Bandpass Filter Response Through Concatenation of Graded-Index and Step-Index Few-Mode Fibers

We propose an all-fiber bandpass filter using a combination of a graded-index few-mode fiber (GI-FMF) and a stepindex few-mode fiber (SI-FMF), in which distinct long-period fiber gratings (LPFGs) are formed. The first LPFG in the GI-FMF performs the mode conversion of the LP
₀₁
mode to the LP
₁₁
mode over broad wavelength range. The second LPFG in the SI-FMF reversely converts the LP
₁₁
mode to the LP
₀₁
mode at the specific resonance wavelength. Such a sequence of mode transition yields a single passband in the transmission spectrum. Though the tunable periodicity is not crucial to the first LPFG, it is beneficial to the second LPFG for changing the passband wavelength.