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 to achieve a high degree of ionization. A substantial number of emission lines is available; the most prominent ones are:

• 514.5 nm (green)
• 488 nm (blue)
• 457.9 nm (blue)
• 351.1 nm (near UV)

They can generate more than 20 W of output power in green light at 514.5 nm, and less at other wavelengths. 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 are similar in construction and pumping to argon-ion lasers and lase on excited Kr⁺ (($4p \rightarrow 4s)$) transitions. They offer several visible colors that argon does not. Typical krypton-ion laser lines include:

• 647.1 nm (red)
• 568.2 nm (yellow)
• 530.9 nm (green)
• 476–413 nm (blue/violet)

and some weaker lines in the near-UV.

Compared with argon-ion lasers, krypton-ion lasers usually deliver lower output power for comparable input power. This is partly due to level lifetimes and partly to less favorable excitation cross-sections.

It is also possible to operate ion lasers with gas mixtures of argon and krypton, allowing lasing on multiple visible spectral lines from both species. In this way, a high-brightness white-light source with excellent beam quality can be obtained. However, the exact spectral composition (and thus the perceived color) can drift with tube temperature, discharge current, and gas mixture evolution.

Helium–cadmium lasers are also ion lasers in the sense that the actual laser-active species is Cd⁺ (a singly ionized metal vapor). At the same time, they fit into the family of metal vapor lasers. The laser tube contains a side arm with metallic cadmium, which is heated to provide the needed vapor pressure.

Unlike high-power argon-ion tubes, helium–cadmium lasers operate at lower power density and behave in several respects more like neutral-atom discharge lasers: Their tubes have longer lifetimes, they need less cooling, and their output powers are typically in the tens to hundreds of milliwatts range. They emit continuously at 441.6 nm (blue) and in the ultraviolet at 325.0 nm or 353.6 nm.

In the discharge, the Cd⁺ ions are produced and excited mainly by collisions with metastable helium atoms created in the discharge, rather than directly in electron collisions. The process involves Penning ionization of Cd, which makes the excitation relatively insensitive to exact energy matching. Similar lasers using zinc or selenium vapors have been demonstrated but are uncommon.

The central component of an ion laser is the discharge tube, often made of a heat- and sputter-resistant ceramic. In most ion lasers (except helium–cadmium), a high DC current flows between end electrodes; in some designs, an electrodeless tube with RF excitation is used to avoid electrode erosion. A longitudinal magnetic field (typically ≈0.1 T) is often applied with an external coil to confine the plasma and stabilize the discharge. Because the plasma and tube get very hot, the tube is almost always water-cooled; only the lowest-power versions can be air-cooled.

A hallmark of (Ar⁺, Kr⁺) ion lasers is the requirement for very high power density in the discharge to maintain enough ions and to pump the upper levels efficiently. Best performance is reached when optical output is above 1 W and the tube is being fed with several kilowatts of electrical power. This demands an effective laser cooling system.

The intense excitation in the discharge populates many excited levels of the gas, but only a small subset of those levels participates in the laser transitions. Moreover, although the quantum defect is small (visible output), the fraction of input power that actually ends up in the upper laser level is small. As a result, the overall wall-plug efficiency of classical argon-ion lasers is typically well below 1% (often ≈0.1%).

Despite the high plasma temperature, the gas density is low, so thermal lensing is relatively weak. With a properly designed resonator and a narrow plasma column, ion lasers can deliver very good beam quality, often close to the diffraction limit.

The combination of high current, high plasma temperature, and intense UV inside the tube leads to limited tube lifetimes — for high-power argon-ion tubes often well below 1000 h. Ion sputtering degrades electrodes and internal parts; UV also attacks windows, seals, and mirror coatings. Together with the high electrical power consumption, this makes ion lasers relatively expensive to operate and is a key reason why they have been widely replaced by diode-pumped and frequency-doubled solid-state lasers.

The laser resonator is usually a simple two-mirror cavity. The fundamental transverse mode is chosen to roughly match the diameter and position of the plasma column, so that single-transverse-mode (TEM₀₀) operation with high beam quality is achieved. Long tube lengths imply relatively small mode spacing (free spectral range), which affects spectral control.

Because the active ions offer multiple usable laser transitions at widely spaced wavelengths, an ion laser can be operated in different ways:

• A single laser line can be selected with an intracavity dispersive element (typically a prism or etalon).
• In multi-line mode, no such element is used and the laser emits on several visible (and sometimes UV) lines simultaneously, maximizing total output power.

Single-frequency operation is in general more difficult than in short solid-state lasers, because the long discharge tube implies a long cavity, thus a small free spectral range, while Doppler and pressure broadening of the plasma lines are relatively large.