Photonic crystal fiber (PCF) can provide both large mode area and low numerical aperture, meanwhile, offer heavy doping rate, good beam quality, and effective dispersion control. Coherent combining is an effective way to further elevate output power of a fiber laser. It is hence very beneficial to develop high-power fiber lasers using coherent combining through a multi-core PCF. In this work, by using multi-core PCF design and coherent combining technique, we studied theoretically and experimentally the multi-beam coherent combining in PCFs. We adopted a symmetrical structure to arrange multiple fiber cores, and have air holes surround them uniformly forming the cladding. Between these cores are solid glass filled. With an appropriate design of PCF structure, the coherent combining of multiple laser beams can bring higher output power up to kilowatts, while maintain good beam quality. Based on the evanescent-wave coupling theory, the modal coupling among seven cores and nineteen cores was studied. It is shown that the evanescent coupling is much stronger than the diffractive coupling if the cores are close enough. In the experiment, we fabricated rare-earth doped double-clad seven-core and nineteen-core PCFs using a stacking-capillary method. The near-field and far-field images were adopted to observe the mode coupling. Since all laser beams pass nearly the same optical length in one PCF, the phase matching can be easily realized. The theoretical and experimental results were compared, which shows that this kind of integrated multi-core PCFs can be very good candidates to achieve high-power coherent beam combining.
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