a Twyman-Green interferometer to measure the interference fringes on the grating surface. Based on the Fourier-transform method and a Zernike polynomial fitting method, the real-time grating surface profile is reconstructed. The results show that substrate thickness increase or area decrease can reduce thermal deformation, the average decreases are 18.3% and 19.9%, respectively. The discussion and analysis of the grating thermal deformation are potentially valuable for designing grating to decrease the thermal deformation and improve the combined beam quality of a SBC system.