# High Repetition Rate, TEM\({}_{00}\) Mode, Compact Sub-Nanosecond 532 nm Laser
## 1 Introduction  
A compact sub-nanosecond microchip laser and an amplifier currently have applications in diverse areas including ranging and imaging, micromachining, material characterization, and environmental monitoring [1, 2, 3]. In particular, as transmitters, high repetition sub-nanosecond lasers have been used in photon-counting laser altimeters [4, 5], or lidars, for high-resolution 2D profiles and/or 3D images of the underlying topography including soil, low-lying vegetation, water surfaces, human-made objects, ocean waves, shallow water bathymetry, 3D underwater imaging, etc. [6, 7, 8]. In a second-generation 3D imaging photon-counting lidar that can be installed in an Unmanned Aerial Vehicle (UAV) developed by Sigma Space Corporation, a compact laser transmitter can deliver a 142 mw, 700 ps, 532 nm beam at the maximum repetition rate of 22 kHz [9]. High beam quality is necessary because a Diffractive Optical Element (DOE) was used to split the 532 nm beam into a 10 \(\times\) 10 array of beamlets with an 80% DOE efficiency. In the photon-counting lidar, the laser should have the characteristics of high repetition rates, short pulse duration, and high beam quality to satisfy the need of measuring efficiency and precision. A microchip laser and a compact amplifier are ideal. Lincoln Laboratory reported passively Q-switched microchip lasers using Nd\({}^{3+}\): YAG as the gain medium and fiber-pumped amplifiers that occupy a volume of less than 0.25 L. The compact laserhas pulse widths between 150 ps and 3 ns, pulse energies up to 250 uJ at 1064 nm, and typical repetition rates between 4 and 20 kHz [2; 3]. [PERSON], [PERSON] et al. reported a compact satellite-ranging transmitter of SLR2000 consisting of a Cr\({}^{4+}\): YAG passively Q-switched Nd: YAG microchip laser, an air-cooled Nd: YVO4 power amplifier, and a SHG module, which generated 532 nm, 470 mW, 270 ps pulses at a 2 kHz rate [10]. [PERSON], [PERSON] reported a master oscillator power amplifier (MOPA) system that combines a 1064 nm microchip laser oscillator with a new diode-side-pumped Nd: YVO4 zig-zag slab amplifier. The 40 W pump power of the amplifier achieved a pulse energy of more than 800 uJ at a 2 kHz pulse rate [11]. Other schemes for high-repetition-rate microchip lasers have also been reported. [PERSON], [PERSON] et al. reported an End-Pumped Composite Nd: YAG Laser Passively Q-switched by Cr\({}^{4+}\): YAG, where the maximum average power was 4.2 W with the laser operating at a 24 kHz repetition rate and a 47.8 ns pulse duration (3.7 kW peak power) at a beam quality of M\({}^{2}\) = 1.34 [12]. [PERSON], [PERSON] et al. researched intracavity frequency doubling by employing a V-type laser resonator, which yielded 532 nm green pulses of 226 uJ energy and 86 ns width at 4.2 kHz [13]. Related to increasing the peak power of a microchip laser, much innovative progress has been reported by the Institute for Molecular Science based in Japan. [PERSON]. [PERSON] et al. presented a microchip laser with a 1.9 MW peak power, 500 Hz, 550 ps, and 532 nm [14], which was pumped into the Quasi-Continuous Wave (QCW) regime by a fiber-coupled 120 W, 808 nm laser diode. [PERSON], [PERSON] et al. reported a novel microchip laser using a high thermal conductivity gain media with the beam of 21.5 mJ pulse energy, 10 Hz, 1064 nm, and an M\({}^{2}\) value between 10.7 and 11.8 [15]. Their research provided novel methods for the thermal restriction of microchip lasers, which would be expected in a high repetition rate. As mentioned above, increasing the average power and the corresponding pulse energies of a sub-nanosecond laser with high repetition rates and high beam quality, which is an advantage of the detection distance and detection probability of lidar, is challenging in compact sizes. This paper proposes a compact sub-nanosecond master oscillator power amplifier (MOPA) laser that combines a seed based on a microchip laser, an off-axis double-pass amplifier, and a SHG module.