A NOVEL CHIRPED FIBER BRAGG GRATING UTILIZING THERMAL-DIFFUSION OF CORE DOPANT

A novel technique is proposed to fabricate a chirped fiber Bragg grati ng utilizing thermal diffusion of core dopant. The chirped grating is written with a uniform period by using UV exposure technique in the fi ber whose effective index of the guided mode varies along its length. Thermal diffusion of the core dopant is employed to realize this chang e of the effective index. Through the thermal diffusion process, the e ffective index of the fiber decreases from its initial value. When the grating is written in the diffused core region, its reflection wavele ngth becomes shorter than that in the non-diffused region. The continu ous change of effective index is required for making a chirped grating . The fiber is heated by a non-uniform heat source. When the uniform g rating is written in this region, the reflection wavelength smoothly c hanges along the fiber length although the grating period is constant. By optimizing the fiber parameters to realize a highly chirped gratin g, we have obtained a typical one whose bandwidth is 14.1 nm at half m aximum and maximum rejection in transmission is 29 dB. Additionally, t he proposed method has an advantage to control the chirp profile with high mechanical reliability.