Significance The high- power laser facility (HPLF) is the most sophisticated active optical system, whose performance is required to approach the physical limits. There are three primary procedures in designing HPLFs for research on inertial confinement fusion (ICF), namely, physical design, optical design, and mechanical design, in which optical engineering plays an important role. Optical engineering of HPLFs needs to follow its specific design principles and key points to achieve high output energy and precision beam quality. Given the final goal and feature of the laser facility, this paper describes the crucial technical issues in optical engineering development and their corresponding solutions from the aspects of the techniques for beam quality control, system alignment, target positioning, and associated technologies. Progress The high-power neodymium glass laser driver is internationally recognized as the most mature laser driver for physical research on ICF. It is also an indispensable and important platform for studies of high-energy-density physics and extreme-condition physics problems, as well as basic research on astrophysics. As one of the most representative giant optical projects in the world, the laser driver integrates advanced optics, laser technology, precision machinery, and computer control. Its scale and overall performance represent a country's highest level of laser technology and engineering. Many great powers of science and technology all over the world have carried out research on laser fusion and implemented the development plans of laser drivers, and several generations of HPLFs have been successively developed and built. The United States built the world's largest experimental device for laser fusion, i. e., the National Ignition Facility ( NIF), in 2009. France is building a large-scale laser nuclear fusion device LMJ, which is similar to the NIF, and 56 laser beams in it have been operating at full power since 2019. Japan, the United Kingdom, and Russia have also built or are building smaller laser fusion devices. At the same time, China has successively built large-scale single-channel laser devices, sixchannel laser devices, and ShenGuang (SG) series laser devices, playing a vital role in the international arena of ICF. With the development of HPLFs in China, the research on the laser driver, unit technology, and component technology has also been rapidly enhanced. HPLFs mainly include nanosecond (ns) laser devices and picosecond (ps) petawatt laser devices. The former outputs a ten-thousand-joule-level high- energy ns laser in a single channel, and the latter outputs a kilojoule-level high peak power ps laser in a single channel. The design of HPLFs consists of energy flow design and beam transmission design. The laser physics design of a laser driver is the first stage, followed by the optical engineering design and precision opticalmechanical structure design, and the final stage focuses on the development of the laser driver. This design logic indicates the important role of engineering optics in the development of laser drivers. An HPLF should meet the requirements of the output ability, beam quality, and beam-control ability proposed by physics. The device mainly contains several core indicators: energy, power, beam quality (focus spot distribution), waveform (time distribution), energy balance and power balance, as well as synchronization and target-hitting accuracy. Among them, beam quality and target-hitting accuracy are closely related to optical engineering design. Due to the large scale of HPLFs, long laser links, and large-caliber optical components, the various development processes need to be combined to satisfy the requirements of the above indicators, and the indicators of each subsystem must be reasonably allocated and strategically controlled throughout the system. The engineering optics of an HPLF has its particular traits. The device contains not only traditional static components but also dynamic components such as laser amplification and control components. Due to the thermal effect, the beam quality will be degraded after the laser passes through the amplifying element, and control measures are required to ensure the beam quality of the system. In addition, the high output energy of the device may cause damage to optical elements, and corresponding measures must be taken to minimize the risk of damage to ensure safe operation. Starting from the main considerations of engineering optics, i. e., optical design and ways to ensure the beam quality and target- hitting accuracy of the system, this paper summarizes the key scientific and technical issues concerning the engineering optics of the existing device. The overall optical design of the device is mainly to " set up the frame", establish the overall optical transmission chain of the device, and give the indicator requirements of components. The beam quality control is a "construction method", and linear and nonlinear transmission run through the entire laser device, which requires a whole picture of consideration. Specifically, the relationship between optical component indicators and beam quality should be clarified; the design, processing, and detection methods of special optical components should be determined; the corresponding optical detection means and active control methods should be matched; the residual wavefront and intensity unevenness of the system should be precisely controlled. In this way, the beam quality can be guaranteed. Target-hitting accuracy is the "foundation", which mainly provides decomposition methods for indicators and effective optical-axis control methods, and it cooperates with precise guidance and alignment technology. Conclusions and Prospects Achieving controllable nuclear fusion and energy gain under laboratory conditions has long been a great dream pursued by scientists, and it is the most challenging major scientific project in the world today. The high- power laser driver for ICF and high-energy-density physical experiments has a powerful output ability of several megajoules of ns laser pulse energy, and it is equipped with tens of kilojoules of ps laser and corresponding output capabilities of 100 ps, femtoseconds ( fs), or short wavelength laser pulses according to different physical demand. At present, powerful HPLFs are one of the focuses of the strong laser competition among the world's major countries. According to the design indicators and design characteristics of high-power laser drivers, this paper sorts out the key scientific and technical problems and corresponding solutions in optical engineering design in detail from the aspects of overall optical design, beam quality control, and target- hitting accuracy control. The research is expected to provide a reference for the engineering design of high-power laser drivers.
