Bragg Gratings in the Germanium-Doped Concentric Rings of a ${\hbox {Yb}}^{3+}$ -Doped Core Solid Photonic Bandgap Fiber

The inscription of Bragg gratings into the germanium-phosphorous co-doped rings of a photonic bandgap fiber with a ytterbium-doped core is reported. These cladding layers are observed to exhibit interesting modal properties, with each ring supporting near-degenerate modes that agree well with numerical modeling. Besides fiber laser applications, there are several potential sensing applications of gratings in such a fiber.