Fabrication Of Optical Waveguide In LiNbO₃ By Femtosecond Laser And Its Transmission Characteristics

Recently, the fabrication of optical waveguide with femtosecond laser has received widespread attention in integrated optics. However, the most relevant investigations only focused on the optical transmission performance of the waveguides fabricated by changing the parameters of femtosecond laser, and paid little attention to the physical mechanism. This thesis mainly studies the formation mechanism of waveguide. Using 76 MHz and 1 kHz femtosecond laser for lithium niobate crystal etching experiments, and then explore its formation mechanism by studying the laser active region of microstructure changes. The work includes the following:(1) Comparing the SEM morphology and EDS distribution, we found that the different microstructures were observed by using the two types of repetition rate femtosecond laser in lithium niobate crystals have. The lithium niobate crystal is damaged under the femtosecond laser irradiation with 1 kHz and high single pulse energy. Due to the femtosecond laser of 76 MHz, the pulse interval is lower than the material absorption time and energy accumulation. The gradient temperature field and the temperature gradient force are produced in the etching area. The action of the molten ion-diffusion and deposition lead to the formation of a laser around a high-density and high refractive index. It is advantageous to the constraint and transmission of light formed good optical waveguide structure.(2) For single laser pulse energy about 3 nJ, the absorption time of the lithium niobate crystal is 1 ?s. We set up the theoretical model of heat accumulation effect for the laser repetition rate and the focus temperature. According to theoretical analysis, we found that heat accumulation effect on lithium niobate crystal would play a key role when the laser repetition rate was of 11.8 MHz. While the repetition frequency was below 11.8 MHz, the thermal effect was almost negligible. Under the condition of low repetition frequency, the etching area detonates due to the higher single laser pulse energy. Meanwhile, the crystal structure changes because of the lower thermal effect. For the high repetition frequency, the pulse interval time is short. The function area of energy accumulation produces high temperature and high pressure, causing the crystal modification and obviously thermal effect.Meanwhile, we simulated that the thermal effect area with the femtosecond laser of 76 MHz were 36.5 ?m in diameter, and conformed to our experiment results the well. When the laser repetition rate changes to 25 MHz, the simulated effect area was 28 ?m in diameter, and the etching area using the femtosecond laser reached 27.5 ?m in diameter at experiments. In order to verify the etching properties and light transmission performance of the annular zone, the linear optical waveguide were prepared.(3) The etching region can be divided into two region which were inner interaction region and thermally modified region respectively. The coupling experiment shows two single mode buried waveguides which are generated in the thermally modified region, and the propagation loss of the two waveguides are below 1.5dB/cm. At the same time, we discovered that the inner interaction region lead to a low transmittance. The Raman spectrum of the etching region was measured, which showed the light-guide region of densification by the thermal effect with high repetition and the large-scale defects in the inner interaction region.