Pixelated fiber with modal sieve has effective mode area of 1250 μm2
The highest-power single-mode fiber lasers, which operate in the kilowatt region, are based on optical fiber that are designed to sustain a large mode area. Because a standard core/cladding fiber design with a very large mode area becomes overly sensitive to even the tiniest physical perturbations and flaws, losing light from the core as a result (light that can go on to destroy the fiber), alternatives have been proposed and fabricated. These include photonic-bandgap fibers of varied structure, as well as Bragg fibers, which have concentric low- and high-refractive index rings at the proper spacing to achieve enhanced light confinement. Bragg fibers have an advantage over photonic-bandgap fibers in that the Bragg ones allow for easy customization of the spacing between rings for tweaking core size. In contrast, photonic-bandgap fibers are limited by the structure’s lattice to relatively large incremental steps.
However, Bragg fibers can also support higher-order modes in the rings that couple to the fundamental mode and lead to very high losses at certain wavelengths. An alternative structure—a fiber made of a single material that has concentric circles of uniformly spaced holes, called pixelated rings, which serve a similar purpose to low-index rings—was proposed by researchers at the Université Lille (Villeneuve d’Ascq, France). Now, the same group has modified the pixelated rings by removing certain holes from each ring to create a “modal sieve” for higher-order modes, greatly enhancing their rejection. Experiments on a 1-m-long fabricated fiber showed single-mode behavior between wavelengths of 1000 and 1700 nm, a low-loss bending radius of 22.5 cm, and an effective mode area of 1250 μm2. Reference: J.-P. Yehouessi et al., Opt. Lett. 40(3), (2015); http://dx.doi.org/10.1364/OL.40.000363.