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Based on a special large-pitch architecture that has already proved its single-mode single-polarization behavior in a passive configuration, two ytterbium-doped versions of such large-mode-area fibers have been fabricated and tested in both laser and amplification configurations for high-power laser source applications. Due to the high sensitivity of large-pitch fiber design to the active-core-to-passive-cladding index mismatch, the realization of a single-polarization structure is highly challenging. However, we report on the preservation of a polarization-maintaining feature. A linear polarization with an extinction ratio of 17 dB is demonstrated for mode field diameters reaching up to 58 μm as long as the single-modeness of the emitted signal is preserved.

In this paper, we demonstrate a single-polarization feature out of passive very-large-mode-area fully aperiodic large-pitch fibers. It has been previously shown theoretically that one of the two polarizations of the fundamental mode is selectively coupled to a cladding mode. This coupling does not require fiber bending, which permits us to avoid any decrease in mode effective area. The coupling is achieved owing to boron-doped silica inclusions implemented into the microstructured cladding and acting as stress-applying parts. This mechanism has been assessed experimentally in this work using fibers of two different core diameters: 60 μm and 140 μm, providing mode field areas of 3637  μm 2 and 14,590  μm 2 , respectively, at 1942 nm. The tested fibers have a length of 45 cm and an outer diameter exceeding 1 mm, yielding rod-type fibers. Each sample has been characterized using an unpolarized laser source emitting at 1942 nm. This laser, based on a thulium-doped large-mode-area step-index fiber, has a spectral bandwidth of about 0.5 nm. After passing through a piece of the passive fiber, a polarization extinction ratio higher than 16 dB has been achieved.