Subnanosecond pulsed lasers with pulse durations ranging from 100 ps to 1 ns have the advantages of both high peak power and high energy, which play an important role in laser processing, laser ignition, photoacoustic imaging, nonlinear optical frequency conversion, and laser-induced breakdown spectroscopy (LIBS). Improving the output power while maintaining good beam quality has always been the focus of research on high-performance lasers, especially for high-repetition-rate lasers that encounter severe thermal effects. Passively Q-switched microchip lasers are commonly used to generate sub-nanosecond laser pulses due to their compact structure, robustness, high beam quality, good spectral purity, and low cost. However, their power scaling is strictly limited by the bonded crystal structure with double-end coatings as the cavity, as thermal management and cavity design are impracticable. Fortunately, the master oscillator power amplifier (MOPA) is an ideal alternative. In this paper, a high-beam-quality subnanosecond MOPA laser system operating at a 1-kHz repetition rate with a peak power of approximately 20 MW based on spherical aberration self-compensation is reported.

The pump absorption in the gain medium of the end-pumped amplification stage leads to a significant thermal lens effect with substantial optical aberrations, making it difficult to achieve high beam quality during high-power operation. As a guide for the experiment, the temperature distributions inside the gain media of the two-stage amplifiers are simulated, and the thermal focal lengths f₁and f₂ are calculated using Seidel aberration theory based on the optical path difference (OPD) caused by thermally-induced refractive index changes. A passively Q-switched Nd∶YAG/Cr∶YAG microchip laser with a cavity length of 6 mm is used as the master oscillator to generate a sub-nanosecond seed pulse. The output power of the microchip laser is 180 mW at 1 kHz, corresponding to the single-pulse energy of 180 μJ. Then, the seed pulse is sent to the amplifier stages through a collimating lens and isolator. Both amplifiers use an identical bonded YAG/Nd∶YAG/YAG crystal as the gain medium and a similar end-pumped dual-pass structure, enabled by a polarizing beam splitter (PBS), quarter-wave plate (QWP), and reflecting mirror. The linearly polarized input and output lasers are orthogonal, while the laser inside the gain medium is circularly polarized to alleviate the impact of thermally-induced birefringence. To avoid laser-induced damage, the pump spot sizes are 0.9 mm and 1.2 mm, corresponding to pump peak powers of 170 W and 200 W, respectively. The reflecting mirrors are set at distances of f₁and f₂ from the gain media, where the sign of the spherical aberration after single-pass amplification is reversed. In this way, the spherical aberration becomes self-compensated after dual-pass amplification. In addition, the thermal lens and reflecting mirror form a 2f imaging system exactly to guarantee good mode matching. This scheme can also be applied to multi-stage amplification systems to further improve output power while maintaining good beam quality.

Simulation results show that the temperature distributions of two end-pumped bonded crystals in the amplifier are axisymmetric along the center of the crystal, where the high-temperature region is concentrated at the front end of Nd∶YAG owing to strong absorption, resulting in a gradient distribution of the refractive index inside the crystals, and additional OPD is introduced to the incident seed pulse. The values of OPD along the radial direction caused by thermal-induced refractive index change are presented (Fig. 2). Using the Seidel aberration theory, the thermal focal lengths f₁=136 mm and f₂=220 mm_{of the two amplifier stages are calculated based on OPD within the laser beam cross-section, which are consistent with the measured values. The two-stage amplifiers adopt a similar end-pumped dual-pass structure but with orthogonal output polarization to maintain the entire system within a tabletop size of 30 cm×70 cm (Fig. 3). The measured seed pulse possesses an average power of 180 mW, pulse width of 517 ps, and beam quality factor of M²=1.53, whereas unstable high-order modes symmetrically surrounding the fundamental mode are observed due to a relatively large pump waist (Fig. 4). The detrimental higher-order modes are effectively suppressed after amplification by optimizing the filling factor. In addition, benefiting from the self-compensation of spherical aberration and good mode matching, a high-beam-quality sub-nanosecond MOPA laser with an average power of 9.7 W and beam quality factor of M²=1.71 is obtained (Fig. 6). The total magnification reaches 54 times. The pulse width is 500 ps, corresponding to a peak power near 20 MW and coefficient of variation (CV) of power of 1.6% (Fig. 7).}

A high-beam-quality sub-nanosecond 1064-nm MOPA pulse laser with a pulse width of 500 ps, repetition rate of 1 kHz, average power of 9.7 W, and peak power of approximately 20 MW is reported. Benefiting from self-compensated spherical aberration and good mode matching, two end-pumped dual-pass power amplifier stages based on bonded YAG/Nd∶YAG/YAG crystals boost the power of the seed laser by 54 times, with a beam quality factor of M²=1.71. Owing to its simple structure, good stability, high average power, and peak power, such a MOPA system is believed to have great potential in LIBS, laser machining, and pumping nonlinear optical frequency converters.