Long period grating resonances in photonic bandgap fiber.

We demonstrate the formation of stress-induced long period gratings (LPGs) in fluid-filled photonic bandgap fiber (PBGF). Based on our experimental results, simulations, and theoretical understanding of LPGs, we identify coupling to a guided LP(11)-like mode of the core and lossy LP1x-like modes of cladding microstructure for a single grating period. The periodic modal properties of PBGFs allow for coupling to the same mode at multiple wavelengths without a dispersion turning point. Simulations identify inherent differences in the modal structure of even and odd bands.