Understanding CO2 lasers

A detailed treatment of a laser system that holds great promise for high-power applications includes design procedures and parameter tradeoff considerations.

In CO2 laser system three gases (CO2, N2 and He) are mixed and fed into one end of a discharge tube at a pressure of a few torr. The gas flows down the end of the tube in about one second and is pumped out the far end with a mechanical forepump. An electrical discharge is maintained between the metallic end flanges of the tube. The ballast resistance is required because of the negative dynamic resistance of the discharge. With a fully reflecting mirror on the left and a partially transmitting mirror on the right, the device becomes a laser which radiates in the far infrared at 10.6 microns.

The CO2 laser is comparable in simplicity, if not in size, to the microwave klystron. Both have a high-Q electromagnetic cavity, a highly efficient power-output coupling technique, and an active medium fed from a simple DC power supply. Also, with slight modifications either can be used as an oscillator or an amplifier.

The possibility of using molecular vibrations for laser action was clearly pointed out by Polanyi in 1961. More recently, Patel has described in some detail the laser action on the vibrational-rotational (V-R) transitions of CO2 in an electrical discharge. Shortly afterwards, progress towards high power and efficiency was achieved by Patel with the addition of rotational (V-R) transitions of CO2 in an electrical discharge. Shortly afterwards, progress towards high power and efficiency was achieved by Patel with the addition of N2 and by Moeller and Rigden with the addition of He. Other molecular systems have been made to lase, but so far CO2 is the most important power producer.

(Figure 1)

Many parameters affect the design and operation of the CO2 laser. The gas discharge can be powered with dc, ac, RF, repetitive pulses, or any combination thereof. The mirrors can be fixed, rotated for Q-switching, or vibrated for reactive Q-switching. The discussion here will be limited to dc excitation with fixed mirrors. Most of the technical data were obtained from Horrigan and Whitehouse;  more detail can be obtained by referring to their reports.

Optimising the CO2  laser

Performance of Air Presto Flyknit Ultra