Back to Basics: Metal vapor lasers

Metal vapor lasers come in two types—neutral and ionized. The active medium in both is the gas phase of a metal, mixed with a buffer gas such as nitrogen. A variety of metal vapor lasers were investigated and even available as products in the 1970s and 1980s. Today only one laser of each type survives as a product: the helium-cadmium laser (HeCd) of the ionized type, and the copper vapor laser (CVL) of the neutral family.

Copper vapor lasers are still the highest-power visible-output lasers available, with commercial devices offering average outputs from ten to hundreds of watts and laboratory systems in the kilowatt range. The lasers are inherently pulsed, because the lower laser energy levels are slow to empty. Typical repetition rates are between 5 and 30 kHz with pulse durations in the tens of picoseconds, but even the output of the slower repetition rates appears continuous to the eye. The two main transitions, both between sublevels of the same states, are in the green at 511 nm and the yellow at 578 nm, with the green line about twice as strong.

Copper vapor lasers require a high-voltage pulsed power supply. A key component of the supply is a thyratron switch, essentially an arc discharge capable of rapidly terminating a high-voltage pulse. A distinctive feature of the CVL is the need to heat the tube to 1500°C to vaporize the elemental copper. Waste heat from the discharge current is recycled back into heating the copper to improve system efficiency.

Commercial fallout

Copper vapor lasers were first offered commercially in 1981 by Oxford Lasers Ltd. (Abingdon, England). Development of CVLs benefited significantly from a worldwide billion-dollar effort in the 1970s to find a more efficient means of separating isotopes of uranium. U²³⁵ occurs naturally in about 0.7% concentration, but concentrations required by light-water reactors are around 5%, and uranium-based fission weapons are 90% U²³⁵ or more. It was found that U²³⁵ could be selectively ionized and separated from the remaining uranium isotopes using high-power dye lasers, pumped by CVLs.

Other methods of achieving isotope separation have proven simpler to implement and laser isotope separation has been phased out, but the improvements in power supplies, tube design and gas mixtures from these programs led to better commercial lasers. In the two decades since their introduction, the average output power of commercial CVLs has increased four-fold.

An interesting variation of the CVL design, popular with some hobbyists and graduate students, has contributed to this dramatic improvement in power. Halides of copper will vaporize at a much lower temperature than elemental copper, so a laser of copper bromide (CuBr), for example, only needs to be heated to 400°C. A double pulse of high voltage first dissociates the CuBr vapor and then excites the upper laser level of the copper atom.

Kinetic enhancement

Copper halide lasers have some drawbacks—the powders used in the lasers are hydroscopic and must be preheated in the laser tube to drive out moisture (which itself has to be removed), and the double-pulse high-voltage supply adds complexity and expense. Surprisingly, however, copper halide lasers proved to have higher output powers than conventional CVL lasers. An investigation into the gas behavior in the CuBr laser has led to a doubling of the average power and efficiency of elemental copper lasers.

The role of the buffer gas in a laser is complex, but essentially it can be beneficial in one or more of the following ways: it can transfer energy to the upper laser level, it can remove heat from the laser, and it can help depopulate the lower laser level. Nitrogen (N₂) is the basic buffer gas for CVLs. Independently of the work on CuBr, it had been found that adding 1% to 2% of hydrogen (H₂) to the N₂ boosted the average power of the laser by 25% or more by speeding up the depletion of the lower level.

The bromine dissociated in the CuBr laser does an even better job of emptying the lower level, allowing longer-duration pulses and higher repetition-rate operation. But bromine behaves badly in the laser tube. Chlorine (in the form of a 0.5% addition of hydrochloric acid to the N₂ and H₂ buffer gas) provides the same benefits, helps couple the electrical pulse more efficiently to the copper vapor, and behaves better in the laser. The benefits of this H₂-HCl-N₂ mixture are called "kinetic enhancement" by its inventors at the Metal Vapor Laser Lab at Macquarie University (Sydney, Australia).¹

Applications of CVLs

These improvements in CVL performance have enabled their main application—micromachining. Nanosecond visible and ultraviolet (UV) pulses can machine a wide variety of materials, including steel, to submicron precision while keeping the thermal damage to neighboring material to submicron dimensions. Hole diameters of less than 20 µm have been drilled with an impressive control of aspect ratio, taper, and angle.

