Lithium Niobate Waveguide Based On Femtosecond Laser And The Design Of The Periodic Gratings

In recent years, waveguide grating has received widespread attention because its easy large scale integration and production, and real-time tune is achieved. Materials selection of waveguide grating has diversity and multifunctionality. Waveguide grating prepared by inorganic crystal materials, such as LiNbO₃, has a faster tuning rate. The main preparation methods are Ti-diffusion, proton exchange, femtosecond laser inscription, etc. Among them, femtosecond laser inscription has received increasing attention due to the advantage of great speed, high precision, low damage threshold, etc. Short period waveguide Bragg grating becomes more challenging because of the shorter period and the more complex preparation process.In this paper, LiNbO₃ waveguide inscription by femtosecond laser is researched while X cut LiNbO₃ acts as the base. Based on transfer matrix theory and photoelectric effect of LiNbO₃ crystal, double side band?DSB? modulation Bragg waveguide grating by one-step preparation is designed, and on the foundation of DSB, tunable waveguide Bragg grating with an electrode configuration and tunable chirp Bragg waveguide grating are designed. Our analytic works in concrete are mainly as follows:First, a brief introduction to LiNbO₃ crystal is showed, including index ellipsoid, photoelectric effect, photorefractive effect, and the theory of LiNbO₃ waveguide inscription by femtosecond laser is introduced in details. Theoretical models of periodic waveguide grating is reasoned and analysed in details. Theory is prepared well for back-end work.Second, the experiment of waveguide inscription by femtosecond laser is completed initially. The theory that laser power and irradiation time influence ablation of LiNbO₃ is analysed.Third, double side band modulation Bragg waveguide grating by one-step preparation is designed, in which spatial modulation is achieved by periodically changing the width of the waveguide. The relationship between the reflection spectrum characteristic and the waveguide width, the difference of waveguide width, the duty cycle of gratings, the changes of waveguide in the shape is simulated.Fourth, on the foundation of double side band modulation, Bragg waveguide grating with an electrode configuration is designed. Tunable chirp waveguide Bragg grating and tunable chirp Bragg waveguide grating are achieved by changing the electrode configuration. And feasibility of the two configurations is theoretically analysed.