High-Efficiency 50 W Burst-Mode Hundred Picosecond Green Laser

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. Due to the sharp increase in space debris (also known as space trash), monitoring and early warning of space debris have attracted considerable attention worldwide owing to a dramatic increase in space garbage, which poses a serious threat to spacecraft operation and human space activities. Moreover, space debris laser ranging technology not only enables real-time space debris orbit measurement with high precision (one or two orders of magnitude higher than that of other ground-based observation equipment), but also provides calibration for other monitoring methods. The use of lasers with a pulse duration of 100 ps can afford improved ranging precision owing to their narrow pulse duration; in addition, their pulses are longer than femtosecond pulses, enabling the detector to respond. Because the laser beam is diffusely reflected by space debris, however, a higher laser pulse energy is needed for the detector to capture the reflected photons. As the pulse energy increases, smaller and more distant space debris can be measured, and raising the repetition rate of the laser shortens the time interval between adjacent diffusely reflected pulses, improving the speed of target acquisition. A recent study showed that using a double-pulse picosecond laser for tracking space debris increases the ranging precision of laser space debris measurement from the decimeter level to the centimeter level. Besides, due to the sub-pulse interval in the burst is short, it is easy to increase the repetition frequency of sub-pulse in the burst to the GHz-level without seriously affecting the conditions of each pulse energy, and it also has obvious advantages in the fields of precision machining and scientific research.

The research group led by Prof. Meng Chen from Beijing University of Technology researched a 1 kHz burst-mode green all-solid-state laser system with a pulse width of 100 ps, fundamental beam power of more than 80 W, double frequency beam power of more than 50 W and conversion efficiency of up to 68%. The laser system is based on the research group's long-term research on semiconductor saturable absorber mirror (SESAM) mode-locking, picosecond pulse stretching, pulse train regeneration amplification, beam shaping, picosecond laser amplification, and high-efficiency frequency doubling. The research results are published in High Power Laser Science and Engineering, Vol. 8, Issue 1, 2020 (Ning Ma, Meng Chen, Ce Yang, Shang Lu, Xie Zhang, Xinbiao Du. High-efficiency 50 W burst-mode hundred picosecond green laser[J]. High Power Laser Science and Engineering, 2020, 8(1): 010000e1).

The laser system developed in this work presents a significant improvement in both single-pulse energy and frequency-doubling conversion efficiency at high repetition rates. The burst-mode pulses of this laser system are obtained by using an improved Michelson interferometer. Therefore, it is simple to adjust the time delay in each burst by changing the arm length of each end mirror, and realize burst-mode pulse with a narrow interval (< 1 ns). Because the pulse envelope is narrow (< 4 ns, 4 pulses in each burst), it’s possible to use a short cavity length regenerative amplifier (RA) to achieve high gain amplification, which will reduce the pressure of the subsequent power amplification, and obtain good beam quality. Benefitting from the use of a burst-mode regenerative amplifier, the relative amplitude of each pulse in the burst can be controlled by adjusting the coupling efficiency between each injected sub-pulse and the regenerative cavity. The traditional burst-mode pulse is generated by using an electro-optic pulse-picker to pick several sub-pulses from the seed beam generated by the mode-locked oscillator cavity. However, the pulse repetition frequency of the seed beam is usually less than 100 MHz (pulse interval > 10 ns), which is limited by the length of the mode-locked oscillator cavity. Moreover, for a given seed beam, the pulse interval in the burst will not be variable. For traditional burst-mode pulse, the length of the burst envelope is not less than 30 ns (4 pulses in each burst), when the repetition frequency of the seed beam is 100 MHz. If an RA scheme is adopted to amplify the long-interval (> 30 ns) burst-mode pulse, the required length of regeneration oscillation cavity should not be less than 4500 mm (30 ns * c / 2). This not only increases the difficulty of adjusting the optical path, but also weakens the overall compactness and reliability of the laser system. Because the traditional burst-mode picosecond lasers are usually designed as a traveling-wave amplification structure, the relative amplitude of the sub-pulses in the burst is not adjustable, and the amplitude variation is ~20 % decrease from first to the last pulse in the burst.

Moreover, the laser system achieves a second harmonic conversion efficiency of up to 68%, and has the characteristics of high energy (> 50 mJ @532 nm), high peak power (> 2 GW / cm2 @ Spot diameter ~ 5 mm) and high average power (> 50 W). Although there have been some reports on high-frequency conversion efficiency lasers, their laser systems have lower average output power or lower pulse energy, which cannot be combined with high average power and high pulse energy.

This work was supported by the National Natural Science Foundation (NSF) of China Union Fund of Astronomy. The laser prototype developed in this work has been applied to the observatory space debris laser research. In order to extend the application scope of the laser system, the future work will focus on further optimizing the optical set-up and improving the output beam quality. Recently, by optimizing the optical path of the system, the directional stability of the output beam is better than ± 10 μrad. In addition, further improve the conversion efficiency of light frequency doubling (e.g., how to break through the current ratio of 1:1 number of photons of second harmonic and fundamental light after light frequency doubling), and using this laser system to carry out other high-efficiency nonlinear frequency conversion (Raman, OPO) is also actively being explored.

Schematic diagram of the burst-mode laser system