High peak power and continuous tuning are main characteristics of transversely excited atmospheric-pressure [TE(A)] CO₂ lasers which have great potential applications in laser spectroscopy, laser chemistry, laser medicine, laser manufacturing, military and so on. Unlike those working at a pressure around 1 atm, tailing phenomenon is mostly eliminated in multi-atmospheric-pressure CO₂ lasers and the pulse width is compressed from hundreds nanoseconds to tens nanoseconds level, so that the total interactive efficiency of the abovementioned applications is further improved. Compared with other pulse witdh compressing techniques such as mode-locking, Q-switching, plasma switching, etc., another advantage of multi-atmospheric-pressure CO₂ laser is the integration of narrow pulse width and continuous tuning. Such a laser can achieve accurate output of arbitrary wavelength in the 9-11 μm range. A typical application is proved to realize stimulated Raman scattering and excite isotope molecules, which may be an alternative high efficiency solution of present huge separation facilities.

The theory consists of the continuously tunable theory and multifrequency dynamic model. The continuously tunable theory means that as the working pressure increases, the discrete spectral lines are gradually broadened until they overlaps each other, so the output becomes continuously tunable. The multifrequency dynamic model can describe the progress of pulsed CO₂ laser at high pressure. In this model, changing the electricity, gas and optical parameters can result in different output laser pulse waveforms, which can be further used to analyze the output characteristics such as energy, pulse width, power and wavelength. In experiment, a multi-atmospheric-pressure pulsed CO₂ laser was set up. The laser mainly consisted of the energy injection unit, the main discharge unit and the resonant cavity. A high voltage DC power supply (≤32 kV) and a triple Marx circuit were used for power supply and pulse modulation in the energy injection circuit. The pre-ionization structures and main discharge electrodes were arranged inside the chamber. The chamber was filled with gas mixture of carbon dioxide, nitrogen and helium. The resonant cavity adopted a plane-concave cavity type, which contained an output coupling window (the coupling mirror made of Ge) and a diffraction grating. The grating was driven by a server motor working at the Littrow angle (with the grating constant of 1/150 mm) to realize a tunable output. By substituting the optimal parameters obtained from the experiment into the theoretical model and analyzing the output pulse characteristics, it is found that the theoretical results are consistent with the experimental values. Thus the experiment is proven reasonable and correct.

The output properties of transversely excited multi-atmospheric-pressure CO₂ lasers are studied in this paper. Under a working pressure of 7 atm, an output with 590 mJ pulse energy and corresponding 35.7 ns pulse width is obtained. It has been reported in 2015 that von Bergmann et al. generated short pulses around 250 mJ and 60-150 ns under the pressure of 1-10 atm. In comparison with former studies, we raised the pulse energy record by nearly 100% and compressed the minimum pulse width record by nearly 50%. Furthermore, a continuously tunable property was observed at a lower pressure of 7 atm. More than 30 mJ average output was detected every 5.86×10^-4 μm from line 10R(32) to line 10R(26). However, the band range measured in experiment was reduced since the loss was bigger than the gain at the edge of four bands as the working pressure rose (Fig. 6). Meanwhile, the modulation degree of the R band is smaller than that of the P band, leading to the smoother output distribution of the R band (Fig. 7). The whole data acquired under lower pressure perform obvious continuous tuning properties and growing tendencies.

A transversely excited multi-atmospheric-pressure CO₂ laser is studied. A gas mixture (VCO2∶VN2∶VHe＝2∶1∶16) at 7 atm was adopted in the cavity. The discharging voltage was 72 kV and the total injection energy was 19.88 J. The resonant cavity contained a Ge coupling window with the reflective index of 36% and a diffraction grating with the grating constant of 1/150 mm driven by a servo motor. The total tuning range of CO₂ laser in the 9.2-10.8 μm band is about 1.43 μm with four bands (9R, 9P, 10R, 10P). New records were achieved at line 10P(20) (10.59 μm) with the corresponding maximum energy of 590 mJ and the minimum pulse width of 35.7 ns. The estimated pulse peak power was about 16 MW. Obvious continuously tunable properties were measured from line 10R(32) to line 10R(26). In the future, output properties under higher working pressures will be studied and the coupling efficiency will be optimized. In combination with the study of multi-atmospheric-pressure CO₂ laser amplifier, an oscillator-amplifier system is in the plan, which may be helpful to the research progress of laser isotope separation.