All-fiber wavelength-tunable and mode convertible bandpass filter for optical interconnections

We report an all-fiber wavelength-tunable bandpass filter with multimode to single-mode converting property via serially concatenated structure of a helicoidal long-period fiber grating (HLPFG), a ring-core hollow optical fiber (HOF), and a multimode fiber (MMF). After passing the cascaded HLPFG-ring core HOFMMF, the bandpass filtering characteristic and the mode conversion between multimode and single-mode were realized at the resonant wavelength of the HLPFG. Due to the novel spectral functionality of the HLPFGfor directional torsion to the helix, the proposed device also showed wavelength-tunable bandpass filtering operation with low polarization-dependent loss.