Objective Type Ⅰ fiber Bragg grating can work well below the temperature of 300 ℃. However, when the temperature further rises, the reflectivity of Type Ⅰ fiber Bragg grating will be reduced. If it operates in high temperature for a long time, the fiber Bragg grating will fail. There is a new type of grating called regenerated fiber Bragg grating attracting worldwide attention that promised to solve the problem. It is essentially formed from the initial Type Ⅰ "seed" grating, precipitating through thermal processing with a structure that is set by the laser written seed. Regenerated fiber Bragg grating is an important optical fiber sensor for high-temperature sensing. But the stability and repeatability of the regenerated fiber Bragg grating under high temperature is less commonly reported, especially for different environmental atmospheres. For this purpose, environmental atmospheres are introduced during the process of regeneration and testing in this paper.
Method Air and nitrogen environmental atmospheres are designed during the regeneration of two type Ⅰ seed gratings written by UV laser. The whole regeneration process is performed in a high-temperature furnace and recorded by optical spectrum analyzer and fiber Bragg grating interrogator (Fig.1). The temperature characteristic of the obtained regenerated fiber Bragg grating is tested by fiber Bragg grating interrogator under air and nitrogen environmental atmospheres (Fig.2). The regeneration results are shown in the form of evolution of central wavelength and transmission depth (Fig.5). The repeatability and stability of the regenerated fiber Bragg gratings are characterized by wavelength (Fig.6-8). And the temperature accuracy is verified at 700 ℃.
Results and Discussions The reflectivity of the two kinds of regenerated fiber gratings is not much different. The one in air has a reflectivity of 21.68%, and the other in nitrogen has a similar value of 21.73%. However, under the temperature cycle of 350-750 ℃, the regenerated fiber grating in the nitrogen environment has good temperature stability, and the temperature measurement accuracy can reach ± 2.9 ℃ at 700 ℃. The regenerated fiber grating prepared under the condition of nitrogen atmosphere is helpful to improve its stability and measurement accuracy.
Conclusion The fabrication methods of regenerated fiber Bragg gratings under two different atmospheres of air and nitrogen are studied. The whole process record of regeneration of seed gratings written by ultraviolet laser at a high temperature of 850 ℃ is obtained, and the temperature characteristics and temperature measurement accuracy of the above two kinds of gratings are systematically studied. The research results show that the regenerated fiber Bragg gratings obtained under the heating atmosphere of air and nitrogen have similar reflectivity, but the growth of regenerated fiber Bragg grating under nitrogen is faster and has good thermal stability, and its temperature measurement accuracy can reach up to ±2.9 ℃. Therefore, changes in the environmental atmosphere will affect the thermal stability and temperature measurement repeatability of regenerated fiber Bragg grating. By optimizing the fabrication method of regeneration, regenerated fiber Bragg grating will play an important role in the field of high-precision high-temperature measurement.
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