Unraveling Modal Interference in Photonic Bandgap Fibers

We address the issue of modal interference in silica hollow core Photonic Bandgap Fibers (PBGFs). Over meter-scale lengths, PBGFs can support several optical modes with widely different group velocities and strong intermodal dispersion. Complex modulation patterns arise in the transmission spectra due to modal interference, which substantially limit the ability to detect weak gas absorption lines in PBGF-based sensor heads. Modal interference in 7-cell and a 3-cell “single mode” PBGFs was investigated in detail and the individual modulation components were identified through frequency-domain Fourier analysis. By comparing the measured group delay of each component with the values obtained from theoretical modal maps, the underlying optical modes were identified for the two fiber structures. Tight control of modal properties and modal interference effects is paramount to gas sensing applications of these fibers.