Objective High -repetition -rate, short -pulse CO2 lasers have broad application prospects in non-metal processing, laser medicine, extreme ultraviolet (EUV) lithography, photoelectric countermeasures, and other fields. Radiofrequency (RF)-excited waveguide CO2 lasers are small, have high efficiency and long life, and are maintenance -free; thus, continuous -wave output CO2 lasers have been widely used. The main technical means for RF-excited waveguide CO2 lasers to achieve high peak power pulse outputs include electro- optical Q -switching, mechanical Q -switching, and acousto-optic Q -switching. Electro-optical Q -switching can achieve pulsed laser output with repetition rates of more than 100 kHz and pulse widths of tens of nanoseconds; however, electro-optical crystals, such as CdTe, are difficult to grow, easy to damage, and expensive, and the driving voltage required by the crystals is more than 1 kV; thus, the technology is relatively complex. The structure of mechanical Q -switching is simple and the cost is low; however, it is limited by the speed of the motor and the stability of the chopper at high speed. It is difficult to obtain a stable pulse output with a high repetition rate, and it is difficult to accurately control and encode the pulse. Acousto-optic Q -switching is normally realized by placing an acousto-optic modulator in the resonant cavity, and the loss in the cavity is modulated by acousto-optic diffraction to achieve a Q -switched pulse output, which has a low device cost and a high damage threshold. Acousto-optic Q -switched RF waveguide CO2 lasers can achieve pulse output with high repetition rate and short pulse width. They have a compact structure and are easy to carry, thus providing high -quality laser sources for photoelectric countermeasures and other fields. Methods The laser designed in this study adopts a semi -external cavity structure. An acousto-optic modulator is placed between the total reflection mirror and the window, and intracavity loss modulation is realized by the acousto-optic diffraction effect. Using rectangular waveguide coupling theory, the relationship between the coupling efficiency at the waveguide port and the curvature radius of the total reflection mirror, the distance from the total reflection mirror to the waveguide port, and the optimal total reflection mirror parameters are obtained. The position of the acousto-optic modulator in the cavity is determined using acousto-optic diffraction theory. Using an experimental method, the pulse output with high repetition rate and short pulse width is realized by optimizing the working pressure and opening time of the Q -switch. The beam quality is measured using the knife-edge method. Results and Discussions The relationship between the waveguide coupling efficiency and the distance from the total reflection mirror to the waveguide port (Fig. 2) and the curvature radius of the total reflection mirror are determined (Fig. 3). It is found that a higher coupling efficiency can be obtained when the total reflection mirror is 60 mm away from the waveguide port with a curvature radius of 8 m. Through experiments, the relationship between the laser output and working pressure is determined. With an increase in the working pressure, the peak power first increases and then decreases (Fig. 5), and the pulse width decreases slightly and then increases (Fig. 6). The highest peak power and shortest pulse width are achieved at a working pressure of 6.5 kPa. This is because an appropriate increase in the working pressure shortens the lifetime of the upper energy level, and thus, the pulse width is compressed. However, when the working pressure is further increased, the ratio of electric field strength to gas particle number density (E/N) deviates from the optimal range, resulting in unstable discharge, a decrease in peak power, and an increase in pulse width. In addition, the results show that the pulse tail length decreases nearly linearly with a decrease in the opening time of the Q -switch (Fig. 8). Therefore, the tail can be effectively removed by optimizing the opening time of the Q -switch. A near -Gaussian tail -free waveform is obtained at the opening time of 0.6 p.s. Finally, the influence of repetition rate on the output is determined when the working pressure is 6. 5 kPa and the opening time is 0.6 p.s. The pulse width increases slightly with an increase in the repetition rate (Fig. 10). The peak power gradually decreases, whereas the average power gradually increases with an increase in the repetition rate. Both tend to stabilize when the repetition rate is greater than 70 kHz (Fig. 11). The laser can achieve repetition rates of 1 Hz- 100 kHz. A maximum peak power of 2809.6 W and pulse width of 108.2 ns are obtained at 1 kHz. At the repetition rate of 100 kHz, the pulse width is 135.1 ns and the peak power is 257 W. At the repetition rate of 70 kHz, the beam quality factors in the x and ydirections Mx2 and My2 are 1.51 and 1.20, respectively (Fig. 12). Conclusions An RF waveguide CO2 laser with a high repetition rate and short -pulse laser output is achieved using acousto-optic Q -switching. In this study, the waveguide coupling loss theory is used to determine the optimal resonant cavity parameters through simulation analysis. The effect of the working pressure on the laser output is analyzed, and the optimal pressure is determined to be 6.5 kPa under the experimental conditions. The main factor affecting the pulse tail, which is caused by the long Q -switch opening time, is determined or investigated. A tail -free pulse waveform with high repetition rate and short pulse width is obtained by optimizing the Q -switch opening time. A laser output with the pulse width of 108.2 ns and peak power of 2809.6 W is obtained at the Q -switch opening time of 0.6 mu s and repetition rate of 1 kHz. The peak power with Q -switch is 345 times that without Q -switch. The beam quality factors in the x and y directions Mx2 and My2 are 1.50 and 1.21, respectively, and the good beam quality is obtained. The study provides a reference for a subsequent realization of high peak power, high repetition rate, and short pulse -width laser output using a large -gain -volume waveguide cavity.
