A comparative analysis has been carried out of the modal composition and radiation-damping spectra in microstructured lightguides based on quartz glass with a large core for transporting powerful light fluxes and differing in the number of cycles of holes around the core. The geometrical parameters of the holey core that promote the propagation regime of the fundamental mode under conditions of differential mode losses have been experimentally determined. It is shown that, in lightguides with a hexagonal cladding structure and less than three cycles of holes, a stable single-mode regime can be achieved for a radiation- damping value greater than 0.1??dB/m at a wavelength of 1550 nm.
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