Study On Wavefront Optimization Based On Transmitting Volume Bragg Gratings

Wavefront quality is one of the most important parameters for laser performance.It directly determines the far-field distribution and harmonic-conversion efficiency of laser beam.In the high-power laser system,the small-scale self-focusing and the thermal-image effects caused by the nonlinear growth from the small-scale wavefront noise,may aggravate the deterioration of laser beam and greatly restrict the output ability of laser system.Therefore,it is of great practical significance to study the optimization of wavefront distortion in high-power laser system.Currently,the main methods to optimize the wavefront distortion in laser beam include the adaptive optics and the focusing spatial filtering.The adaptive optics is used to correct the low-frequency wavefront distortion and the spatial filtering is used to eliminate the high frequency wavefront distortion.But they both have the complex structure,large scale and high cost.Additionally,due to the intrinsic shortcoming such as pinhole-closure effect,the traditional spatial filtering is difficult to satisfy the requirements of high efficiency,compactness and high stability for the future high-power laser system.Due to the excellent Bragg selectivity,high diffraction efficiency and high damage threshold,the volume Bragg gratings?VBGs?recorded in photo-thermo-refractive?PTR?glasses can directly control the near field of laser beam in the process of transmission and amplification,and are widely used in the advanced beam-control technology.In this thesis,the transmitting volume Bragg gratings are used as the wavefront optimization elements.The relationship between the angular selectivity of the VBGs and the wavefront optimization ability is studied theoretically and experimentally.Firstly,combining the coupled-wave theory and Fourier optical principle,the relationship between the grating parameters and the diffraction characteristics of the grating is analyzed.The wavefront optimization calculation model of the volume Bragg grating is established.The optimization effect on the specific wavefront distortion with different angular selectivities has been stimulated.Results show that,the wavefront distortion with 100%modulation can be effectively optimized with the VBGs.The better angular selectivity of VBGs means the better PV and RMS values of diffracted beams.Meanwhile,the encircled energy can also be improved.From the PSD function,when the angular selectivity of the VBGs is smaller than the spatial frequency of wavefront distortion,the wavefront distortion can be effectively filtered.When the angular selectivity of the VBGs is larger than the spatial frequency of wavefront distortion,the wavefront distortion cannot be completely filtered.However,it can be suppressed to a certain extent.The experiment on the wavefront optimization with three VBGs of different angular selectivities is demonstrated.The results show that the wavefront distortion with the spatial frequencies of 0.4mm^-1 and 0.5mm^-1with 100%modulation can be effectively suppressed by VBG with angular selectivity of0.38mrad,and can be partly suppressed by VBG with angular selectivity of 0.89mrad and1.24mrad.The suppression effect is inversely proportional to the angular selectivity of the grating.The experimental results are in good agreement with the theoretical results.