Long-period fiber-grating-based bandpass filter using self-aligned absorptive core mode blocker

We present an all-fiber bandpass operation by using an absorptive core mode blocker self-aligned between two long-period fiber gratings (LPFGs). The first LPFG converts the core mode into the cladding modes at resonant wavelengths, and the second LPFG operates in reverse direction. The core mode blocker attenuates the nonresonant core mode from the first LPFG but passes the resonant cladding modes to the second LPFG. This bandpass filter exhibits a passband of 20-nm bandwidth at a center wavelength of 1542 nm with nonresonant light suppression greater than 15 dB. Thermal tuning coefficient is found to be −0.51 nm/°C for the temperature range from 10 to 70 °C.