Abstract
1 Introduction
Burst-mode picosecond green lasers with a high pulse energy and high average power have important applications in many fields. One of the most promising applications is space debris laser ranging[
Various methods have been proposed to obtain high-pulse-energy and high-average-power ultrashort green lasers for high-energy ultrafast laser systems; these include beam shaping techniques[
CPA technology is used mainly in femtosecond laser systems, where a broader linewidth in the gain medium is useful[
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Beam combining, including coherent beam combining and spectral beam combining, is another way to obtain high-power lasers. To realize a high peak power using fiber amplifiers, in 2017, Želudevičius et al.[
In this study, we designed a high-pulse-energy, high-average-power burst-mode picosecond laser system for space debris laser ranging. It consists of a semiconductor saturable absorber mirror (SESAM) mode-locked picosecond oscillator, VBG pulse stretcher, pulse splitting system designed to obtain a nanosecond pulse interval in the burst, a burst-mode regenerative amplifier (RA) and a ring pattern beam reshaping system consisting of two aspheric lenses, two-stage power amplifiers and a high-efficiency frequency doubler. Moreover, it adopts the collinear output of the fundamental beam and second harmonic generation (SHG), and both frequency pulses can be used simultaneously for space debris laser ranging to improve the measurement accuracy or operated independently. In particular, the fundamental beam, which can operate during the day, reduces the effect of the sun. Further, the relative amplitude of each pulse in the burst, as well as the pulse interval, can be easily adjusted, which is convenient for experimental research on space debris ranging. The aspheric lens system reshapes the Gaussian beam output by the RA to a ring profile with high conversion efficiency, which reduces the difficulty of subsequent amplification; consequently, an average power of up to 50 W at 532 nm is obtained, which corresponds to a pulse envelope energy of 50 mJ, and the second harmonic conversion efficiency reaches 68%.
2 Experimental setup
The seed laser, which is shown schematically in Figure
To realize high-power, high-energy picosecond pulsed lasers, a diode-side-pumped Nd:YAG RA and a two-stage traveling-wave amplification are adopted for the amplifier system, as shown in Figure
The frequency conversion module, which is used to adjust the beam size and divergence, is placed behind the 4f imaging system to realize the highest conversion efficiency. To further improve the conversion efficiency, a thin-film polarizer at 1064 nm is used to purify the polarization direction of the amplified fundamental light. The second harmonic waves are generated by an LBO crystal (
3 Experimental results
Owing to the properties of the RA[
Figure
Frequency doubling is performed by controlling the fundamental beam with a small divergence angle into a 15-mm-long LBO at
4 Conclusion
In conclusion, we experimentally demonstrated a burst-mode picosecond laser system with a high pulse energy, high average power and high frequency doubling efficiency for space debris laser ranging. In this system, pulse stretching, beam splitting and reshaping by aspheric lenses are used to realize high-power laser output. After two-stage master traveling-wave amplification, the power of the fundamental beam is as high as 80 W with a power fluctuation of 0.56% over 30 min. By properly passing the beam through a type-I noncritical phase-matching LBO crystal, a 50 W second harmonic laser with less than 1% power fluctuation over 30 min is obtained at a pulse repetition rate of 1 kHz with a conversion efficiency of 68%. The laser system, which emits co-channel burst-mode pulses with a narrow linewidth and high intensity at 1064 and 532 nm, provides a promising light source for high-precision laser ranging of small space debris.
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