With the development of single-photon detection technology, human exploration of Earth and space has enhanced the study of Earth's surface changes, such as those in ice, terrain, and vegetation, and improved the understanding of the impact of glaciers on sea level change. These detections have led to new requirements for the light source of LiDAR technology, including lasers with high repetition rates, narrow pulse widths, and narrow linewidths for improved detection distance and accuracy, enabling better observations of changes in surface characteristics. Lasers with all-fiber structures are more compact and stable, which have higher photoelectric conversion efficiency and longer lifespan. The optical fiber structure is more conducive to the multi-beam ground detection. This is because a certain detection blind zone between the beams exists, and the detection of multiple beams reduces the distance interval between the beams, resulting in a certain degree of gridded high-precision detection. All-fiber lasers are expected to enable thousand-beam laser ground detection and direct ground measurement.

In this study, a continuous seed light of 62.6 mW is used, which is modulated into pulsed light by an electro-optical modulator through a rectangular pulse signal from a signal generator. First, the pulsed seed light is amplified by a gain optical fiber to obtain pulsed light with a wavelength of 1064.43 nm, a linewidth of 0.037 nm, and a peak power of approximately 9.33 W. Then, through a gain fiber for two-stage two-pass amplification, the pulsed light with a linewidth of 0.037 nm and a peak power of approximately 383.5 W is obtained. After the first two-stage amplification, an acousto-optic modulator (AOM) is connected to filter out the continuous wave components of the front stage and improve the contrast of the pulsed light. The third-stage amplification is done through a PLMA-YDF-15/130 double-clad gain fiber to obtain pulsed light with a linewidth of 0.046 nm and a peak power of 7.11 kW. The main amplification stage uses the PLMA-YDF-25/250 and photonic crystal fiber (PCF) for amplification effect comparison. The PCF amplified linewidth is smaller than that of the PLMA-YDF-25/250, with no spontaneous radiation, stimulated Raman scattering, or other nonlinear effects. It obtains a wavelength of 1064.44 nm, pulse energy of 298 μJ, and pulse width of 1.34 ns for lasers with a linewidth of 0.05 nm. The corresponding maximum peak power of the laser is approximately 223 kW. The temperature-matched lithium triborate (LBO) is used for the frequency doubling of fundamental frequency light at an energy of 298 μJ, resulting in a green light output of 155.5 μJ. The frequency doubling conversion efficiency is 52%, and a beam quality of Mx2=1.28 and My2=1.26 are also obtained.

To simplify the optical path and maintain the stability of the output, the forward amplification method is selected (Fig. 1). The main amplification stage uses the PLMA-YDF-25/250 and PCF for comparison. Under varying pump currents (Fig. 6), the former exhibits slightly higher optical conversion efficiency than the latter. At a current of 5.6 A, the PLMA-YDF-25/250 exhibits self-phase modulation effects, as shown by the spectral comparison (Fig. 7). Because the mode field area of the PCF is larger than that of PLMA-YDF-25/250, the threshold of nonlinear effects is increased, and other nonlinear effects, such as amplified spontaneous radiation and stimulated Raman scattering, are not observed at 298 μJ (Fig. 8). The optical path design uses a temperature-matched LBO crystal for frequency doubling on fundamental frequency light of 1064 nm, resulting in 155.5 μJ green light output with a frequency doubling conversion efficiency of 52% (Fig. 9).

In this study, a master oscillator power amplifier (MOPA) structure combined with photonic crystal fiber is used to obtain stable fundamental frequency light with a repetition rate of 10 kHz, a wavelength of 1064.44 nm, a linewidth of 0.05 nm, an energy of 298 μJ, and a peak power of approximately 223 kW. After the temperature-matched LBO frequency doubling, the resulting 155.5 μJ green light with a frequency doubling efficiency of 52% and beam quality of Mx2=1.28 and My2=1.26 can be used as a light source for space detection lidar.