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 HOF-MMF, 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 HLPFG for directional torsion to the helix, the proposed device also showed wavelength-tunable bandpass filtering operation with low polarization-dependent loss.