We present the selective excitation of the fundamental mode in an anti-resonant hollow-core fiber (ARHCF) fusion-spliced with a commercial large mode area (LMA) fiber. By designing and fabricating a single-ring ARHCF that is mode-matched to a LMA fiber and by splicing the two using a
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laser-based splicer, we achieve a coupling efficiency of 91.2% into the fundamental mode. We also demonstrate an all-fiber integration of an ARHCF with a commercial ytterbium-doped fiber in a laser cavity for beam delivery application. Coupling of the single-mode laser output beam into the fundamental mode of the ARHCF is demonstrated with 90.4% efficiency (
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We present the first report on a compact, temperature-insensitive, multi-axial mechanical force sensor based on a single-core antiresonant hollow-core fiber (ARHCF). Single-core antiresonant fibers are inherently few-moded in a short length and show characteristic multimode interference pattern in their transmission spectrum. We report here a simple technique that enhances the interaction between the interfering modes in such fibers, giving rise to up to four-fold improvement in the peak-to-peak amplitude of the interference pattern. The enhanced interference pattern is shown to be responsive to external mechanical forces, like longitudinal and transverse strain and curvature, with distinguishable linear responses. Transverse and longitudinal mechanical forces affect different attributes of the interference pattern, making the proposed sensor suitable for their simultaneous sensing. The temperature sensitivity of the sensor is found to be 3.3 pm/C suggesting negligible thermal crosstalk while measuring the effect of mechanical forces. The sensor has a compact configuration and is inherently insensitive to polarization of light used.
We proposed and demonstrated mode cleaning in a high-power fiber laser by integrating an anti-resonant hollow-core fiber (AR-HCF) into a multimode laser cavity of an ytterbium (Yb)-doped fiber (YDF). An in-house mode-matched AR-HCF was fusion-spliced to a commercial multimode LMA-YDF, ensuring efficient fundamental mode coupling. The AR-HCF inflicts a high propagation loss selectively on higher-order modes, facilitating fundamental mode operation. Thus, the AR-HCF works as an efficient spatial mode filter embedded in the multimode fiber laser cavity and reinforces preferential amplification of the fundamental mode. Beam quality factor enhancement was achieved from M2 = 2.09 to 1.39 at an output power of 57.7 W (pump-power limited). The beam quality can be further improved by refining the AR-HCF fabrication. The proposed technique has a great potential to be exploited in other multimode fiber laser cavities involving erbium- or thulium-doped fibers and obviates the need for complicated specialty active fiber designs. Compared with the commonly used fiber bending technique, our method can achieve an efficient higher-order mode suppression without inducing mode-field deterioration.
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