Advances in 2 μm single-longitudinal-mode all-solid-state pulsed lasers (cover paper·invited)

Significance The 2 µm single-longitudinal-mode (SLM) all-solid-state pulsed laser has attracted much attention for its applications in lidar, gas monitoring, laser medicine, material processing and scientific research, owing to its high stability, narrow spectral linewidth and other advantages. For instance, the 2 µm SLM laser features high atmospheric transmittance and eye-safety, making it an ideal emission source for Doppler wind lidar. Moreover, the 2 µm laser covers the absorption peaks of various gases such as H₂O, CO₂ and CH₄, enabling it to be used as the emitter of differential absorption lidar for atmospheric greenhouse gas monitoring. By combining the 2 µm laser with other sensors, a comprehensive atmospheric environment monitoring system can also be established. In the field of material processing, the 2 µm laser can interact with many materials, greatly simplifying the processing steps. Furthermore, the 2 µm laser has diverse applications in medical surgery, such as tissue cutting, stone crushing and eye surgery. Through the characteristics of its working wavelength, the 2 µm laser can achieve precise tissue treatment, while reducing the damage to the surrounding tissue, offering a safer and more effective option for medical surgery. The 2 µm SLM all-solid-state pulsed laser also plays a vital role in the field of military defense.

The 2 µm laser output can be obtained by using nonlinear frequency conversion or directly pumping gain medium doped with Tm³⁺ or Ho³⁺. However, the linewidth of the 2 µm laser output generated by nonlinear frequency conversion is relatively wide, so it is extremely difficult to achieve SLM laser output. In contrast, compared with the nonlinear frequency conversion technique using 1 µm lasers as the pump source of optical parametric oscillators, Tm³⁺ or Ho³⁺ doped Q-switched lasers typically involve using a special resonator design or introducing mode selection elements, which have more compact structure and higher stability in achieving a 2 µm SLM pulsed laser.

With the significant development of laser technologies such as laser pump technology, single longitudinal mode selection technology, and high energy laser pulse technology, the 2 µm SLM all-solid-state pulsed laser is developing towards smaller size, better performance, and more stable output performance. In recent years, researchers at home and abroad have designed and fabricated various 2 µm SLM all-solid-state pulsed lasers. According to the specific application scenario, the most suitable SLM selection scheme is chosen, and researchers have obtained 2 µm SLM pulsed lasers with different characteristics and successfully applied them to several fields. However, there are still some technical challenges to be overcome in the development of the current 2 µm SLM all-solid-state pulsed laser technology. In this paper, the common 2 µm single-mode all-solid-state pulsed laser technologies with the ring cavity, twisted-mode cavity, volume Bragg grating and injection-seeded method are analyzed and summarized.

Progress This paper reviews the research progress of 2 µm SLM all-solid-state pulsed laser technology, in conjunction with its applications across various fields. It introduces the working principles and characteristics of SLM selection techniques such as the ring cavity, twisted-mode cavity, volume Bragg grating, and injection-seeded method. The laser output characteristics of different structures, including central wavelength, output energy, pulse width, full width at half maximum (FWHM) of the spectrum, pulse repetition rate, and beam quality factor, are summarized based on different SLM selection techniques. The results indicate that the 2 µm SLM all-solid-state pulsed laser has made significant strides in single pulse energy, spectral line width, and stability. It can achieve high-energy SLM laser output with a line width on the order of MHz and pulse repetition frequency on the order of kHz. However, the output pulse width remains wide (on the order of nanoseconds), the structure is complex, and the thermal effect is pronounced. Finally, the paper analyzes the current technical bottlenecks, provides corresponding solutions, and prospects the future development of 2 µm SLM all-solid-state pulse lasers.

Conclusions and Prospects Driven by the escalating demand for practical applications, 2 µm SLM all-solid-state pulsed lasers are evolving rapidly towards miniaturization, enhanced stability, high efficiency, narrow spectral linewidth, and substantial output energy. Future development trends are expected to focus on further advancements in output performance and the exploration of innovative methods for realizing 2 µm SLM all-solid-state pulsed lasers. Moreover, with the progression of laser technologies such as longitudinal-mode selection, pulse width compression, and thermal management, coupled with the continuous exploration of new gain media and laser structures, the comprehensive performance of 2 µm SLM all-solid-state pulse lasers is anticipated to be further improved to cater to diverse application requirements.