Reduction of transmission spectrum shift of long-period fiber gratings by a UV-preexposure method

We examined the photosensitivity and stability in an ultraviolet (UV)-preexposure method that locks the photosensitivity of refractive index by hydrogen loading in the fabrication of long-period fiber gratings (LPFGs). Photosensitivity and stability comparisons are made between the UV-preexposure method, a typical hydrogen-loading method, and a UV-exposure method without hydrogen loading. The UV-preexposure method can enhance the photosensitivity to induce effective refractive index modulations up to 1.6×10^-4, which is sufficient for fabricating LPFGs in a practical manner under the hydrogen-loading condition at a relatively low pressure of 2.9 MPa. The sensitivity is preserved in terms of the length of time at least over 10 000 h. The spectrum changes after the completion of the LPFG writing are more stable than under the typical hydrogen loading. Therefore, the proposed UV-preexposure method is one of the most useful techniques for fabricating precise and stable fiber gratings, because there are no free hydrogen molecules in the fiber when the fiber gratings are written