Research On Key Techniques Of Laser Far-field Focal Spot Diagnosis Based On Iterative Phase Retrieval

Inertial Confinement Fusion(ICF)is a frontier field of domestic and foreign basic scientific research because of its important value for the development of new energy in the future.High-energy laser driver is one of ICF driving source technologies,and a series of high-energy laser fusion driver devices have been built at home and abroad.The device is composed of hundreds of large-aperture optical elements.The wavefront errors introduced by the surface error of elements,system assembly error,and thermal distortion make the laser far-field focal spot in the target point presenting the distribution characteristics of"main lobe"and"side lobe".The size and distribution of the focal spot will ultimately affect the effect of laser targeting.Therefore,how to measure the intensity distribution of laser far-field focal spot with high precision is of great significance for the performance evaluation and operation of the high-energy laser fusion drive device.Due to the limitation of the dynamic range of the photoelectric sensor,the direct measurement of the high-energy laser far-field focal spot adopts the array camera method and the schlieren method successively.The array camera method is limited by the dynamic range and the size of the detector,resulting in the loss of"side lobe"information of the focal spot.The blocking device used in the schlieren method is difficult to completely block the"main lobe"of the laser far-field focal spot,and the schlieren method has high requirements on the light path control system.The indirect measurement method indirectly reconstructs the laser far-field focal spot by measuring the near-field complex amplitude of the high-energy laser.The iterative phase retrieval method can theoretically realize the measurement of the full frequency band information.Therefore,this dissertation provides an effective technical approach for high-precision diagnosis of laser far-field focal spot by studying the key technology of laser far-field focal spot diagnosis based on iterative phase retrieval.The main research contents of this dissertation are as follows:1.A modified scaling angular spectrum(MSAS)method is proposed for numerical simulation in long-distance propagation.The calculation model of MSAS method is theoretically deduced,and the selection requirements of the optimal calculation window and the calculation complexity are also analyzed.In theoretical simulation,the MSAS method is compared with the two traditional angular spectrum(zero-padding angular spectrum,PAS and band-limited angular spectrum,BLAS)methods in long-distance and accurate Gauss-quadrature numerical integral(NI)method.The simulation results of the proposed method are consistent with the simulation results of NI method.When the propagation distances are ₁z(28)100mm and z₂(28)500mm,its signal-to-noise ratios(SNRs)are 1.66?and 2.68?respectively higher than that of the BLAS method.And the calculated speeds of the MSAS method are 7.77?and 51.54?respectively faster than that of the PAS method when the propagation distances are ₁z(28)100mm and z₂(28)500mm.Finally,the experimental results verify the correctness of the proposed MSAS method.2.An improving physical parameters estimation method in single-beam multiple-intensity measurement reconstruction(PE-SBMIR)is proposed.Compared with the traditional single-beam multiple-intensity measurement reconstruction(SBMIR)method,the parameter estimation model of distance is introduced in SBMIR.And the parameter optimization algorithm for the PE-SBMIR method based on the combination of Quantum genetic algorithm(QGA)and nonlinear optimization algorithm(NOA)is given.The parameter optimization algorithm is used to accurately estimate the propagation distance parameters,avoiding the influence of the distance errors caused by the translation of the detector and the straightness of the motion mechanism on the accuracy of the SBMIR method.Through theoretical simulation,the best detected step length and the number of detected step under the simulation conditions in this dissertation are given.Compared with the SBMIR method,the simulation results show that the measured accuracy requirements for the distance parameters of the PE-SBMIR method can be reduced from the order of micrometers to the order of hundreds of micrometers.Meanwhile,the proposed method has good noise resistance.The experimental results show that the resolution of the PE-SBMIR method is improved from 44.19?m to 17.54?m compared with the SBMIR method.3.A new phase retrieval method using sequential phase modulations(PRSPM)is proposed.The requirements and the complexity parameter model of the modulation phases are given.The PRSPM method modulates the phase of the light field to be measured in sequential modes to ensure its convergence.Compared with the SBMIR method,this method does not introduce the influence on the accuracy of phase retrieval resulted by the distance errors of the movement of the detector and the straightness of the motion mechanism.Through theoretical simulation,the retrieved results of the proposed method are analyzed under the modulation phase mode coefficient,the number of modes and the modulation phase of different modes.The maximum RMS(Root mean square)value of the proposed PRSPM method between the retrieved and the theoretical phases is2.82?10^-4.Finally,the effectiveness of the proposed PRSPM method is also proved by building an experimental device and its resolution is 35.08?m.4.A reconstructed method of laser far-field focal spot based on iterative phase retrieval is proposed.The complex amplitude of the laser near field retrieved by the PE-SBMIR and PRSPM method is respectively substituted into the laser far-field focal spot reconstruction model to obtain the distribution of laser far-field focal spot.The chirp-z transform(CZT)is introduced to replace the fast Fourier transform(FFT),which can avoid the under-sampling of the focal spot.At the same time,the calculation complexity is lower than the zero-padding FFT of the dimensional expansion.The simulation results show that the PIB(Power in the bucket)curves of the reconstructed focal spots under the two methods completely coincide with the PIB curve of the theoretical focal spot.An experimental device is built to reconstruct the laser far-field focal spot using the above two methods.The correlation coefficients between the reconstructed focal spots and the theoretical focal spot using the PE-SBMIR and PRSPM methods are 0.9976 and 0.9827,respectively,while the correlation coefficient between the measured focal spot using the traditional long-focal lens imaging method and the theoretical focal spot is 0.9477.Both simulation and experimental results show that the iterative phase retrieval method can provide a technical means for the high-precision measurement of the laser far-field focal spot.