Study On The Laser Near-field Characteristics And Beam-control Technology Based On Volume Bragg Gratings

“Power constraint” in high-power laser system is always one of the important questions in the process of the laser development. The spatial filtering is one of the key components in the laser system. Owing to its focusing characteristics, the traditional pinhole spatial filtering cannot satisfy the requirement for the future high-power laser system in compactness, availability and low-cost. The volume Bragg gratings(VBGs) based on photothermorefractive glass(PTR) can realize the near-field filtering for the high-power laser and overcome the drawbacks of pinhole spatial filtering due to its high diffraction efficiency, excellent angular selectivity and high damage threshold, which may be widely used in the future high-power laser system. However, there are still some scientific and technical problems in the integrated application of VBGs, such as the wavefront characteristics, decrease of fill factor and optical-axis deflection, etc.Based on above problems, the near-field characteristics and beam-control technology based on VBGs have been further studied in this dissertation, which can provide some new method and technology storage for the development of high-power laser system. The main research results of this dissertation are as follows:(1) The near-field beam-control technology based on the Hump Volume Bragg gratings(HVBGs) is proposed. The physical model and the diffraction characteristics of HVBGs are analyzed with the coupled-wave theory. The ability to filter the spatial modulation with different periods in the laser beam is discussed. The stimulation results show that the near-field band-stop filtering(BSF) based on the HVBGs has the ability to suppress the rapid increase of specific medium-high spatial frequencies in the laser system, which may be used in improving B-integral in high-power laser system.(2) The BSF based on two cascaded VBGs is proposed and studied. Theoretical and experimental results show that when the aiming beam is incident on the VBGs deviated from the Bragg condition, some certain spatial modulations with medium and high frequencies satisfy the Bragg condition and can be effectively diffracted. Then, the VBGs can be regarded as a near-field band-stop filter. The affecting factors for the BSF have been studied with the coupled-wave theory. The characteristic spatial frequency and bandwidth of the BSF are determined by the deviation from Bragg condition and angular selectivity of VBGs, respectively. This technology can be used plug-and-play in the high-power laser systems.(3) The wavefront-optimized technology based on VBGs has been proposed. Based on both theoretical and experimental analyses, the influence of VBGs’ angular selectivity on the wavefront-optimized ability for the wavefront distortion has been discussed. Experiment results show that: the medium-high wavefront distortions in the laser beam can be effectively optimized with the VBGs. The peak-valley value and root-mean-square value are reduced 9.93 times and 5.05 times, respectively. The focusing characteristics of laser beam are obviously improved. The Strehl ratio of the beam increases 2.23 times and the far-field distribution is uniform. This technology may relax the B-integral constraint in the high-power laser system.(4) Combined the coupled-wave theory with the exact measurement of the refractive index and loss coefficient of VBGs, the design method of structural parameters for loss volume Bragg diffraction elements is established. This method can obtain the optimum structural parameters of VBGs according to actual requirements in the laser system, which lays a theoreticalfoundation for the preparation of the volume Bragg diffraction elements and the varied near-field beam-control technology.(5) The FM-to-AM conversion introduced by the diffraction of sinusoidal phase modulated beam through VBGs is studied. The non-uniform spectral diffraction of sinusoidal phase modulated beams through the traditional angular filter due to its spectral selectivity results in the FM-to-AM conversion. Based on coupled-wave theory, the VBGs with better angular selectivity and broad spectral selectivity have been designed and satisfy the filtering requirement of different phase modulated beams in thehigh-power laser system. It is of great significance in the near-field beam-control technology in the high-power laser systems.The results in this dissertation may has potential applications in the optical detection and high-power laser system development, such as increasing laser output, wavefront optimization, near-field beam control and far-field characteristics improvement, etc.