The inertial confinement fusion (ICF) high-power laser facility is an important driver of laser fusion ignition and high-energy-density physics. To achieve fusion ignition, the energy control accuracy, time-power curve controllability, spectral control, and control of near-and far-field beam quality directly affect the results of fusion-ignition physics experiments. High-power laser devices have been developed in China for more than 60 years, and the front-end and pre-amplification technologies of high-power laser facilities have evolved from single-time waveform control and synchronous static control to the high stability and global active controllability of laser beams. This paper describes the development of precision control technology for front-end and pre-amplification laser systems in SG-II series facilities.

A high-power laser driver is primarily composed of a front-end system, pre-amplification system, main amplification system, and terminal system. The front-end and preamplification systems serve as the driver source, which is typically referred to as the laser injection system. The front-end system serves three major functions: (i) provide seed pulses to the device to satisfy various requirements with the corresponding functions and accurate synchronization; (ii) control the laser time domain, frequency domain, and signal-to-noise ratio (SNR); (iii) perform detection and abnormal feedback control. Similarly, the pre-amplification system serves three major functions: (i) pre-amplify the pulse energy to realize the J-level energy output of the shaping pulse; (ii) precisely control the near-field distribution of the beam and gain spectral pre-compensation; (iii) control the service capabilities of the device, which provides output laser pulses with a certain repetition rate and energy for the subsequent auto-collimation system of the optical path, wave-front correction, and system-operation monitoring.

The ICF high-power laser device includes multiple main nanosecond laser pulses and short picosecond pulses. The former is intended primarily for achieving high-density compression of the target sphere, which requires high spatial uniformity and time symmetry. The latter is primarily used in diagnostic lasers or ignition heating pulses. Synchronization between nanosecond main lasers and picosecond short-pulse lasers is the core requirement for the active diagnosis of the evolution of the high-energy-density physical state as well as for investigating rapid ignition and high-efficiency implosion. The National Laboratory on High Power Laser and Physics, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences (hereinafter referred to as the front-end research group) has developed a high-precision synchronization scheme with a homologous clock-lock, (a phase-locked frequency technology), which achieved a long-short pulse synchronization accuracy of <20 ps (PV) and 3 ps (RMS) (2 h test results). Moreover, the electric-scale signals and high-precision synchronous trigger signal systems required for multichannel physical test diagnosis have been established. The time jitter between the signal and main laser is <3 ps (RMS) and 20 ps (PV), and a certain amount of delayed tuning is satisfied.

A high-power laser system is used to drive the ignition of the ICF, for which precise control of the time–power curve is the main requirement. To improve the accuracy of pulse shaping and the system efficiency, a feedback closed-loop control system is typically adopted to realize the closed-loop control of the pulse-time waveform. The front-end research group utilized an AWG and closed-loop feedback control, in addition to neural-network algorithms, to realize an accurate online calibration of the transfer function of the front-end pulse-shaping system (Fig. 1). Consequently, a 1 Hz real-time noise reduction of shaping signals, as well as various types of isentropy-like double-slope pulse operations are realized.

Meanwhile, transverse stimulated Brillouin scattering must be suppressed (TSBS) to protect large-aperture optics and ensure the uniformity of the focal spot; hence, spectral-broadening technology is adopted in high-power laser systems. However, after spectral broadening, the discrepancy of the transmission rate of the spectral phase or amplitude modulates the time waveform, which is an FM-to-AM effect. To improve the temporal-power curve, a full single-polarization front-end system based on a single-polarization transmission fiber and a group-velocity dispersion (GVD) compensation unit were utilized in the front-end system to solve the FM-to-AM conversion caused by the polarization mode dispersion (PMD) and GVD (Fig. 4). Real-time monitoring of the FM-to-AM conversion was performed to observe the modulation depth caused by different modulation frequencies to provide feedback and control the FM-to-AM conversion caused by the GVD. Additionally, fidelity-amplification technology was developed for the full spectral range, thus solving the gain narrowing of the amplification system and resulting in a fundamental-frequency time-domain modulation degree of less than 5%@0.3 nm (3 GHz+20 GHz).

Subsequently, the FM-to-AM degree of the nanosecond terminal and target surface was evaluated. The results show that the combination of the dispersion grating used in smoothing via spectral dispersion (SSD) and the focusing system is equivalent to an 8 GHz (3 dB bandwidth) Gaussian low-pass filter (Fig. 7). The modulation degree on the incident surface of the wedge focusing lens (WFL) is 19%, whereas that on the target is 4.9%, and the high-frequency components can be effectively filtered out.

For near-field control, the front-end research group developed a near-field intensity control scheme (Fig. 10) and a series of light-field control devices, such as passive-dielectric-film binary optical elements (aperture: 50 mm×50 mm; damage threshold: 7 J/cm²)(Fig. 12), active optically addressed spatial light modulators (aperture: 300 mm×300 mm; damage threshold: 300 mJ/cm²) (Fig. 13), and optically oriented liquid-crystal shaping devices (aperture: 62 mm×62 mm; damage threshold: 2 J/cm²) to control the near-field intensity distribution of high-power laser devices. To increase the irradiation energy density to 1 J/cm², spatial-light modulator technology based on high-damage-threshold photoconductive materials is being developed to adapt to more application scenarios of high-power lasers. To remain relevant with the development of ICF laser drivers and the continuous expansion of the functions of the preamplifier system, the front-end research group has developed a repetition-frequency preamplifier system (Fig. 18) based on LD pumping, which can achieve an output energy of 3 J/Hz, an energy stability level of 0.5% (RMS), and an output beam near-field modulation of ≤1.21.

Since the invention of lasers, laser-fusion drive technology has been developed for more than 60 years. To satisfy the application requirements of laser fusion ignition, precision control technology for lasers has been developed and has progressed from the initial time-domain smoothness and coarse synchronization technology to high-precision, high-stability, long-range centralization, thus providing controllability in time, space, and frequency domains, as well as in polarization. Additionally, the development of precision control technology for laser fusion drivers necessitates the development of a laser injection system for high-power laser devices.

Owing to the success of ignition in the United States, laser fusion has evolved from exploratory technology research to the realization of laser fusion energy. Regarding the challenges of laser technology, in addition to the high repetition frequency of laser technology, one should improve the beam-target coupling efficiency of lasers and the key control device to achieve an efficient repetition-frequency operation.