The Dynamic Analysis Of Mechanical Q-switched CO₂ Laser

This thesis is composed of three parts, the first part is setting up a mechanical Q-switched CO2 laser. The second part is the theoretical and experimental study of the stability of this laser. The last part is that We made a theoretical analysis for kinetic processes of the Q-switched CO2 laser. We put special emphasis on the effect of the non-laser vibro-rotational levels on describing the dynamic processes of this laser. The calculated pulse waveforms based on the vibro-rotational model are in good agreement with the experiment.The first part is that we get the short laser pusle by using the mechanical Q-switched methed. A fast chopper with the rotation rate of 12000r/m is used to set up a mechanical Q-switched CO2 laser. The structure of mechanical Q-switched CO2 laser is shown in Fig.1. The repetition rate of the laser pulse we get is from 100 to 800Hz, the maximum peak power is 1152 W, with a pulse duration of about 200ns. The tailing phenomenon is obvious.The second part is the theoretical and experimental study of the stability of this laser. In the experiment, we study the changes of the output power when we shift the situation the focus mirrors with the same distance. Fig2 and Fig3 Grating Chopper Focusing mirrors:M1,M2 ZnSe window Laser tube Output mirrorFig.1 the structure of mechanical Q-switched CO2 laserare the output power of the CW CO2 laser and the Q-switched laser respectively when we changed the situation of the focus mirrors.Fig2. the output power variation of CW laser vs the situation of the focus mirrors Fig3. the output power variation of pulse laser vs the situation of the focus mirrorsIn the theoretical analysis, the basic theory of the CO2 laser and the theory of optical transmission matrix. Fig4 is the power distribution and the stability of the CO2 laser.In Fig4, the hyperbolic curve is the dividing line of the stability of the laser cavity. The part between the two hyperbolic curves is the stable area, the resonant cavity in the area is stable, or is unstable. The vertical line in Fig4 is a curve actually, it is the variations g1 and g2 byL. The point M is the maximum output power of the CW laser, it is 8.8W, L is changed 0.02㎜. We can find that the output power in the stable area is larger than the unstable Fig4 the power distribution and the stability of the CO2 laser.area.From the unstable area to the stable area , and then to the unstable area again, the output power is from the minimum to the maximum point M, and then became small and small. It gives a good agreement with the theoretical analysis.In the last part , we made a theoretical analysis for kinetic processes of the Q-switched CO2 laser. We put special emphasis on the effect of the non-laser vibro-rotational levels on describing the dynamic processes of this laser. We determined the dynamic parameters of this Q-switched CO2 laser by the fitting of the experimental and the theoretical analysis results. It is clear that the simple two-level atom model is too constrained to describe the behavior of such laser, and the four-level model using center-manifold techniques seems to be a particularly attractive candidate model for describing it. The effect of the rotational levels involved in the population dynamics of the two relevant vibrational levels with the same relaxation rate on the output pulse is obvious and the calculated pulse waveform of the Q-switched CO2 laser is consistent with the experiment. Fig5 is the structure of the CO2 laser energy level, the level one and two are the laser level, J is the number of rotational levels involved in the population dynamics of the two relevant vibrational levels and denotes the cross relaxation rate of each rotational sublevel into the lasing sublevel of the corresponding vibrational manifold Fig.5 Energy level structure related with laser dynamics for a CO2 molecule.The numerical calculation of this model can be achieved by using Runge-Kutta method, which allows o...