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Metal Vapor Lasers

Metal vapor lasers are a kind of gas lasers which utilize a metal vapor as laser gain medium. More specifically, the laser-active agents are metal atoms, or sometimes metal ions.

Available emission wavelengths of metal vapor lasers range from the infrared to the ultraviolet.

The metal vapor is often contained by a quartz tube, having a electrodes, laser mirrors and optical windows at its ends.

In contrast to other gas lasers, their gain medium is not ready for operation at room temperature, but needs to be created at substantially increased temperatures by evaporating the used metal, or sometimes a metal compound, e.g. a halide. The evaporation is usually achieved with a sufficiently intense arc discharge. One sometimes applies two current pulses, where the first one creates the metallic vapor and the second one more strongly excites the metal atoms such that laser operation becomes possible. In some cases, excitation energy is transferred to the metal vapor by an excited noble gas (e.g. helium in a metastable state).

Some metal vapor lasers, particularly alkali metal vapors, are optically pumped.

The operation principle of electrically pumped metal vapor lasers often works only in pulsed operation, i.e., these are mostly pulsed lasers, although with some types one can also realize continuous-wave operation. The pulse durations are typically in the nanosecond region.

One of the technical problems is the risk that metal is deposited on the optical windows, deteriorating their transparency. One may use a condensation trap to prevent that.

Types of Metal Vapor Lasers

The most important types of metal vapor lasers are described in the following. Others are based on the metal elements lead, calcium, gold, selenium, manganese or thallium. These are used only rarely, and more and more replaced with other lasers, such as diode lasers.

Copper Vapor Lasers

Copper vapor lasers are the most powerful metal vapor lasers. They are excited with intense current pulses and generate nanosecond pulses at 510.6 nm (green) or 578.2 nm (yellow). The copper temperature needs to be rather high (around 700 °C) in order to achieve a sufficiently high vapor pressure (around 5 μbar) and density. Preheating of the laser is often necessary before laser operation can start. One often uses some copper halides instead of elementary copper in order to ease the generation of the vapor.

The used laser transitions are examples of self-terminating laser transitions: the lower laser level is only slowly depopulated. Therefore, such lasers generally work only in pulsed operation (not in continuous-wave operation).

Compared with other metal vapor lasers, the achieved gain can be relatively high. Particularly for high pulse repetition rates (e.g. 100 kHz), the average output power can exceed 100 W or even well over 1 kW. With their powerful visible output and robust setup, they are suitable for example for application in laser shows. They have also been used for pumping of pulsed dye lasers (e.g. for laser isotope separation) or amplifiers and titanium–sapphire lasers. Another application is photodynamic therapy.

Metal vapor lasers can be realized relatively easily, even home-built with simple parts.

There are also metal vapor lasers utilizing Cu^+ ions for obtaining and ultraviolet laser, which may even be used in continuous-wave operation.

Alkali Vapor Lasers

There are alkali vapor lasers, using e.g. a cesium or rubidium cell in an oven as the gain medium. Such metals can be relatively easily vaporized – not only for lasers, but also for metal vapor lamps and spectroscopic gas cells.

The laser transition occurs on the D₂ line, which is in the visible or the near infrared region, depending on the used alkali metal (see Table 1). Pumping is done on the D₁ line, i.e., at an only slightly shorter wavelength. Population transfer from the pump level to the upper laser level is obtained by collisions with molecules of a buffer gas (e.g. ethane).

Table 1: Wavelengths of the relevant spectral lines of alkali metals.

The quantum defect of such lasers (particularly for the lighter metals) is quite small, and at the same time the quantum efficiency can be rather high. Therefore, the power efficiency can be fairly high. The upper-state lifetime, is relatively short (some tens of nanoseconds) because the laser transition is not a forbidden transition, but the emission cross-section is also very high.

Alkali vapor lasers can be pumped with laser diodes. The output beam quality can be much higher than that of the pump source, so that such a laser acts as an efficient brightness converter.

Helium–cadmium Lasers

Helium–cadmium lasers are in some respects (continuous operation, high beam quality, moderate power density, long lifetime) similar to helium–neon lasers, although the laser active cadmium needs to be vaporized before laser operation can start. They 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.

Typical applications of helium–cadmium lasers are 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 versions are harder to replace.

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Encyclopedia articles:

• gas lasers
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