We propose a novel structure for a gain-flattening filter, in which a conventional long-period fiber grating (LPFG) and a phase-shifted LPFG are written closely. The transmission characteristics are investigated in the case of a closely arranged configuration. For the fabrication of a phase-shifted LPFG, a UV-trimming method to control the amount of the phase shift is examined theoretically and experimentally. Finally, a 67-mm-long LPFG gain-flattening filter designed for an erbium-doped fiber amplifier is fabricated and the measured spectrum is shown.
Michiko Harumoto, Masakazu Shigehara, and Hiroshi Suganuma, "Gain-Flattening Filter Using Long-PeriodFiber Gratings," J. Lightwave Technol. 20, 1027- (2002)