Improving Beam Quality Of Coherent Laser Combining Using Diffractive Telescopes Array

V High power and great beam quality are the eternal targets in the study of laser technology. However, the fundamental-mode beam will become multimode beam as the laser power increases, which leads the beam quality drop. In addition, the power increasing will cause some other problems, like thermal damage, thermal lens effect and nonlinear effect in the laser gaining media, which all directly affect the beam quality of output laser.The proposition of coherent beam combination(CBC) offers an efficient way to obtain the laser with high power and great beam quality. The phenomenon of coherence in CBC makes the combination spot in far field consist of a main lobe and many side lobes. The energy of the spot is so dispersive that the beam quality is quite low. The fill factor of the input beams array is the determining parameter for energy focusability of the central main lobe. The larger fill factor is, the more concentrative the energy and the better the beam quality will be. Based on these analyses and we bring forward a method to increase the fill factor by using diffractive micro-lenses array, and design a set of diffractive telescopes array considering the diffractive optical element has advantages, of being easy to obtain high fill-ratio lens array and convenient for equipment. The six coherent incidence laser beams generated by passive phase locking can be expanded to be tangent with the diffractive telescope array. Thus, the fill factor is increased considerably and the aim of improving the beam quality is reached.The diffractive telescopes array consists of six coincident units. The structure of each one is Galileo telescope system, which can cut down the total length without real focus to breakdown the air. The surface is designed as the theory of multi-level Fresnel phase lens. It is impossible to fulfill the speed and accuracy in computing the beam propagation and focus because of the huge dates in the fine structure of diffractive optical element. So, we firstly bring out a novel arithmetic of matrix multiplication to solve this problem. On this basis, we propose the concept of fidelity to study the expanding result of the unit diffractive telescope. By making the different sections of the surface with different steps, we do analysis of the improved structure. The result indicates that the fidelity rises to 95.82% with the minimum lithography line-width above 2μm of each section. Then, we take detail analysis for the improved effect of the beam quality. The diffractive telescopes array makes the fill factor of input beams array increase from 0.3571 to 1, which allows the number of side lobes decrease absolutely, and the beam quality is improved above 2 times. At last, we fabricate the 8-level diffractive telescopes array and make the experiment. The preparatory result shows that, the far-field combination spot measured is much smaller, higher energy focusability and better beam quality in the condition of expanding the input beam by the diffractive telescopes array.