Many materials have been proved to be suitable for passively Q-switched lasers in the 3 μm waveband, and only a few relatively stable materials such as Fe^{2 +}∶ZnSe crystals can achieve a large energy output. However, as the Fe²⁺∶ZnSe crystal has a low damage threshold (1.5-2.0^{J/cm2@100 ns) in the 3 μm waveband, it can easily be damaged when operating at high peak power and high repetition frequency. Hence, to reduce the risk of damage during normal operation, it is necessary to analyze the pulse characteristics of Fe2+∶ZnSe crystals in passive Q-switched lasers to reduce the possibility of damage to the saturable absorber and realize laser operation at a high peak power and high repetition rate.}

Based on the measurement results, a high-repetition-rate Fe²⁺∶ZnSe crystal passively Q-switched laser system with 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.

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.

Lasers with high repetition rate and nanosecond pulse width around 3 μm waveband are required to improve the conversion rate of optical parameters and reduce the thermal effect when they are used in mid-infrared parametric pumping and hard tooth tissue ablation. The Q-switched technology is widely used to generate lasers with high peak power and narrow pulse width. Currently, the high-peak-power laser output at 3 μm waveband has been obtained using electro-optic Q-switched laser technology. However, because of the thermal depolarization effect of polarized laser under high-power pump, the repetition frequency of electro-optic Q-switched technology cannot be increased. The high repetition frequency can be achieved using acousto-optic Q-switched technology, but the large laser pulse energy cannot be realized owing to the limitation of the diffraction efficiency of the acousto-optic device. Mechanical Q-switched technology cannot produce stable laser pulses because it is difficult to accurately control the motor during high-speed operations. Theoretically, a passively Q-switched laser can achieve nanosecond laser pulses with high repetition frequency and high peak power as long as the damage threshold of the optical components is sufficiently large. Moreover, as a passively Q-switched laser has a compact cavity structure, its use is advantageous in laser applications.

Using output mirrors with different reflectivities, the values of the output pulse width of a Fe²⁺∶ZnSe saturable absorber are theoretically calculated (Fig.1 and Table 1). The values provide theoretical guidance for the design of passively Q-switched lasers. The pulse widths under two initial transmittances of the Fe²⁺∶ZnSe crystal (91.9% and 93.6%) and two reflectivities of the output mirror (30% and 40%) are measured using Er,Cr∶YSGG laser crystal rods with two sizes (Ф3 mm×100 mm and Ф4 mm×100 mm) pumped by a xenon lamp (Fig.2). The theoretical calculation results are verified through relevant experiments.

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.

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.