LASING  MEDIUM (CO₂ LASER)

    CO₂ lasers use a mixture of carbon dioxide, nitrogen, and helium in the approximate ratio 1 : 2 : 8, with each gas in the mixture assuming a specific role in this laser. The quantum system of CO₂ laser uses a scheme similar to that of the HeNe laser, in which the pump level for this four-level system is in a separate species from the lasing atom. Nitrogen (N₂) becomes excited with energy from the discharge and the first vibrational energy level of that molecule provides a pump energy level that matches very closely the ULL in the CO₂ molecule (the first asymmetric stretch mode, 001). This is identical to the role of helium in the HeNe laser. A large quantity of nitrogen (i.e., a higher percentage than CO₂) ensures that CO₂ molecules in the ground state are pumped rapidly to the ULL.

       Lasing occurs as a result of a transition between two vibrational energy levels in the carbon dioxide molecule, the levels resulting from the various modes of vibrations possible. Transitions can terminate at possible lower levels, as shown in Figure 1.1, with the most common (and powerful) transition resulting in the production of radiation at 10.6 μm. From that level, depopulation takes place in a two-step process by which either LLL decays to a lower energy state, corresponding to the bending motion of the molecule (010) and finally, to ground state. The addition of helium to the gas mixture ensures that CO₂ molecules at the LLL are depopulated quickly required for a sizable population inversion Helium also serves to conduct heat from the discharge to the walls of the tube since helium conducts heat much better than most gases do. This provides a means of decreasing the thermal population of energy levels of the CO₂ molecule (which lie quite close to ground state), again helping to ensure that an inversion occurs.

     As well as “purely” vibrational levels, rotations of the CO₂ molecule are responsible for the output spectrum of this laser, since rotational levels serve to split each major vibrational level into a cluster of multiple closely spaced levels. As a result, the actual laser output is a series of closely spaced wavelengths covering the range 9.2 μm. to almost 11 μm, centered around 9.6 and 10.6 μm. The 10.6-μm transition in a normal CW laser, for example, consists of over 20 transitions in the wavelength range 10.44 to 11.02 μm. With a diffraction grating added to the cavity for tunability, the large range of outputs makes the laser useful as a source for IR spectroscopy.

Figure 1.1. Energy levels in the carbon dioxide laser.

    Water cooling is required for most CO₂ lasers not just to remove discharge heat but also to reduce the thermal population of the lower energy levels, which are very close to ground level. The output power of most CO₂ lasers is quite sensitive to plasma temperature, and a blocked or restricted cooling water line can easily result in a decrease in output power. To this end, many lasers have thermal sensors on the water cooling jacket of the tube, designed to shut down the laser should the temperature reach 40 to 508C. While the plasma tube would probably tolerate much higher temperatures, laser output would drop drastically at these temperatures.