The Argon Gas Laser System
The Argon laser was invented in 1964 by William Bridges at Hughes Aircraft and is one of a family of Ion lasers that use a noble gas as the active medium. This laser is used in many applications such as:
- Forensic Medicine
- Entertainment
- General Surgery
- Ophthalmic Surgery
- Holography
- Optical "pumping" source
This image shows all the wavelengths of light emitted by the Argon laser operating in multi-mode. Every wavelength is a monochromatic light source of itself and each wavelength has a very narrow bandwidth. The two dominant wavelengths, of 514nm "Green" and 488nm "Blue" make up about 67% of the total beam output power. Single line operation is also possible by inserting prisms, diffraction gratings and other optical devices to "filter out" the unwanted wavelengths. Of course, when single line operation is required, the total output power decreases dramatically as well.
The laser resonator is made up of two mirrors. One is highly reflective (HR) and other is a partially reflective mirror (OC). From this optic (the Output Coupler) the beam emerges as laser light. The Brewster's Angle optic mounted at both ends of the tube, minimizes reflection loses while creating a polarized beam. When the laser is first turned on, a delay allows for temperature stabilization. Then a pulse of high voltage (8 kilovolts DC) ionizes the argon gas. Upon ionization, high DC current (45 Amps) and about 600 volts DC across the tube maintains a sufficient discharge to keep the gas ionized. The typical Argon laser tube has a tungsten bore which has a high melting point and allows the laser to operate at higher power levels with longer tube life.
Argon lasers with average powers of over three watts require tap water cooling and separate three phase electrical line feeds.
Safety Note:
Argon laser emissions are hazardous to view. Both diffuse and direct exposures beyond the applicable MPE (Maximum Permissible Exposure) limit can cause permanent damage to the retina.
CO2 Laser
Class I CW CO2 Lasers
The Class I CW CO2 laser was the first CO2 laser developed, and continues to be the most common.
Below is a diagram of such a laser. Common characteristics of this CO2 laser class include the following:
- Water or oil cooling by use of a double-walled glass plasma tube.
- Gas flow at a low rate (1-20 liters per minute, depending upon size and output of laser).
- Dc excitation, coaxial with gas flow and laser beam.
- Low-current operation (3-100 mA).
- Gas pressures of 10 to 30 torr.
- Tube diameters of 1.0-2.0 cm.
- Available output powers up to about 50 W per meter of tube length.
Simple coaxial flowing CO2 laser.
The primary factor that limits output power of these lasers is their inability to efficiently remove waste heat from the gas. Cooling is principally achieved by helium (He) collisions with tube walls. Air cooling of CO2 laser tubes is possible, but this results in an elevated wall temperature and greatly reduces laser efficiency. Smaller CO2 lasers and those used in research often employ water cooling.
Industrial CO2 lasers usually use recirculating oil and oil-to-water heat exchangers for better system stability and reduced maintenance. An increase of tube current beyond the recommended operating value results in more heat than can be effectively removed from the system in this manner. Increases in tube diameter also decrease cooling efficiency by increasing the path length necessary for He atoms to reach the walls from the center of the tube.
Thus, the only effective method of increasing output power of this type of CO2 laser is to extend the active length. For best results, this must also be accompanied by an increase in gas flow rate. In most larger systems the gas is recirculated with a few percent being replaced on each cycle.
