Argon Ion Lasers
Argon ion lasers are ion lasers, which typically generate multiple watts of optical power in a green or blue output beam with high beam quality. They are the most common ion lasers, and can produce the highest output powers of those. Usually, they are used in continuous-wave operation, although pulsed operation (typically with microsecond pulse durations) is possible.
Argon Ion Laser Tubes
The core component of an argon ion laser is an argon-filled tube, made e.g. of beryllium oxide ceramics, in which an intense electrical discharge between two hollow electrodes generates a plasma with a high density of argon (Ar^+) ions. A solenoid (electromagnet coil) around the tube (not shown in Figure 1) is normally used for generating a magnetic field (with a strength of e.g. 0.1 T), which increases the output power by better confining the plasma. It is particularly important for high-power and UV versions. Brewster windows are used at the ends.
The outer walls of the tube are generally water-cooled, as they would otherwise get far too hot; low-power versions, however, (e.g. with <100 mW output) can be air-cooled.
The intense electric current is associated with a substantial gas pumping effect. In a simple cylindrical laser tube, this leads to a substantial pressure gradient along the tube, which is detrimental for the gas discharge. Therefore, one often uses a tube with a gas return path. Earlier versions had a return tube visible from outside, made long enough to avoid an electric discharge in that region. More modern designs have the return path integrated in the tube, not seen from outside.
The laser resonator is normally a simple two-mirror resonator, little longer than the laser tube. The fundamental mode radius approximately matches the diameter of the plasma, so that single transverse mode operation is achieved. A dispersive prism can be inserted for selecting one of the laser lines (see below).
A resonator frame, made of low-thermal-expansion material, keeps the mirrors stably aligned despite the varying temperatures. Active mirror positioning and an adjustable optical aperture for mode control may also be used.
The obtained laser gain is quite high – of the order of 10 dB/m; it rises steeply with increasing current density. Parasitic resonator losses e.g. from Brewster windows are therefore not of particularly high concern; one can use an output coupler mirror with relatively high transmissivity.
Typical Output Power and Power Consumption
A typical high-power argon ion laser, containing a tube with a length of the order of 1 m, can generate 10 W or 20 W of output power in the green spectral region at 514.5 nm or a couple of somewhat shorter wavelengths It requires several tens of kilowatts of electric power. (The voltage drop across the tube may be 100 V or a few hundred volts, whereas the current can be several tens of amperes.) The dissipated heat must be removed with a water flow around the tube; a closed-circle cooling system often contains a chiller, which further adds to the power consumption. The total wall-plug efficiency is thus very low, usually below 0.1%.
Due to the extreme conditions in the high-density electric discharge, the lifetime of an argon laser tube is unfortunately limited to at most a few thousand hours. Problems result from ion sputtering, causing a deterioration of the electrodes and also a loss of gas pressure. Another challenge arises from intense ultraviolet light, which degrades optical materials and seals.
Due to the sophisticated construction, replacement tubes are quite expensive. This together with the high electricity consumption leads to quite high operation cost.
Low-power Argon Lasers
There are also simpler and smaller air-cooled argon ion lasers, generating some tens or hundreds of milliwatts of output power from several hundred watts of electric power. They do not need water cooling, also normally no electromagnet. However, the natural operation characteristics of argon ion lasers favor devices with output powers of several watts: power efficiencies are worse for low-power devices.
The laser can be switched to other wavelengths such as 457.9 nm (blue), 488.0 nm (blue–green), or 351 nm (ultraviolet) by rotating the intracavity prism (on the right-hand side). The highest output power is achieved on the standard 514.5-nm line. Without an intracavity prism, argon ion lasers have a tendency for multi-line operation with simultaneous output at various wavelengths. All these transitions are 4p → 4s transitions of Ar^+ ions between different sublevels, with two possible lower laser levels, both being quite short-lived. Ultraviolet Ar ion lasers can also use Ar2+ ions as laser-active agents.
Note that while single-line operation is relatively easily achieved, single frequency operation is difficult because of the substantial Doppler-broadened linewidth of the laser transitions in combination with the small free spectral range of the long laser resonator.
There are similar noble gas ion lasers based on krypton instead of argon. Krypton ion lasers typically emit at 647.1 nm or on some other lines mostly in the visible spectral region – green, yellow or blue.
Due to the substantial heating of the tube, a relatively long warm-up time is often required for achieving stable operation. That also contributes to the high electricity consumption.
Laser safety issues arise both from the high output power of typical ion lasers and from the high voltage applied to the tube.
Multi-watt argon ion lasers can be used e.g. for pumping titanium–sapphire lasers and dye lasers, or for laser light shows. However, they have been more and more replaced with frequency-doubled diode-pumped solid-state lasers. The latter are far more power efficient and have longer lifetimes, but are more expensive. An argon laser may thus be preferable if it is used only during a limited number of hours, whereas a diode-pumped solid-state laser is the better solution for reliable and efficient long-term operation.
In earlier years, argon ion lasers with their ability to generate multiple watts at various wavelengths in the visible spectral region were often chosen for laser shows.
Other applications were in Raman spectroscopy, particle flow velocimetry, forensic medicine, holography, wafer inspection and lithography in the semiconductor industry, lithographic fabrication of CD masters, and laser printers. There are also medical applications such as retinal phototherapy. One may also treat diabetes-related retinal detachment of the retina.
|||E. I. Gordon and E. F. Labuda, “Gas pumping in continuously operating ion lasers”, Bell Sys. Tech. J. 43 (4), 1827 (1964); https://doi.org/10.1002/j.1538-7305.1964.tb04114.x|
|||W. B. Bridges, “Laser oscillation in singly ionized argon in the visible spectrum”, Appl. Phys. Lett. 4 (7), 128 (1964); https://doi.org/10.1063/1.1753995; erratum: Appl. Phys. Lett. 5 (2), 39 (1964); https://doi.org/10.1063/1.1754038|
|||E. F. Labuda, E. I. Gordon and R. C. Miller, “Continuous-duty argon ion lasers”, IEEE J. Quantum Electron. 1 (6), 273 (1965); https://doi.org/10.1109/JQE.1965.1072226|
|||P. K. Cheo and H. G. Cooper, “Ultraviolet ion laser transitions between 2300 and 4000 Å”, J. Appl. Phys. 36 (6), 1862 (1965); https://doi.org/10.1063/1.1714367|
|||J. P. Goldsborough, E. B. Hodges and W. E. Bell, “RF induction excitation of CW visible laser transitions in ionized gases”, Appl. Phys. Lett. 8 (6), 137 (1966); https://doi.org/10.1063/1.1754523|
|||O. Svelto, Principles of Lasers, Plenum Press, New York (1998)|
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