Tunable long-period fiber grating

Long-period fiber grating (LPFG) having grating periodicity in the hundreds of microns, functions as a band rejection filter: This device is useful for gain equalizing applications of fiber amplifiers. The transmission characteristics are mainly determined by the initial fabricating conditions such as grating periodicity and UV exposure time. Therefore, it was very difficult to tune the transmission characteristics with wide range after the LPFG was made. Many works have been reported to tune the resonant wavelength and depth of the dip point. But, tuning range of resonant wavelength was not so wide and resonant depth was not easy to control after the grating fabrication. In this paper, we have proposed, analyzed and evaluated the novel and simple method to tune the wavelength characteristics of LPFG after the grating fabrication. In this device, the grating is divided into two regions. One region is the conventional LPFG, and the other region is the modified LPFG surrounded outside the cladding with a medium having higher or lower refractive index than that of the cladding. In case of the higher refractive index, only resonant depth could be controlled over 20dB without resonant wavelength shift. In case of the lower refractive index, resonant wavelength could be moved around 40nm while resonant depth change is small. The experimental results were compared with the theoretical analysis. Both results showed good agreements.