Gas laser

A gas laser uses atoms, ions or molecules in the gas phase as its gain medium. Most are pumped by an electrical discharge that accelerates electrons, which excite the gas by collisions; a few are pumped chemically, by electron beams or optically. Because gas atoms interact only weakly, their transitions are narrow and their wavelengths are fixed by atomic or molecular structure, so a given gas laser emits at well-defined lines: 632.8 nm for the helium-neon laser, 488.0 and 514.5 nm for argon-ion, 441.6 and 325.0 nm for helium-cadmium, 337.1 nm for the nitrogen laser, 10.6 µm for the CO₂ laser and 193–351 nm for excimer lasers. Powers range from below a milliwatt to tens of kilowatts.

Families and excitation

Gas lasers are usually grouped by the species that lases.

• Neutral atom lasers: helium-neon, and metal-vapour lasers such as copper vapour (510.6 and 578.2 nm). In the HeNe, electrons excite metastable helium, which hands its energy to neon in a near-resonant collision.
• Ion lasers: argon and krypton ion lasers run a high-current arc through the gas and lase between levels of the singly ionized atom. Helium-cadmium lasers lase on Cd⁺.
• Molecular lasers: CO₂ and CO lasers on vibrational transitions in the infrared, and the pulsed nitrogen laser on an electronic transition in the ultraviolet.
• Excimer lasers: rare-gas halides such as KrF that exist only in the excited state, giving automatic population inversion and nanosecond ultraviolet pulses.
• Chemical lasers: hydrogen fluoride and deuterium fluoride lasers pumped by an exothermic reaction, built mainly for high-power research.

Linewidth

In a low-pressure gas, each line is broadened mainly by the Doppler effect from thermal motion. The full width at half maximum is

$$\Delta\nu_D = \frac{\nu}{c}\sqrt{\frac{8 k_B T \ln 2}{m}}.$$

For neon (20.18 u) at 400 K and 632.8 nm this gives 1.5 GHz. For CO₂ (44 u) at 400 K and 10.6 µm it gives about 61 MHz, the lower frequency and heavier molecule both narrowing the line. For argon ions in a discharge with an ion temperature of a few thousand kelvin, taking 3000 K gives about 3.6 GHz at 514.5 nm. The gain bandwidth sets how many longitudinal modes can oscillate and how short a mode-locked pulse can be; a Gaussian pulse filling a 3.6 GHz line lasts at least $0.44/\Delta\nu \approx$ 120 ps, and the mode-locked ion lasers of the 1970s and 1980s were used mostly to pump dye lasers. The laser's own linewidth can be far narrower than the gain width: a single-mode stabilized HeNe holds its frequency to within a few MHz or better.

Efficiency

Most atomic and ionic gas lasers are inefficient. The HeNe upper level lies about 20.7 eV above the ground state and the photon carries 1.96 eV, so even with ideal pumping less than 10% of the excitation energy could leave as light; wall-plug efficiencies are below 0.1%. Water-cooled argon-ion lasers convert on the order of 0.1% or less of their electrical input into light, so multiwatt units draw kilowatts to tens of kilowatts of electrical power and need a water supply. CO₂ lasers, at 10–20% wall-plug efficiency, are the exception, because they lase between low-lying vibrational levels.

Where gas lasers stand now

Diode and diode-pumped solid-state lasers have replaced gas lasers in most low- and medium-power roles: solid-state and semiconductor sources at 488 and 532 nm replaced argon-ion lasers in flow cytometry and pumping, and red laser diodes replaced HeNe tubes in barcode scanners and alignment. Gas lasers remain where their wavelength, beam or line quality is hard to match: CO₂ lasers for cutting and marking non-metals, excimers for deep-ultraviolet lithography and eye surgery, HeNe lasers for interferometry and metrology where a stable, well-known wavelength is needed, and HeCd lasers for photoresist exposure, holography and fluorescence work.

Pitfalls

Gas lasers need high voltages, from about a kilovolt for small HeNe tubes to tens of kilovolts for pulsed systems, and the stored energy in their supplies is dangerous after switch-off. Tubes age as gas is lost or contaminated and as electrodes sputter; sealed-tube lifetimes are specified in operating hours and vary widely by type. Many lines coexist in one gas mixture, so a laser meant for one line may also emit weaker lines unless the mirrors or a prism suppress them.

Common questions

What was the first gas laser?

The helium-neon laser, operated by Javan, Bennett and Herriott at Bell Laboratories in 1960 at 1.15 µm. It was also the first laser to run continuously.

Why do gas lasers have better beam quality than many other lasers?

The gain medium is optically uniform and has little thermal lensing, and many gas lasers use long, narrow tubes that act as apertures, so low-power gas lasers readily run in a single transverse mode with $M^2$ close to 1. High-power CO₂ and excimer lasers are often multimode.

Are gas lasers still used?

Yes, though in fewer roles than in the 1980s. CO₂ and excimer lasers are industrial workhorses, and HeNe lasers are still made for metrology, alignment and teaching.

References: A. E. Siegman, Lasers (University Science Books, 1986); O. Svelto, Principles of Lasers, 5th ed. (Springer, 2010); A. Javan, W. R. Bennett Jr. and D. R. Herriott, Phys. Rev. Lett. 6, 106 (1961); W. B. Bridges, Appl. Phys. Lett. 4, 128 (1964).