The ability of CVLs to maintain diffraction-limited beam quality at high repetition rates and peak power in excess of 100 kW is a key advantage in these applications. In addition, CVL beam profiles can be modified to fit the requirements of the job. A top-hat profile, for example, is better suited to drilling a high aspect ratio than is a Gaussian beam with its extended wings. Taken together, these qualities point to an emerging application for CVLs in the telecom industry.

When an optical fiber is illuminated from the side by a suitably patterned UV source, it establishes a three-dimensional grating along the length of the fiber—a Bragg grating—that selectively reflects a narrow range of wavelengths that would otherwise be transmitted. These fiber Bragg gratings (FBGs) are widely used in optical communications to stabilize the outputs of diode lasers and fiber amplifiers, to provide gain flattening of erbium-doped fiber amplifiers, in add/drop multiplexing for wavelength-division multiplexing, and to compensate for fiber dispersion.

Illumination of the fiber by light between 240 and 260 nm alters the germanium-oxygen bonds in the fiber, changing the local index of refraction.² The UV source requirements include long coherence length and good beam-pointing stability, along with the usual need for reliability. The frequency-doubled output of a CVL at 255 nm using cesium lithium borate meets these requirements. Since it is expected that FBGs will be produced in large numbers for the next generation of all-optical networks, the future of this application looks very bright.

Cadmium lasers

The helium-cadmium laser (1966) is one of many lasers discovered by William Silvast while at the University of Utah (Salt Lake City). The two main transitions are at 442 and 325 nm. The stronger blue line has a gain of about 0.002/cm—a factor of 104 less than the green line in copper. Output is in the range of tens to hundreds of milliwatts. However, cadmium needs to be heated to only 250°C in order to vaporize.

The helium buffer gas plays basically the same role for HeCd as it does in the helium-neon laser. Excited helium in the discharge couples extremely well to the upper laser level of the cadmium ion. Helium-cadmium lasers typically have less than 10 Torr of helium constrained in a bore somewhat larger than 1 mm in diameter, with vaporized cadmium at about 0.1% of the helium concentration.

One of the challenges in making HeCd lasers operate continuous-wave (CW) was dealing with cataphoresis. This is the term given to the migration of the positively charged metal ions toward the cathode where they may condense, depleting the supply of vapor and contaminating tube components and optical surfaces. A critical breakthrough was learning to put cataphoresis to good use in distributing vaporized cadmium evenly throughout the discharge, which made possible CW operation of the UV line.

Holography

Commercial holography (as distinct from holograms made by artists or hobbyists) uses photoresist to engrave a hologram onto a glass plate. The etched glass is electroplated and used in an embossing machine to mass-produce the holograms, a process called "embossed holography." The crucial step is the exposure of the photoresist, which is sensitive to wavelengths between 420 and 450 nm.

Embossed holography requires a laser operating TEM₀₀ with a polarization extinction ratio of 100:1 or better. Higher powers minimize the effects of vibration and temperature changes by limiting exposure times. As a rule of thumb, the maximum possible width of a hologram is about equal to the coherence length of the laser. Gas lasers are ideally suited to these requirements.

The 442-nm output of a HeCd laser is several times more efficient at exposing the photoresist than the 458-nm line of an argon-ion laser, which once competed for this market. This greater sensitivity offsets the inherently lower power of the HeCd compared to argon-ion. With TEM₀₀ output of better than 100 mW and coherence length exceeding 25 cm, HeCd lasers have come to dominate the commercial holographic application. Other embossing processes that use photoresist, such as mastering CDs, also make use of HeCd lasers.

In the early years of their development, a large number of metal vapor lasers using other elements were investigated and even offered commercially. A neutral metal vapor laser using gold produced several watts of red light and looked promising as part of a cancer treatment. A selenium laser very similar to the HeCd was available in the 1970s and could simultaneously produce more than 25 visible lines, resulting in white-light laser output. However, competing technologies and problems with reliability led to the demise of all but the strongest members of each type of laser.

Next month the series will review developments in ultrafast lasers.

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