CO₂ laser

The carbon dioxide laser is a gas laser whose gain comes from transitions between vibrational levels of the CO₂ molecule. The strongest band is centred at 10.6 µm (28.3 THz, 0.117 eV per photon), with a second band near 9.4 µm (0.132 eV); within each band dozens of individual rotational lines can lase, and a grating in the cavity selects one of them. Sealed tubes deliver from a few watts to a few hundred watts, and fast-flow systems for cutting and welding reach several kilowatts to tens of kilowatts of continuous power. Wall-plug efficiency is typically around 10–20%. C. K. N. Patel first demonstrated it at Bell Laboratories in 1964.

Energy levels and the gas mixture

The discharge runs in a mixture of CO₂, nitrogen and helium at pressures from tens of millibar in continuous lasers up to about one atmosphere in pulsed TEA (transversely excited atmospheric) designs. Electrons in the discharge excite the first vibrational level of N₂ very efficiently, and because that level (2331 cm⁻¹) lies close to the CO₂ asymmetric-stretch level 00°1 (2349 cm⁻¹), collisions transfer the energy to CO₂ with little loss. Lasing takes CO₂ from 00°1 down to the symmetric-stretch level 10°0 (10.6 µm band) or the bending overtone 02°0 (9.4 µm band). Helium cools the gas by conduction and helps empty the lower levels, keeping the system close to an ideal four-level laser and sustaining the population inversion.

The quantum efficiency follows from the level energies. For the 10.6 µm line the photon carries 943 cm⁻¹ of the 2349 cm⁻¹ stored in the upper level:

$$\eta_q = \frac{943}{2349} \approx 0.40$$

The remaining 60% ends up as heat in the gas, which is why high-power CO₂ lasers circulate the gas through heat exchangers or rely on diffusion cooling to closely spaced electrodes.

Linewidth and tuning

At 400 K the Doppler width of a 10.6 µm line is

$$\Delta\nu_D = \frac{\nu}{c}\sqrt{\frac{8 k_B T \ln 2}{m}} \approx 61\ \mathrm{MHz},$$

using the CO₂ mass of 44 u. At the pressures of a sealed tube, collisional broadening is comparable, and in atmospheric-pressure lasers it dominates and broadens each line to gigahertz widths. The lines remain discrete until pressures of roughly 10 atm, where they overlap enough for continuous tuning.

Optics and beam delivery

Ordinary glass and fused silica are opaque at 10.6 µm, so CO₂ systems use zinc selenide lenses and windows, germanium, and for some applications alkali halides such as KCl, together with gold- or copper-coated mirrors. There is no practical low-loss silica fiber for this wavelength, so beams are delivered through articulated mirror arms or hollow waveguides.

The long wavelength sets the focused spot size. For a collimated Gaussian beam of diameter $D$ focused by a lens of focal length $f$, the spot diameter is about $4\lambda f/(\pi D)$. With $f$ = 100 mm and $D$ = 20 mm this gives 67 µm at 10.6 µm, against 6.8 µm for a 1.07 µm fiber laser with the same optics. The tenfold difference in beam parameter product at equal beam quality is one reason fiber lasers have taken over thin-metal cutting.

Where it is used

In laser machining, CO₂ lasers cut and engrave wood, acrylic, paper, textiles, glass and thick steel plate. Organic materials, water and most glasses absorb 10.6 µm strongly, while metals reflect much of it. Surgical CO₂ lasers exploit strong absorption in tissue water for cutting and skin resurfacing with shallow thermal damage. Pulsed CO₂ lasers also drive the tin-plasma extreme-ultraviolet sources used in lithography, and the band sits in the long-wave infrared atmospheric window used for lidar and gas sensing.

Pitfalls

The beam is invisible and is blocked by ordinary glass; alignment uses a coaxial visible diode or burn paper. The cornea absorbs 10.6 µm strongly, so the main eye hazard is a corneal burn, and little reaches the retina. Even clear polycarbonate eyewear absorbs the wavelength, although it must be rated for the power and exposure. Sealed tubes slowly lose power as CO₂ dissociates into CO and O₂ in the discharge; catalysts in the tube and small gas additives such as CO, H₂ or xenon slow this. ZnSe lenses contaminated with smoke or debris absorb the beam and can crack.

Common questions

Why is the CO₂ laser so much more efficient than most other gas lasers?

The lasing transition is between low-lying vibrational levels, so each photon uses about 40% of the upper-level energy, and the nitrogen transfer step pumps that level selectively. Atomic and ionic gas lasers such as the helium-neon laser have quantum efficiencies below about 10% (1.96 eV out of the 20.66 eV neon upper level for the HeNe) and wall-plug efficiencies below about 0.1%.

Can a CO₂ laser cut metal?

Yes. Kilowatt CO₂ lasers with an oxygen or nitrogen assist gas cut steel plate up to about 25 mm thick and were the standard sheet-metal cutting tool for decades. For thin sheet and reflective metals such as copper and aluminium, 1 µm fiber lasers are now usually preferred.

What is the difference between the 10.6 and 9.4 µm lines?

Both bands share the upper level and end on different lower levels. Some polymers, glasses and ceramics absorb more strongly near 9.4 µm, which can give cleaner marking or drilling in those materials.

References: C. K. N. Patel, Phys. Rev. 136, A1187 (1964); A. E. Siegman, Lasers (University Science Books, 1986); O. Svelto, Principles of Lasers, 5th ed. (Springer, 2010).