Carbon-dioxide laser

A carbon dioxide laser, often called a CO2 laser, is a powerful device that produces a beam of infrared light. This light is invisible to the human eye because it exists in the infrared spectrum. These lasers are among the highest-power continuous-wave lasers available today. They are also highly efficient, meaning they can convert up to 20% of their input power into useful light output.

Because they can produce such intense energy, they are essential in many scientific and industrial fields.

The mechanism of a CO2 laser relies on a specific sequence of energy transfers within a gas mixture. The active medium is a gas discharge contained in a tube. This mixture typically contains 10–20% carbon dioxide (CO2) and 10–20% nitrogen (N2). It also includes a few percent of hydrogen (H2) or xenon (Xe), with helium (He) making up the rest. To create the laser beam, a process called population inversion must occur. First, electron impacts excite the vibrational modes of the nitrogen molecules. Because nitrogen is a homonuclear molecule, it cannot release this energy as light. Instead, the energy stays stored in the nitrogen for a short time.

Next, these excited nitrogen molecules collide with carbon dioxide molecules. This collision transfers the vibrational energy to the CO2, pushing it into an excited state known as the asymmetric stretch. The CO2 molecules then release this energy as infrared light. They do this by dropping to different vibrational states, producing wavelengths centered around 9.6 or 10.6 micrometers (μm). To keep the process running, helium atoms are used to cool the CO2 molecules. In sealed lasers, the helium atoms strike the walls of the tube to release heat. In flow-through lasers, a pump moves the hot gas mixture out of the system entirely.

There are two main ways to construct these machines. A sealed laser uses a fixed amount of gas inside a discharge tube. A flow-through laser is different because the gas is continuously pumped through the device. These lasers can produce different levels of power. They can operate in a continuous wave (CW) mode, ranging from milliwatts to hundreds of kilowatts. They can also be "Q-switched," which uses a rotating mirror or an electro-optic switch to create massive bursts of energy. These peak powers can reach up to gigawatts (GW).

Building a CO2 laser requires specialized materials because the infrared light behaves differently than visible light. Standard glass is often unsuitable for the windows and lenses. Instead, manufacturers use materials like germanium or zinc selenide. For very high-power tasks, gold mirrors and diamond windows are preferred. Diamond is especially useful because it is extremely hard and has high thermal conductivity. This allows it to stay cool even in dirty or high-heat environments. Historically, scientists used salt, such as sodium chloride, for lenses. However, salt was not ideal because it would degrade when exposed to moisture in the air.

These lasers are incredibly versatile and are used in many different industries. In manufacturing, they are used for cutting, welding, and engraving metal. They are also used in selective laser sintering to fuse plastic powder into solid parts. In the military, they are used for rangefinding through LIDAR techniques. This works because the atmosphere is quite transparent to infrared light, allowing up to 80% transmission at these wavelengths. Even in semiconductor manufacturing, CO2 lasers help generate extreme ultraviolet light.

In the medical field, CO2 lasers are highly effective for soft-tissue surgery. This is because biological tissue contains a lot of water, which absorbs this specific frequency of light very well. This absorption allows doctors to perform procedures like skin resurfacing or removing vocal-fold lesions. Using a laser can result in less bleeding, shorter surgery times, and a lower risk of infection compared to traditional tools. They are used in many specialties, including dentistry, gynecology, and veterinary medicine. In some dental cases, the laser can even ablate hard tissue at temperatures as high as 5,000 °C.