Laser-diode (LD)-pumped solid-state lasers operating in the electro-optic (EO) Q-switching mode have broad application prospects in various fields, including ranging, imaging LIDAR, laser processing, remote sensing, and pumping nonlinear optical frequency converters. High repetition rate, short pulse width, high peak power, and high beam quality are the basic requirements for most of these applications to improve signal-to-noise ratio (SNR), measurement capability, ranging accuracy, and conversion efficiency. A compact size is also essential, especially for air-, space-, and rocket-borne systems. The diode-end-pumped electro-optically Q-switched laser has good beam quality and a compact structure; however, the pulse energy is quite limited because of the small mode size in the cavity. Thus, master oscillator power amplifier (MOPA) technology is necessary. In this study, a compact electro-optically Q-switched Nd∶YAG MOPA system based on split pumping and self-compensated spherical aberration is proposed to provide an effective technical approach for miniaturized high-repetition-rate lidar sources.

The electro-optically Q-switched Nd∶YAG MOPA system operating at 1 kHz is shown in Fig. 1. To maintain a compact size and reduce cost and complexity, the laser oscillator and amplifier shared an LD pump source with maximum peak power of 170 W. The pump beam was separated by a beam splitter into two parts, 30% (10.6 mJ) and 70% (24.28 mJ), which were incident onto the oscillator and amplifier, respectively. This structure does not require additional pulse synchronization electronics; therefore, it is simple, compact, and stable. The oscillator produces a high-quality seeding beam with considerable pulse energy, which then proceeds to a specially designed double-pass amplifier for power amplification. By placing a 1064-nm reflector (M6) at the focus of the thermal lens in the amplification stage, the thermally induced spherical aberration by the gain medium during the first pass undergoes a sign reversal at M6. Upon passing through the gain medium for the second time, the thermally induced spherical aberration is compensated such that the phase distortion of the wavefront is eliminated and an acceptable beam quality is guaranteed. This structure is similar to that of a 2f imaging system, and ensures good mode matching between the forward and backward laser beams. The double-pass amplifier not only alleviates the thermal effects and improves beam quality but also enhances conversion efficiency.

The pump energy ratio of 3∶7 for the oscillator and amplifier was mostly dependent on seeder performance; that is, the pulse energy, pulse width, and beam quality should be optimized before optical damage occurs. Using a T=50% output coupler, the maximum output energy of the laser oscillator was 2.68 mJ, and the pulse width was 4.4 ns (Fig. 2). The beam quality factors in the two orthogonal directions were Mx2=1.28 and My2=1.23, respectively [Fig. 5(a)]. The double-pass end-pumped Nd∶YAG amplifier boosted the pulse energy to 9.37 mJ, corresponding to an energy amplification factor of 3.5 times and optical-to-optical conversion efficiency of 26.86% [Fig. 3 (b)]. The peak power was 2.02 MW. Benefiting from the self-compensation of the spherical aberration, the beam quality of the double-pass amplifier was found to be well-controlled compared with that of a single-pass amplifier. The beam quality factors were Mx2=1.49 and My2=1.61 in two orthogonal directions at the maximum output [Fig. 5(b)]. A single pulse energy of 4.98 mJ at 532 nm was achieved by a type-I phase-matched LBO crystal, with a pulse width reduced from 4.63 ns to 3.5 ns. The root mean square (RMS) power instabilities of the oscillator, double-pass amplifier, and frequency-doubled laser within 1 h were 0.223%, 0.094%, and 0.488%, respectively [Fig. 6 (a)].

In summary, an electro-optically Q-switched Nd∶YAG MOPA system based on split pumping was demonstrated, producing high-energy, high-stability, and high-beam-quality nanosecond laser pulses in a compact structure. The oscillator enabled a single-pulse energy of 2.682 mJ and a pulse width of 4.4 ns at a repetition rate of 1 kHz. An end-pumped double-pass Nd∶YAG laser amplifier with self-compensated spherical aberration was utilized, which amplified the single pulse energy to 9.37 mJ with a pulse width of 4.63 ns, corresponding to a peak power of 2.02 MW and optical-to-optical conversion efficiency of 26.86%. The RMS power instability reached 0.094% within 1 h. Using an LBO crystal for frequency doubling, the single-pulse energy at 532 nm was 4.98 mJ, and the pulse width was 3.5 ns. This MOPA laser system is believed to be ideal for high-precision ranging and high-resolution LIDAR applications.