Double-pulse narrow-linewidth lasers have the characteristics of outputting double pulses, narrow spectral line width, high peak power, and long coherence length, so they can be widely employed in many fields, such as laser ranging, laser communication, atmospheric environment monitoring and fine-tuning. In recent years, double-pulse laser has been gradually applied to the medical field, and has great advantages in the treatment of epidermal pigmented lesions. Compared with the single-pulse laser, two laser pulses are applied successively at the same position of the target tissue by using the double-pulse laser, which is equivalent to increasing the energy density acting on biological tissue without increasing the laser power. This method can not only improve the therapeutic effect, but also reduce the damage to skin tissue [1], [2], [3], [4]. The existing technical means for realizing double-pulse laser output are mainly based on active Q-switching and passive Q-switching, but the overall research band is concentrated in the mid-infrared band, and there are few studies on the double-pulse laser output in the visible light band. And so far, there is no report on the double-pulse narrow-linewidth laser in the visible light band. In 2016, Tamer F. Refaat et al. demonstrated the capability of airborne dual-pulse 2μm IPDA lidar for atmospheric remote sensing of carbon dioxide. This instrument depends on an advanced 2μm dual-pulse laser transmitter generated by single pulse pumping, which has the ability to tune wavelength and lock each pulse individually while monitoring their energies. These features improve the spatial and temporal resolution of carbon dioxide measurement [5]. In 2021, Zhang Jiale et al. achieved the purpose of electro-optic Q-switching by using RTP double-crystal method and proved an actively Q-switched quasi-three-level Nd:YLF laser. The resulting pulse widths were 58.6 ns and 60.3 ns, and the corresponding pulse energies were 0.84 mJ and 0.73 mJ at the central wavelength of 908.3 nm [6]. However, the Q-switching methods adopted in the above study have the complex cavity structure and difficulty to achieve, but we can obtain the double-pulse laser output with controllable pulse interval and pulse amplitude by employed the double-step acousto-optic Q-switching technology. Meanwhile, although the double-pulse laser is widely applied in many fields, the linewidth of the solid-state double-pulse laser output in the multi-longitudinal mode operation mode is too large, which sometimes is not suitable for some practical situations, so the technical means are needed to compress the linewidth to overcome this inherent defect. Common methods of compressing the linewidth include the insertion of double F-P etalon, short cavity to enhance mode competition, twisted-mode cavity to eliminate the spatial hole burning effect and volume Bragg grating (VBG) with dispersive cavity [7], [8], [9]. Compared with other methods, inserting double F-P etalon has the advantages of easy accessibility, obvious mode selection effect and tunable output wavelength.

Therefore, in this paper, the Pr:YLF crystal was used as the gain medium, which can directly obtain the laser output in the visible light band (479 nm, 522 nm, 604 nm, 607 nm, 639 nm, 720 nm) without frequency doubling [10], [11], [12]. Pr:YLF crystals are essential to promote the research of lasers in the visible light band due to its low phonon decay and long fluorescence lifetime [13]. Then, on the basis of using the two-step acousto-optic Q-switching technology to obtain the equal-amplitude double-pulse laser output, the double F-P etalon was inserted into the resonator to achieve the purpose of compressing the linewidth. At the repetition rate of 10 kHz, the pulse interval of the equal-amplitude double-pulse laser output was 40 μs, the pulse energy was 7.8 μJ, and the pulse width was 113.0 ns and 140.2 ns, respectively.