What are Argon Ion Lasers?

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- GoPhotonics

Nov 5, 2022

The argon ion laser is a class of noble-gas ion lasers that operates in the visible and ultraviolet regions. The argon ion laser emits visible wavelengths ranging from 408.9 to 686.1 nm, and ultraviolet wavelengths ranging from 275 to 363.8 nm. The argon ion laser was invented in 1964 by William bridges. This continuous wave (CW) lasers are known as ion lasers because it consists of ionized species of argon noble gas. In an argon ion laser, the energy level transition of argon ions contributes to the laser actions.  In the visible spectral region, powers of up to 100 W are produced using an argon ion laser.

A typical Argon ion Laser as shown in the figure above consists of a long and narrow discharge tube made of beryllium oxide filled with Argon gas having two Windows at its ends inclined at Brewster’s angle i.e., the angle of incidence at which light with a particular polarization is perfectly transmitted through a transparent surface, with no reflection. A DC power supply is attached for the power requirements. 

The narrow discharge tube act as an optical resonator or cavity as two mirrors are placed at each end of the tube facing perpendicular to the length of the tube. One of the mirrors is a partially reflecting mirror and the other is a 100% reflecting mirror.

Laser Structure explanation


Argon ion lasers operate in high-temperature plasma tubes with a bore diameter of 1-2 mm and lengths ranging from 0.1 m to 1.8 m. Plasma tubes are designed to operate with bore temperatures of the order of 1,000°C, produced by the very high-power inputs required for laser operation. The series of tungsten disk bores are held in place by a copper support ring, as shown in the figure. The heat generated from the bore is cooled by the copper support ring. 

The bore region is segmented because tungsten bore disks are conductive and a single tungsten tubing for the bore would cause the electrical current to be conducted through the tungsten rather than through the discharge. The series of tungsten bore disks are enveloped with alumina ceramic and a water jacket combined with a magnetic coil. The magnetic coil provides a magnetic field in the direction of the discharge current which slows down the electron movement toward the tube walls. A heated cathode is provided for efficient electron emission into the discharge. 

Commercial argon lasers are manufactured in three sizes: the high­power, large-frame, and the medium-power, small-frame water-cooled lasers; and the low-power air-cooled lasers. The high­power, large-frame lasers offer CW powers up to 10 W on the strong transition at 514.5 nm and 100 W at various wavelengths. The power of a photon is inversely proportional to its wavelength. The lasers are 2 m long and have a separate power supply. These lasers require input powers of 60 kW or more and water cooling at flow rates of 5 gal/min at a pressure of 60 lb/in2 gauge. The medium-power, small-frame water-cooled lasers provide output powers of up to 5 W multi-line and 2 W single-line. They require up to 10 kW of input power and cooling rates of 2 gal/min at a pressure of 25 lb/in2 gauge. The air-cooled argon ion laser produces approximately 10 mW at 488.0 nm. The lasers require input powers of the order of 1 kW and have a laser head with dimensions of the order of 35 cm in length by 15 cm square.

Excitation Mechanism


In the CW argon laser, the excitation is a two-step process, as shown in Figure above, on pumping by an electrical discharge the plasma electrons within the gas discharge collide with the laser species i.e., it first collides with ground-state neutral argon atoms (Ar) to produce ground-state argon single ions (Ar+) also known as intermediate level. 

And the second step involves electron collisional excitation from the argon ion ground state (Ar+) to the upper laser levels (u). Here a population inversion occurs between the upper laser levels and the lower laser levels, it can be achieved by electrical pumping in which the atoms in the ground state are excited to higher states by absorption of pump discharge. Thus, the excited argon atoms radiatively decay to a lower energy level (l) emitting photons of 488 nm wavelength. Similarly, other wavelengths are emitted when the transition occurs between different energy levels in an argon ion laser.

Argon ion laser parameters


Applications

Argon ion lasers are used primarily for phototherapy of the eye. Phototherapy of the eye involves the dissolution of a small streamer (a long, narrow strip) of blood that develops within the eyes of people with diabetes. If these blood streamers are not removed, the patient can become blind.  The blood streamer occurs within the light-sensitive surface, the retina. To remove these streams, a physician directs an argon laser at them through the lens of the eye. The blue and green wavelengths are highly absorbed by the blood streamers and thus dissolve them.

Argon lasers are also very effective in pumping CW dye and titanium sapphire lasers. The laser wavelengths are in the pump absorption region of both of these high-density laser materials, and the laser has the power to provide sufficient gain for pumping such lasers. The high intensity of the argon laser and the blue and green wavelengths make it suitable for use in printers. 

The laser can be focused on a small spot, and the high intensity enables rapid scanning rates for printing purposes. Cell cytometry involves the use of argon ion laser light to count various types of living cells. The laser beam is directed into a flowing sample of the material containing the cells to be counted, and the scattered light that is detected when a cell passes through the region of illumination and recorded on a detector and counted.

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