Currently, nanosecond pulsed 3 μm lasers are of interest for many scientific research and practical applications. For mid-infrared optical parametric oscillators (OPOs), pumping sources with longer wavelengths are desirable to reduce the quantum loss in the parametric conversion. Moreover, pumping sources with short pulse duration and high peak power can improve the conversion efficiency to the mid-infrared wavelength (3‒12 μm range) and obtain greater output power or energy. Another important application of nanosecond pulsed 3 μm lasers is related to the distinctive features of water and hydroxyapatite, i.e., extremely high absorption in the vicinity of the 3 μm wavelength range. Therefore, pulsed lasers in this wavelength range are widely employed for medical ablation surgery, particularly for dental and orthopedic applications. Further, lasers with high repetition rate can improve the ablation efficiency of hard tissue and speed up the treatment process. If the laser pulse duration is less than the thermal diffusion time, unnecessary thermal damage to the surrounding healthy tissue can be reduced. Therefore, it is a common endeavor to achieve a stable 3 μm laser output with a high peak power and short pulse duration at a high repetition rate.

Bulk LiNbO₃ crystals have excellent acousto-optical (AO) properties and can be used as an ideal AO medium, exhibiting higher transmission in the 3 μm wavelength range, lower acoustic attenuation coefficient (1 dB/cm @ 1 GHz), and higher damage threshold (>200 MW/cm²). More than 30 years ago, scientists attempted to use LiNbO₃ crystals to create an AO Q switch, but it failed to work at 3 μm wavelength. Recently, we innovated and developed a LiNbO₃-based AO Q switch, and its effectiveness was verified in our previous study. In this work, the output characteristics of a LiNbO₃ AO Q switch at a high repetition rate are investigated in an Er, Cr∶YSGG laser. Hence, the output characteristics of an AO Q-switched Er, Cr∶YSGG laser pumped by a flash lamp at a high repetition rate are studied. The thermal focal lensing effect in the gain medium is compensated using a plane-convex resonator (PCR), which significantly improves the beam quality and output capacity of the laser at a high repetition rate. A stable output of the LiNbO₃ AO Q switch in the Er, Cr∶YSGG laser is realized.

To effectively compensate for the thermal focal lensing effect, the thermal focal lengths of Er, Cr∶YSGG laser crystals are calculated theoretically. Because the thermal focal length of the laser crystal is related to many factors, the corresponding theoretical calculation cannot be completely accurate. Hence, the thermal focal length of the Er, Cr∶YSGG laser crystal is measured using the critical resonator stabilization method in a plane-parallel resonator at 100 Hz. The theoretical calculation and actual measurement results are presented in Fig.2. According to the design theory of the resonator with the embedded thermal lens, the curvature radius of the convex mirror in the plane-convex resonator should be -166.3 mm. Therefore, convex mirrors with curvature radii of -100, -150, and -200 mm are adopted as rear mirrors in the respective experiments to measure and collect laser pulse energy. It can be seen from Fig.3 that the compensation effect of the convex mirror with a curvature radius of -150 mm is better than those with -100 mm and -200 mm curvature radii.

To explore the influence of the reflectivity of the output coupler(OC)mirror on the output performance of the LiNbO₃ AO Q switch, the reflectivity is set at 60%, 70%, and 80% for experimental research in the plane-convex resonator, and the results are shown in Fig.4. The optimum reflectivity of the OC mirror of the LiNbO₃ AO Q-switched Er,Cr∶YSGG laser is 70%. To explore the influence of the thermal focal lensing effect on the output performance of the LiNbO₃ AO Q switch, a comparison experiment of Q-switching between the plane-parallel resonator and the plane-convex resonator is performed. It can be seen from Fig. 5 that the structure of the plane-convex resonator can improve the output performance of the LiNbO₃ AO Q-switched Er, Cr∶YSGG laser in a certain pump energy range. The diffraction efficiency is varied by changing the radio frequency driving power(RFDP) added to the Q switch, and the output performance of the laser is explored. As can be seen from Fig.6, when the repetition rate is 100 Hz, the maximum pulse energy and minimum pulse duration are 4.36 mJ and 76.8 ns, respectively, when the RFDP is 30 W. Moreover, thebeam qualityfactor (M²) and the stability of the LiNbO₃ AO Q-switched Er, Cr∶YSGG laser are examined. As shown in Fig.7, for the pulse energy of 4.36 mJ, the beam quality factor (MX2) in the X direction and the beam quality factor (MY2) in the Y direction are 8.3 and 7.7 at 100 Hz. Further, as shown in Fig.8, the degree of energy stability (standard deviation) of the laser is 2.55%.

The results show that the designed LiNbO₃ AO Q switch can realize nanosecond pulse output at a high repetition rate in a 2.79 μm Er, Cr∶YSGG laser. The plane-convex resonator structure can effectively compensate for the thermal lensing effect of the gain medium, optimize beam quality, and improve the output performance of the laser. Increasing the radio frequency driving power of the AO Q switch can increase the pulse energy and compress the pulse duration, thus improving the output performance of the LiNbO₃ AO Q-switched Er, Cr∶YSGG laser.