We fabricated a three-dimensional microstructured optical waveguide (MOW) in a single-crystal using femtosecond-laser writing and phosphoric acid etching techniques, and observed excellent midinfrared waveguiding performance with low loss of ∼0.5 dB∕cm. Tracks with a periodic arrangement were written inside the yttrium aluminum garnet (YAG) crystal via femtosecond laser inscription, and then etched by the phosphoric acid (H 3 PO 4 ) to form hollow structures. The evolution of the microstructure

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... of tracks was investigated in detail. The function of the MOW was analyzed by different numerical methods, indicating the proposed MOW can effectively operate in quasi-single-mode pattern in the midinfrared wavelength range, which agrees well with our experiment results.