Question

What is the best way to design an argon laser that emits at 501.7 nm?

Answer 1

An ion laser is the best way to go. An ion laser consists of a plasma with a glow discharge at high current density passing through it. The most common ionized gas laser utilizes ionized argon as the laser medium. A partial energy level diagram relevant to the argon ion laser is shown. A number of laser transitions are indicated. The strongest laser transitions are at 488 and 514.5 nm.

The lowest energy level shown in the figure is the ground state of the argon ion, which lies 16 eV above the ground state of the neutral argon atom. In addition, the upper energy levels lie about 20 eV above the ionic ground state. Thus, a considerable amount of energy must be supplied to the neutral argon atom to raise it to the upper laser level for laser operation.

The argon laser has higher gain than the helium–neon laser, and much larger amounts of power may be extracted from it. The output power scales nonlinearly with the current density, so that it is desirable to operate argon lasers with a narrow bore and a high electrical current. Current densities above 100 A/cm² may be employed in argon lasers. The high current density produces heating and erosion of the plasma tube walls, and thus it strongly influences the design of argon lasers.

Materials used to define the bore of an argon laser must be resistant to both high temperature and to sputtering by the electrical discharge. A common design involves use of electrically isolated, radiation-cooled annular segments of material to confine the discharge, enclosed in a vacuum envelope. The material that is most commonly used now for the segments is tungsten. The envelope is usually alumina ceramic or quartz. Argon lasers require active cooling. Air cooling may be used in relatively low power models, but water cooling is employed at higher levels of power.

A typical design for a segmented bore argon laser tube is shown.

Each bore segment has a hole in its center to define the discharge region, and holes around the central region to allow for gas return. The figure also shows the wavelength selecting prism, which allows operation on a single one of the wavelengths available. The prism is rotated to select the desired wavelength. Without the prism, multiple wavelengths are present in the output simultaneously, and the laser is said to operate multiline. The solenoid supplies a longitudinal magnetic field, which confines the electrical current and increases the current density.

The argon gas is depleted in the course of operation, so that gas refill reservoirs are included in the design of many argon lasers. When the argon pressure drops below a specified value, the operator activates the refill system to restore the pressure to the specified value.

A commercial table is also given for configurations: