Jeff Hecht, Contributing Editor
Laser Focus World

The ability to generate continuous-wave (CW) powers of several milliwatts up to tens of watts at a variety of visible wavelengths, and up to several watts in the ultraviolet (UV), has made rare-gas ion lasers useful in applications from pumping tunable lasers to putting on laser light shows. They are used regularly to detect latent fingerprints, create masters of video disks and compact disks, make medical measurements, and perform fluorescence studies. New applications for ion lasers continue to emerge, including producing three-dimensional (3-D) models of parts in a process called stereolithography and serving as light sources in confocal microscopes.

Engineers have steadily enhanced the performance of rare-gas ion lasers. Lifetime, durability, and output power have improved, while costs have been reduced and shorter UV wavelengths made available. Limited efficiency of ion lasers remains a serious drawback, and developers of diodepumped neodymium lasers have set their sights on replacing low-power argon lasers with frequency-doubled Nd:YAG lasers. But they are not there yet. The high cost of pump diodes leaves low-power ion lasers with healthy price advantage, and frequency-doubled Nd:YAG lasers have yet to match the highest powers from ion lasers. Meanwhile, other solid-state laser advances have helped create a new market for ion lasers-pumping tunable Ti:sapphire lasers.

Many lasers operate on ionic transitions, but an ion laser is normally meant to be one in which the active medium is a an ionized rare gas. The most important com mercial ion laser is based on ar gon, with strong emission lines in the blue-green and weaker lines in the UV and near-IR (see table). Krypton lasers have weaker output but produce a broader range of visible wavelengths, plus additional LJV lines. Mixing argon and krypton gas produces a multigas laser that emits on lines of both gases.

Ion lasers are excited by passing a high-current discharge through a tube containing about 1 torr of pure laser ps or a mixture of argon and krypton in mixed-gas lasers. The discharge is concentrated in a small-diameter bore at the center of the tube, which is where laser action occurs. An initial spike of a few thousand volts breaks down the gas, then voltage drops to 90-400 V while the current jumps to 10-70 A in the sustained discharge. An external magnet producing a magnetic field parallel to the bore axis can help confine the discharge to the bore.

Laser action takes place on transitions far above the ground level of the ion and even further above the neutral ground state (see Fig. 1). The two principal visible argon lines are 514.5 nm (green), which is stronger in larger lasers, and 488 nm (blue), which is stronger in smaller air-cooled argon lasers. Argon lasers are used in a variety of applications requiring visible light, including pumping of tunable lasers, reprographics, displays, and mastering of videodisks. Singly ionized argon (Ar, or Ar II in spectroscopic notation) also produces several weaker blue-green laser lines and a near-infrared line. Argon ions in the lower laser level release energy by spontaneous emission in the far ultraviolet and drop to the ion ground state. This energy-level structure limits the wall-plug efficiency of visible argon lasers to no more than 0.1%.

Higher discharge currents can produce doubly ionized argon (Ar, or Ar III), which has UV laser lines. The most useful UV lines are at 275, 300-306, 334, 351, and 364 nm. Because higher discharge currents are needed, the wall-plug efficiency of these lasers is limited to 0.01%. The shorter lifetime of the UV argon-ion laser is also a result of the high discharge current. Harmonic generation of the visible argon lines can yield even shorter wavelengths.

Krypton generates only 10%-30% as much power as argon used in the same tube. The strongest line of singly ionized krypton (Kr) is at 647.1 nm (red), but yellow, green, and violet lines can produce up to one half as much power, as shown in the table. The violet lines are important for dyelaser pumping; the red and yellow lines are important for displays. Doubly ionized krypton (Kr) has three near-UV lines. Laboratory lasers apparently operating on   Kr lines have generated CW powers totaling 2.5 W at 242 to 266 nm and wavelengths as short as 219 nm.1 These lasers, however, have yet to be commercialized.

Mixed-gas lasers are mainly used in displays and entertainment, where it is important to balance output of the red, yellow, and blue-green primary lines. Early mixed-gas lasers lost krypton faster than argon. The yellow line would fade first, then the red line, so after 1000 h, what started as a mixedgas laser behaved more like an argon laser. Recent improvements, including sealing hard-coated mirrors directly to the tube and adding gas-fill systems, have extended multigas laser lifetime to thousands of hours.

FIGURE 1. Energy levels of singly ionized argon (Ar'), simplified to show only the major bluegreen transition, are far above the neutral ground state. Lower laser level drops to the ground state of Ar' spontaneously with emission of a 74-nm extreme ultraviolet photon.

FIGURE 2. 
(seen below)
Typical low-power air-cooled argon-ion-laser tube shows characteristic long discharge bore and fat cathode region, which serves as a gas reservoir. Air removes excess heat from cooling fins attached to the tube.

Another rare gas that can be operated as an ion laser is neon. Singly ionized neon (Ne) can emit 1 W of CW multiline UV, divided among three lines: 332.4 nm (0.5 W), 337.8 nm (0.2 W), and 339.2 nm (0.3 W). The HeNe laser operates on neutral neon lines, so it is not an ion laser. The only ion laser that normally operates pulsed is xenon, which emits at several lines between 488 and 540 nm. Neon- and xenon-ion lasers are available commercially, but are not in wide use.

Plasma-tube design
The conditions inside an ion-laser plasma tube are extremely harsh. High current densities lead to sputtering of materials in the laser tube and bore, eroding the surfaces, contaminating gases, and trapping some gas atoms. The strong electron flow in the discharge pushes neutral atoms toward the positively charged anode, while positively charged ions migrate toward the negative cathode. The inefficiency of laser operation generates waste heat that must be removed from the laser tube. Much of the relaxation energy appears as short-wavelength UV radiation, which can damage optical surfaces and glasses. Laser engineers have used various approaches to solving these problems.

To achieve reasonable lifetimes, ionlaser tubes are made of ceramic or metal-ceramic structures. For low- to moderate-power operation, a typical design has a beryllia (BeO) bore, with Brewster-angle windows that couple to external optics (see Fig. 2). Reserve gas is kept in a reservoir, which can be an expanded region at the end of the bore or a separate tank. Excess heat is removed by blowing air over the tube or flowing water along its ceramic outer jacket.