A CO2 laser is a type of gas laser that generates a highly intense beam of infrared light. Invented in 1964, it remains one of the most widely used laser technologies across various fields. Its significance stems from its ability to precisely interact with many materials, including biological tissues, making it a valuable tool in both industrial and medical applications.
Generating the CO2 Laser Beam
The creation of a CO2 laser beam begins within a sealed tube containing a specific gas mixture. This active medium primarily consists of carbon dioxide (CO2), nitrogen (N2), and helium (He) gases.
An electrical discharge, supplied by a high-voltage power source, excites the gas molecules. This electrical energy preferentially excites nitrogen molecules to a metastable vibrational energy level. These excited nitrogen molecules then efficiently transfer their absorbed energy to the carbon dioxide molecules through collisions. This energy transfer elevates the CO2 molecules to higher vibrational energy states.
When these excited carbon dioxide molecules return to a lower energy state, they emit photons. This process is significantly amplified through stimulated emission. In stimulated emission, a photon passing by an excited CO2 molecule can trigger the emission of an identical photon, having the same frequency, phase, and direction. This cascading effect creates a coherent and powerful laser beam. The primary wavelength produced by CO2 lasers is in the infrared spectrum, typically around 10.6 micrometers (µm).
Essential Components
A CO2 laser system relies on several components to generate and deliver its powerful beam. At its core is the laser tube, a sealed enclosure containing the gas mixture where the laser light is initially produced. This tube often includes a water-cooling system to manage the heat generated during the electrical discharge, which helps maintain stable power output and prolongs the tube’s operational life.
The optical resonator is a pair of mirrors positioned at either end of the laser tube. One mirror is fully reflective, while the other is partially reflective, allowing a portion of the amplified light to exit as the laser beam. These mirrors cause photons to bounce back and forth, amplifying the light through repeated stimulated emissions until it reaches sufficient intensity.
A power supply provides the electrical energy necessary to excite the gas mixture within the laser tube, initiating the laser action. This can be a direct current (DC) voltage or radio frequency (RF) waves. Finally, a beam delivery system, such as an articulated arm or focusing lenses, guides the laser light from the laser source to the target material, allowing for precise application.
Interacting with Tissue
The CO2 laser’s interaction with biological tissue is primarily based on the strong absorption of its 10.6 µm wavelength by water. Since biological tissues are largely composed of water (often over 70%), this high absorption leads to a highly localized effect. When the laser beam strikes tissue, the water within the cells rapidly absorbs the laser energy.
This rapid absorption causes intracellular and extracellular water to heat almost instantaneously, leading to vaporization. This process, known as photo-thermal ablation, results in the precise removal of tissue. As water turns into steam, the tissue volume expands, creating a microscopic explosion that effectively cuts or ablates the target area.
The precise nature of this interaction means that the laser’s effect is confined to a very shallow depth, typically between 20 and 100 micrometers. This minimizes thermal damage to surrounding healthy tissue. The heat generated also causes coagulation of small blood vessels, which helps to reduce bleeding during surgical procedures. This combination of precise tissue removal and simultaneous coagulation makes the CO2 laser a valuable tool in various medical applications, including dermatology and soft-tissue surgery.