Investigation of Long-Period Grating Resonances in Hollow-Core Photonic Bandgap Fibers

The mode-coupling processes of long-period gratings (LPGs) fabricated in hollow-core photonic bandgap fibers are investigated. The LPGs are formed by periodic structural deformations induced by local heating generated by use of a pulsed CO₂ laser. Highly polarization-dependent grating resonances are observed in the transmission spectrum of the LPGs and found to be due to coupling from the fundamental to higher order core modes. The mode-coupling process is understood based on coupled local-mode theory: A single deformation of the holey cladding modifies the local-mode field profiles and induces fractional energy transfer between the fundamental and the higher order modes. Resonant coupling between a phase-matched fundamental/higher order mode pair is excited by having multiple such deformations arranged periodically along the fiber. The LPG spectrums are numerically modeled by solving the coupled local-mode equations at different wavelengths and found in agreement with the experimentally measured results.