Study On Coherent Beam Combination Technology Of High-energy Short-pulse Lasers

With the scientific needs for the high-energy super-high density physics research, the coherent combination of multi-way short pulses with hundreds kilojoule output energy to obtain extreme physical state has become the goal of the researchers. The ultimate output power of single laser is limited by nolinear effect, facet fracture, thermal lens and compressor grating size. Meanwhile, coherent combination of multiple laser beams provides an effective approach for achieving high-energy super-high density focal spot, and the goal for coherent combination on the large-scale solid-state laser facility has been proposed by the main high-energy short-pulse research institute in the worldwide range. The size and threshold of the compression gratings determine the maximum output energy for all short-pulse laser systems using the chirped-pulse-amplification (CPA) technology. Production of gratings with big size is technically difficult and financially impractical, so most researchers consider that the most efficient and financial method is using phased array grating to increase the grating size. Because the arrayed grating is applied here, the single laser beam divided into two ways should be coherently combined firstly, which is the notable character of high-energy short-pulse laser facility.At present, the coherent combination research of high-energy short-pulse laser is focusing on concept exploring and preliminary design. The general researches on the issue are studied in this paper, the coherent combination based on the CPA technology is systematically analyzed and the corresponding physical model is setup. The numerical simulation results with practical considerations give the error ranges of chief influencing factors, which is the foundation for designing the coherent combination technology programme. The key technology of phase-locking is demonstrated in the laser coherent combination. As the results, the following progresses have been made:1. Systematic study about the coherent combined focusing of mulit-way high-energy short-pulse lasers is made by theoretical method. Based on diffractive optics, the influencing factors, including near-field amplitude distribution, beam layout, laser waist radius, f number, phase difference between beams, beam directivity, wavefront distortion, pulse spectrum width and dispersion, on the spatial and temporal coherence characteristics are analyzed both qualitatively and quantitatively, and the corresponding error tolerances and physical models are also given which is very helpful for designing multi-channel short-pulse coherent combination.2. The coherent combined technical route is described to produce ultra-high peak power density. Because of the low repetitive frequency of pulse laser, the pulse laser soure is instead by countinuous laser source when collimating and adjusting laser line. In the experimental demonstration, the key techniques of phase-locking between beams have been tested to achieve 2-way countinuous laser beams coherent combination using stochastic parallel gradient descent (SPGD) algorithm under the vibrating laboratory condition, which can be drawn lessons by engineering facility.3. The high-energy short-pulse laser compressor uses the array grating, which makes the single beam seperate into two-way beams. The phase between the beams stochastically changes by the external vibrating effect. Therefore, the coherent combination of single beam lines should be solved firstly. Combining the coherent combinatino theory and grating compressor phase character, the model for array optical elment is setup. The geometric tracing and the Huygens - Fresnel diffraction theory are used to handle the far-field spatial distribution of array grating compressor and array parabolic mirror. The array grating compressor is dispersion compensation component, and the analytical expressions of a pulse delay and pulse broadening are deviated from Treacy compressor configuration to describe the temporal characteristics of the combined waveform,4. In order to better carry out engineering studies on optical array phase-locking, an optical-electromechanical integrated model is established to analyze the far-field focal spot under vibration motivation of different supporting structures. The focal-spot shape is improved in the closed-loop feedback mode. Finally, the closed-loop system consisted of the displacement sensors and piezoelectric actuators and the automatic phase-locking by the far-field CCD camera and actuators are achieved on the optical supporting shelf. After phase locking, the root-mean-square error is limited within several nanometers and the stablization time is over one hour, which meets the engineering requirements for arrayed system.5. The Von Karman disturbing spectrum is preferably used to describe the wavefront distortion, and the coherence between beams is determined by phase relationship. For the supplement of coherent combination cases of large-size high-energy laser beams, the Von Karman disturbing spectrum and random phase screens are used to build an integrated model to simulate coherent combination, non-coherent combination and partial-coherent combination under the distorted wavefronts, and the simulations result in good agreement with the reported experimental results. The integrated model is very convenient for simulating and optimizing the multi-way short-pulse combined focal spot.Considering the current study of the high-energy short-pulse laser facility, the studies on combined focusing and tiling technique in this paper more focuses on 2×2 array unit. Because the Quad (2×2) structure is taken as basic grouping unit in such large-scale laser facility, starting from the basic unit can be more closely combined with the actual situation and has good scalability and application value.