Objected Lasers w ith high repetition rate and nanosecond pulse w idth around 3 μm w aveband are required to im prove the conversion rate of optical param eters and reduce the therm al effect w hen they are used in m id - infrared param etric pum ping and hard tooth tissue ablation. T he Q- sw itched technology is w idely used to generate lasers w ith high peak pow er and narrow pulse w idth. Currently, the high- peak- pow er laser output at 3 μm w aveband has been obtained using electro- optic Q- sw itched laser technology. H ow ever, because of the therm al depolarization effect of polarized laser under high - pow er pum p, the repetition frequency of electrooptic Q- sw itched technology cannot be increased. T he high repetition frequency can be achieved using acousto- optic Q- sw itched technology, but the large laser pulse energy cannot be realized ow ing to the lim itation of the diffraction efficiency of the acousto - optic device. M echanical Q- sw itched technology cannot produce stable laser pulses because it is difficult to accurately control the m otor during high- speed operations. T heoretically, a passively Q- sw itched laser can achieve nanosecond laser pulses w ith high repetition frequency and high peak pow er as long as the dam age threshold of the optical com ponents is sufficiently large. M oreover, as a passively Q- sw itched laser has a com pact cavity structure, its use is advantageous in laser applications. M any m aterials have been proved to be suitable for passively Q- sw itched lasers in the 3 μm w aveband, and only a few relatively stable m aterials such as Fe2 + ∶ ZnSe crystals can achieve a large energy output. H ow ever, as the Fe2+ ∶ ZnSe crystal has a low dam age threshold (1. 5 ‒ 2. 0 J/cm2@ 100 ns) in the 3 μm w aveband, it can easily be dam aged w hen operating at high peak pow er and high repetition frequency. H ence, to reduce the risk of dam age during norm al operation, it is necessary to analyze the pulse characteristics of Fe2+ ∶ ZnSe crystals in passive Q- sw itched lasers to reduce the possibility of dam age to the saturable absorber and realize laser operation at a high peak pow er and high repetition rate. M ethods U sing output m irrors w ith different reflectivities, the values of the output pulse w idth of a Fe2+ ∶ ZnSe saturable absorber are theoretically calculated (Fig. 1 and T able 1). T he values provide theoretical guidance for the design of passively Q- sw itched lasers. T he pulse w idths under tw o initial transm ittances of the Fe2+ ∶ ZnSe crystal (91. 9% and 93. 6%) and tw o reflectivities of the output m irror (30% and 40%) are m easured using Er, Cr∶ Y SG G laser crystal rods w ith tw o sizes (Ф3 m m × 100 m m and Ф4 m m × 100 m m) pum ped by a xenon lam p (Fig. 2). T he theoretical calculation results are verified through relevant experim ents. Based on the m easurem ent results, a high- repetition- rate Fe2+ ∶ ZnSe crystal passively Q- sw itched laser system w ith a concave-convex resonator structure is developed to compensate for the thermal focal length. The Er,Cr∶ YSGG crystal has dimensions of Ф3 mm×100 mm. Concave mirror M1 is used as the all-reflection mirror (R1=200 mm), M2 is used as the output mirror (R2= -216 mm), and the reflectivity of the convex surface is 70% at 2. 79 μm. Results and Discussions The results in Table 1 and Fig. 3 show that the pulse width of the output lasers with different initial transmittances narrows with an increase in the reflectivity of the output mirror. A passively Q-switched laser output with large energy and narrow pulse width can be obtained more easily when the initial transmittances of the saturable absorber are lower. The experimental results verify the accuracy of the calculation. Moreover, the pulse width of the laser output has little relation with the size of the laser crystal rod, and the pulse widths obtained with the crystal rods with two different sizes are similar. From the beam diameter in the cavity, it is observed that a larger crystal rod diameter changes the mode volume in the cavity and increases the output laser energy; however, the laser energy density in the cavity does not increase because the bleaching process of the saturable absorber is not affected by the increase in the beam diameter. By optimizing the internal component layout of the 60 Hz Er, Cr∶ YSGG passively Q-switched laser, the high repetition frequency and high peak power of the 2. 794 μm passively Q-switched laser can be achieved. Figure 5 shows the experimental waveforms of the 60 Hz passively Q-switched laser with two Fe2+∶ZnSe crystals. The single pulse energy of the lasers is 4. 7 mJ and 7. 0 mJ, with pulse widths of 97. 0 ns [Fig. 5 (a)] and 72. 6 ns [Fig. 5 (c)], respectively. Conclusions The results show that a saturable absorber with low initial transmittance can achieve a low pulse width, whereas a saturable absorber with high initial transmittance can compress the pulse width by enhancing the reflectivity of the output mirror. Based on these results, the xenon lamp pumping Er, Cr∶ YSGG laser is optimized, and Fe2+ ∶ ZnSe passively Q-switched laser pulses with high repetition rate (60 Hz) and high peak power (7. 0 mJ) are realized. © 2023 Science Press. All rights reserved.
