As an important frontier issue, inertial confinement fusion (ICF) has been concerned by researchers for a long time. Because of its strategic significance in national defense security and energy security, major countries in the world have invested a lot of resources to carry out research on ICF, and the most representative high-power laser drivers include the National Ignition Facility (NIF) in the United States, the Laser MegaJoule (LMJ) in France, and the Shen Guang series (SG) laser device in China.
In December 2022, NIF provided energy up to 2.05 MJ in the form of laser pulses to the target, generating a record-breaking energy output of 3.15 MJ. It was a milestone event in ICF research that for the first time more energy was produced from the self-sustaining fusion reaction than the energy put into it. A series of experimental data indicate that a key breakthrough has been made in the ICF research, and the improvement of the output efficiency of the device requires further improvement in laser energy and irradiation uniformity, which is essentially an improvement in beam quality (Fig. 1).
Since 2010, our group has paid attention to the influence of beam quality on the output performance of high power laser drivers. Many factors such as material purity, density uniformity, machining accuracy, installation and calibration process, thermal distortion, and use environment of optical components will affect beam quality. It was demonstrated that uneven beam energy distribution would induce nonlinear effects which makes damage to the optical elements and results in further degradation of the beam quality. Therefore, precise optical measurement methods are needed to timely discover the beam degradation characteristics and improve the beam quality with appropriate optical compensation means (Fig. 2).
Optical elements are generally measured by interferometers. Zygo interferometer is mature and highly instrumented, which can accurately measure large aperture optical elements. However, a large aperture interferometer demands large space and a high price. Moreover, it is difficult to manufacture the optical standard parts of the interferometer when measuring aspheric optical elements, which affects the measuring accuracy and application range.
Non-interference wavefront measuring instruments represented by Hartmann sensor are generally used to measure beam quality. Hartmann sensor is composed of a microlens array and CCD, which can record the intensity and phase of the beam. With a simple structure, small size, fast measurement speed, and good anti-interference effect, Hartmann sensor has been widely used in NIF. When combined with the deformable mirror, Hartmann sensor can measure and control the wavefront distribution of the target beam in real time and correct the wavefront distortion (Fig. 5). The measurement accuracy of Hartmann sensor is limited by the number of the microlens array and the size of a single measurement unit, and thus the spatial resolution is low. Moreover, when the phase gradient of the wave front changes greatly, the signal crosstalk will appear in the focal plane, and the phase offset cannot be accurately judged.
In view of the limitations of traditional measurement methods, researchers have turned their attention to computational optics. Coherent diffraction imaging (CDI) has made considerable progress. Based on the coherent diffraction principle, CDI can reconstruct the amplitude and phases of illumination and object to be measured simultaneously by iterative calculation. Due to the merits of a simple lightpath, low hardware requirements, and high imaging accuracy, CDI has been applied to the measurement of optical components and beam quality at the same time. Ptychography iterative engine (PIE) and coherent modulation imaging (CMI) are typical CDI techniques. PIE records a series of diffraction patterns by scanning multiple positions and then reconstructs the complex amplitude distribution of the sample and probe beam through iterative calculation. CMI uses the code plate with known distribution to reconstruct the complex amplitude distribution of the beam to be measured through iterative calculation.
In 2000, Matsuoka et al. firstly applied CDI to phase measurement of TW-level femtosecond laser pulses with an accuracy of λ/30 (PV) and λ/200 (RMS), which is higher than the measurement accuracy of phase profilometer (Fig. 26). In 2012, CDI was successfully applied to the focal spot diagnosis process of OMEGA EP device. By accurately measuring the phase error of the system, the far-field focal spot similarity was increased from 0.78 to 0.94 (Fig. 29).
A lot of optical detection work on SG-Ⅱ devices using CDI technique has also been implemented. First, our research group carried out a lot of theoretical research on CDI. 1) Our research group proposed a single-shot PIE scheme, which improved the data acquisition speed of PIE and realized single-shot 3D imaging after solving the problem of highly tilted illumination. 2) By designing a multi-step phase plate and combining a multi-mode algorithm, a multi-mode CMI algorithm was developed, which could reconstruct the complex amplitude distribution of multiple beams with different wavelengths to be measured from a single diffraction pattern. 3) Combining the advantages of CMI single exposure and PIE high precision reconstruction, our research group developed beam splitting coding imaging technology, which greatly improved the reconstruction accuracy of single exposure imaging technology.
Second, computational imaging technology is applied to the detection of optical components of high power laser drivers and wavefront of the target beam. Optical element detection mainly includes phase measurement, thermal distortion measurement, stress measurement, and damage measurement of large aperture optical elements. The detection of the target beam wavefront mainly includes near-field complex amplitude distribution, focal spot complex amplitude distribution, time domain waveform distribution, as well as the measurement of the interaction between laser and matter in the ultrafast event.
Third, the research group also established an analytical model of CDI, which proved the uniqueness of CDI solution mathematically, laying an important mathematical foundation for the development of CDI as a measuring instrument.
In general, a theoretical system of computational optical imaging based on coherent diffractive principle was established in the optical detection of high power laser drivers. A series of related instruments have been developed for the detection of optical elements and the detection of the wavefront of the target beam, which provides important technical support for the efficient operation of high power laser drivers.
昌成成, 潘良泽, 徐英明, 吴丽青, 陶华, 刘登, 陈飞, 刘诚, 朱健强. 计算光学成像在惯性约束聚变中的应用及技术进展[J]. 光学学报, 2023, 43(22): 2200001. Chengcheng Chang, Liangze Pan, Yingming Xu, Liqing Wu, Hua Tao, Deng Liu, Fei Chen, Cheng Liu, Jianqiang Zhu. Application and Progress of Computational Optical Imaging in Inertial Confinement Fusion[J]. Acta Optica Sinica, 2023, 43(22): 2200001.