The influence of laser energy on the FBG reflection spectrum is as follows. As shown in Fig. 3(a), the higher the laser energy, the greater the red shift of the wavelength at the same exposure time. The corresponding wavelength at the exposure time of 125 s is 1549.61 nm and 1549.75 nm for the laser energy of 700 mW and 800 mW, respectively. The higher the laser energy, the higher is the reflectivity; however, the bandwidth also increases significantly. The corresponding maximum reflectivity values of FBG under the two conditions are 14.3 dB and 15 dB, respectively, as shown in Fig. 3(b). Simultaneously, the corresponding bandwidths of approximately 0.62 nm and 0.74 nm are achieved, as shown in Fig. 3(c). In addition, a higher laser energy leads to more side lobes on both sides of the main resonant peak in the spectrum, as shown in Fig.4.
Fiber Bragg gratings (FBGs) exhibit the advantages of small size, reflection operation, high sensitivity, and wavelength encoding. Further, the FBGs induced by femtosecond laser pulses have unique advantages, such as high-temperature resistance and high-temperature stability, which are particularly suitable for sensing applications under extreme operating conditions, and thus, these FBGs have been widely used in aerospace and other fields. Fabricating FBGs via phase mask technology has the merits of effective processing and high device consistency and has been demonstrated to be the most promising industrial technology scheme. However, limited research has been conducted on the spectral characteristics of FBGs. We fabricate FBGs via the femtosecond laser phase mask method and extensively study the influence of laser energy and exposure time on the wavelength, reflectivity, and bandwidth of the FBGs. The factors influencing the FBG spectrum are theoretically and experimentally analyzed. This study offers an experimental basis and guidance for the fabrication of high-quality FBG devices using femtosecond laser technology.
In this study, FBGs fabricated via the femtosecond laser phase mask method are experimentally demonstrated. The light path of the grating processing system comprises a femtosecond laser system, aperture, optical attenuator, cylindrical lens, and phase mask. The spectrum testing system comprises an amplified spontaneous emission (ASE) broadband light source and an optical fiber spectrum analyzer. The laser system outputs 800 nm femtosecond laser pulses. A diaphragm and optical attenuator are used to control the spot diameter and adjust the laser pulse energy, respectively. Meanwhile, a cylindrical lens is used to focus the light beam, and a phase mask to form interference fringes and periodic structures in the fiber, and writing the gratings. In addition, a spectral testing system is used to monitor the spectral changes of the FBG in real time. In the experiment, when the laser energy is set to 600 mW and the longest exposure time is set to 240 s, the influence of exposure time on the FBG reflection spectrum is studied. Moreover, the relationship between the spectral properties and exposure time is analyzed under laser energy values of 700 mW and 800 mW.
The influence of the exposure time on the FBG reflection spectrum is as follows. As shown in Fig. 2(a), when the exposure time increases, the FBG reflectivity gradually increases and then remains unchanged, and the wavelength undergoes a red shift. At the initial exposure time of 20 s, the FBG reflectivity reaches 11.6 dB rapidly, and then, at the exposure time of 75 s, reaches a maximum of approximately 14.03 dB, as shown in Fig. 2(b). Meanwhile, the bandwidth of the FBG increases with the exposure time, which increases from 0.41 nm to 0.73 nm in the exposure timer ange of 5?240 s, as displayed in Fig. 2(c). The FBG wavelength has a red shift as the exposure time increases and the wavelength changes by 0.36 nm, from 1549.33 nm to 1549.69 nm, as the exposure time increases from 5 s to 240 s, as shown in Fig. 2(d).
In this paper, FBGs fabricated using the femtosecond laser phase-mask method are proposed. The effects of laser energy and exposure time on the wavelength, reflectivity, and bandwidth are studied. The experimental results demonstrate that to obtain FBGs with a small bandwidth, high reflectivity, and high spectral quality, appropriate laser energy and exposure time should be selected. In particular, the laser energy should not be considerably large. Considering the writing efficiency and reflection spectral quality, an excellent FBG device can be obtained by selecting the laser energy of approximately 600 mW and exposure time of less than 1 min.
