We demonstrated a kind of long-period fiber grating (LPFG), which is manufactured with\na thermal diffusion treatment. The LPFG was inscribed on an ultrahigh-numerical-aperture (UHNA)\nfiber, highly doped with Ge and P, which was able to easily diffuse at high temperatures within a\nfew seconds. We analyzed how the elements diffused at a high temperature over 1300 ââ??¦C in the\nUHNA fiber. Then we developed a periodically heated technology with a CO2 laser, which was able\nto cause the diffusion of the elements to constitute the modulations of an LPFG. With this technology,\nthere is little damage to the outer structure of the fiber, which is different from the traditional LPFG,\nas it is periodically tapered. Since the LPFG itself was manufactured under high temperature, it can\nwithstand higher temperatures than traditional LPFGs. Furthermore, the LPFG presents a higher\nsensitivity to high temperature due to the large amount of Ge doping, which is approximately\n100 pm/ââ??¦C. In addition, the LPFG shows insensitivity to the changing of the environmentââ?¬â?¢s refractive\nindex and strain.
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