Near-infrared Femtosecond Laser Fabrication Of Fiber Bragg Grating With A Phase Mask

All-fiber Tm³⁺-doped fiber laser has been demonstrated more excellent application prospects in industrial, medical, scientific and military fields. Fiber Bragg Grating (FBG) is the most important component in all-fiber Tm³⁺-doped fiber laser. All-fiber laser based on FBG has many excellent characteristics such as narrow linewidth output, simple structure, stable performance, easy maintenance and so on. Recently, a new technique inscribing FBG in the non-photosensitive fiber with near-infrared ultrashort pulse laser attracts wide attention from academics and engineering. Compared to the traditional technique inscribing FBG with UV-pulsed laser in photosensitive fiber, this method has a greater value. In this paper, near-infrared femtosecond laser fabrication of fiber Bragg grating with a phase mask is studied theoretically and experimentally.Based on coupled-mode theory which is the basic equation of fiber Bragg grating, the spectral characteristics of sinusoidal refractive index modulation single-mode uniform FBG are analyzed, and the impact of structural parameters of FBG on its performance is obtained. Second-order FBG reflection spectrum with this form of index modulation is simulated. Simultaneously, in order to remedy the defect of coupled-mode theory, Rouard is introduced to analyze the characteristics of non-uniform FBG. The reflection spectrum of Gaussian apodizing index modulation FBG is obtained by Rouard's method.The design of the inscribed system is based on grating equation and Bragg equation. The effects of phase-mask parameters on interference fringes are analyzed. The rational principle for the design of phase mask is attained, and the optimal distance between the fiber and phase mask is calculated. The experiment system of the inscribed equipment is set up and FBGs are inscribed successfully in Tm³⁺-doped fiber and photosensitive fiber. The reflectivity and wavelength are measured, and the reflectivity is more than 95%.The method tuning the period of phase mask using lens is theoretically discussed, this method can improve the limitation of phase mask method. The variation trend of FBG wavelength with tuning parameters is proposed. The analysis results show that FBG wavelength can be continuously tuned within a certain range using this method. Through experiment, FBGs with different wavelengths are attained using the same phase mask by adjusting the distance between phase mask and fiber, as well as the distance between phase mask and tuning lens.With the inscribed FBGs, all-fiber Tm³⁺-doped fiber lasers are achieved by using two different methods. All-fiber Tm³⁺-doped fiber laser using pump combiner is the best way to achieved high power all-fiber fiber laser. In the end, acousto-optical Q-switch operation of Ho: YAP laser at 2.1μm wavelength pumped by all-fiber Tm³⁺-doped fiber laser is achieved.