Ion lasers are a type of gas lasers where ions are used as laser-active agents. Usually, some positively charged noble gas ions such as Ar^+, Ar2+ or Kr^+ play the key role. They are formed by excitation processes in an intense electric dc arc discharge, while neutral atom gas lasers usually use a glow discharge, normally with much lower power density. The degree of ionization is typically a few percent, and the gas pressure of the order of 100 Pa.
Ion lasers belong to the most important visible gas lasers, and some are even suitable as ultraviolet lasers. In contrast to that, most other gas lasers emit in the infrared. The shorter emission wavelengths of ion lasers are related to the fact that the ionization energies of ions are substantially larger than those of neutral atoms – often well above 15 eV. This is particularly true for doubly charged ions like Ar2+, which are suitable for ultraviolet lasers.
Most ion lasers are used in continuous-wave operation. Pulsed operation is in principle possible, but in quasi-continuous mode with pulse durations in the microsecond region, not in the form of Q switching.
Types of Ion Lasers
Argon Ion Lasers
Argon ion lasers use a typically about 1 m long water-cooled tube with an argon plasma, made with an electrical discharge with high current density in order to achieve a high degree of ionization. They can generate more than 20 W of output power in green light at 514.5 nm, and less at some other wavelengths such as 457.9, 488.0, or 351 nm. Their power efficiency is fairly low, so that tens of kilowatts of electrical power are required for multi-watt green output, and the cooling system has corresponding dimensions. There are smaller tubes for air-cooled argon lasers, requiring hundreds of watts for generating some tens of milliwatts. Argon ion lasers can be used e.g. for pumping titanium–sapphire lasers and dye lasers, and are rivaled by frequency-doubled diode-pumped solid-state lasers.
For more details, see the article on argon ion lasers.
Krypton Ion Lasers
Krypton ion lasers are similar to argon ion lasers and mostly use 5p → 5s of Kr^+ ions. They can emit at 647.1 nm (red) and some other wavelengths such as 413.1 nm (blue), 530.9 nm (green) or 568.2 nm (yellow), but various other lines in the visible, ultraviolet and infrared spectral region are also accessible. Compared with argon ion lasers, their output power is typically smaller. That is partly due to the longer lifetimes of the lower laser levels.
Argon/Krypton Ion Lasers
It is also possible to operating ion lasers with both argon and krypton, allowing for lasing on multiple visible spectral lines. That way, a high-power white light source with excellent beam quality can be obtained. However, the spectral composition and thus the color tone changes during operation.
Helium–cadmium lasers, in contrast to helium–neon lasers, are also ion lasers, since their laser-active species is the Cd^+ ion. At the same time, they can be considered as metal vapor lasers. Here, the laser tube has a side arm containing metallic cadmium, which is heated to achieve the appropriate vapor pressure.
Helium–cadmium lasers are in various respects more similar to neutral atom lasers, for example concerning the low output power and moderate power density required. The laser tube lifetime is also correspondingly much longer than that for argon iron lasers, for example. These lasers emit continuously at 441.6 nm (blue), 325.0 nm or 353.6 nm (ultraviolet), with optical powers of the order of 100 mW.
The laser transition occurs in Cd^+ ions, which become excited in collisions with metastable helium atoms (which are generated by the glow discharge). The collision process also involves the ionization (Penning ionization) of Cd to Cd^+. This is important because the freed electron can take away some variable amount of energy. A consequence of that is that the efficiency of the process does not depend on a close match of excitation energies.
Similar lasers have been constructed with zinc or selenium instead of cadmium, but are not common.
The central part of an ion laser is a tube, often made of a highly resistant ceramic material. In most cases (exception: helium–cadmium lasers), a high electric current is applied through electrodes at the ends, or sometimes in an electrodeless tube with RF excitation. The confinement of the current and thus the relatively hot plasma is usually improved with a magnetic field (with a strength of e.g. 0.1 T), generated with a coil around the laser tube. The outer walls are generally water-cooled, as they would otherwise get far too hot; low-power versions, however, can be air-cooled.
A common characteristic of ion lasers (except for helium–cadmium lasers) is that they require excitation with a rather high power density. Best operation conditions are achieved in a regime where the output power is at least about 1 W, and several kilowatts of electrical power are consumed. The large amount of waste heat needs to be reduced with an effective laser cooling system.
The intense excitation leads to a population in many excited levels of the atoms and ions in the gas, including quite highly excited levels. Only a relatively small fraction of the used energy leads to population in the upper laser level, and in addition the quantum defect is quite small. Therefore, the wall-plug efficiency is usually rather low – e.g. of the order of 0.1% for argon ion lasers.
The high power density and high plasma temperature of a high-power ion laser is unfortunately associated with a relatively limited lifetime of the expensive laser tube – often well below 1000 h. (Ion sputtering substantially contributes to the aging, e.g. by degrading electrodes; also, the intense UV light in the tube degrades optical materials and seals.) This together with the high electricity consumption leads to rather high operation expenses. Therefore, such lasers have one more been replaced with diode-pumped and frequency-doubled solid-state lasers.
The laser resonator is normally a simple two-mirror resonator, where the fundamental mode radius approximately matches the diameter of the plasma, so that single transverse mode operation is achieved.
The used ions offer a number of laser transitions with significantly different wavelengths. While some ion lasers operate on a single laser line, which is selected with a dispersive optical element in the resonator (often a prism), others are intentionally operated in multi-line mode, where maximum output power is achieved.
Single-frequency operation is generally difficult to achieve with ion lasers, since the required long tube length leads to a small free spectral range of the laser resonator, while substantial Doppler broadening of the laser transition impedes narrow-linewidth operation.
Applications of Ion Lasers
In the early times of lasers, there was only a very limited choice of visible lasers, and therefore ion lasers with their ability to generate multiple watts at various wavelengths in the visible spectral region where often chosen for applications like laser shows.
For a long time, argon ion lasers have also been used for pumping dye lasers and titanium–sapphire lasers because it was difficult or at least quite expensive to make solid-state lasers with sufficiently high output power and beam quality. Note that Ti:sapphire lasers require a particularly high pump intensity.
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.
Helium–cadmium lasers are used in fields like spectroscopy and holography. While blue helium-cadmium lasers are more and more replaced with cheaper and more compact blue gallium-nitride-based laser diodes, the 325-nm ultraviolet versions are harder to replace.
Alternatives to Ion Lasers
Generally, ion lasers are more and more replaced with diode-pumped solid-state lasers and optical parametric oscillators (OPOs) because those are far more power-efficient and typically exhibit substantially longer lifetimes. High-power green lasers with high beam quality can be realized e.g. with Nd:YAG lasers or vanadate lasers and frequency doublers.
OPOs are mostly required when special emission wavelengths are required, which are hard to get directly from lasers.
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