Researchers at IMRA America (Ann Arbor, MI) have created a single-mode optical fiber with the largest core ever. The core, with a diameter of 84 µm and an effective mode area (Aeff) of 3600 µm2 at 1 µm, is restricted to single mode due to its low numerical aperture of 0.013 (a refractive-index difference of about 6 × 10-5). Such low numbers were achieved by “freezing in” thermal stress when fabricating the fiber, creating a central parabolic index profile. To do this, the pure-silica fiber preform contained fluorine-doped silica rods in a hexagonal lattice with the central rod missing; the drawn fiber contained, well within the central all-pure-silica region, the stressed portion.
Using the same fabrication method, the researchers created a fiber with an even larger core diameter of 252 µm. While not strictly single-mode, the fiber demonstrated single-mode operation with a mode-field diameter of 149 µm and an Aeff of 17,400 µm2 at 1.03 µm and 31,600 µm2 at 1.5 µm. The output beam from this fiber had a beam quality M2x of 1.04 and M2y of 1.18. Contact Libin Fu at [email protected].