Helium-neon laser

The helium-neon (HeNe) laser is a low-pressure gas laser in which an electrical discharge through a helium-neon mixture produces gain on transitions of the neon atom. The familiar red line is at 632.8 nm in air (632.991 nm in vacuum, 473.61 THz, photon energy 1.96 eV), and commercial tubes deliver from about 0.5 to 35 mW in a near-diffraction-limited TEM₀₀ beam. Other neon lines are available with suitable mirrors: 543.5 nm (green), 594.1 nm (yellow), 611.9 nm (orange), and infrared lines at 1152.3, 1523.1 and 3391.3 nm. First operated in 1960 at 1.15 µm by Javan, Bennett and Herriott, it was the first gas laser and the first laser of any kind to run continuously, and the 632.8 nm version remained the most common laser in laboratories until diode lasers displaced it for most low-power tasks.

How the gain arises

The gain medium is a glass capillary, typically 1–2 mm bore, filled with helium and neon in a ratio of roughly 5:1 to 10:1 at a total pressure of order 1 torr (about 130 Pa). A DC discharge, started at several kilovolts and then running at roughly 1–2 kV and a few milliamperes, excites helium atoms into the long-lived metastable levels 2³S (19.8 eV) and 2¹S (20.6 eV). Collisions transfer this energy to neon, whose 3s₂ level (Paschen notation) lies within about 0.05 eV, roughly two $kT$ at room temperature. The neon atoms accumulate there faster than in the lower 2p levels, creating the population inversion; the 632.8 nm line is the 3s₂ to 2p₄ transition. The lower level empties by fast decay to the 1s levels, and the 1s atoms must reach the tube wall to relax further, which is why the bore is narrow and why gain falls as the bore grows.

The gain is small, of order a few percent per pass, so the mirrors must have very high reflectivity: the output coupler usually transmits around 1 %. The 3.39 µm transition shares the upper level and has much higher gain; mirrors designed to reflect poorly at 3.39 µm, and in long tubes magnetic fields or absorbing elements, keep it from depleting the red line.

Spectral and beam properties

Thermal motion of the neon atoms broadens the gain by the Doppler effect to about 1.5 GHz FWHM at a discharge temperature near 400 K (1.3 GHz at 300 K). The longitudinal mode spacing $c/2L$ is 500 MHz for a 30 cm cavity, so such a tube oscillates on two or three modes simultaneously, and fringe visibility in an unequal-path interferometer falls and revives with a period in path difference equal to twice the cavity length. Frequency-stabilized versions run on a single mode, either by balancing two orthogonally polarized modes against each other with a heater on the tube or by locking to an iodine absorption line; the iodine-stabilized HeNe at 632.991 212 58 nm is one of the recommended realizations of the metre, with relative uncertainty of about 2 × 10⁻¹¹.

A typical beam has a waist radius near 0.4 mm and a half-angle divergence of

$$\begin{aligned} \theta &= \frac{\lambda}{\pi w_0} = \frac{632.8\ \mathrm{nm}}{\pi \times 0.4\ \mathrm{mm}} \\ &= 0.50\ \mathrm{mrad}. \end{aligned}$$

Tubes with an internal Brewster-angle window emit linearly polarized light; tubes with mirrors sealed directly to the capillary are "randomly polarized", with modes of orthogonal polarization whose power shares shift as the tube warms.

Uses

HeNe lasers served for alignment, barcode scanners, interferometric displacement measurement, holography, particle sizing and as the reference in scanning wavemeters. Their advantages over a diode laser of equal power are a clean Gaussian beam without astigmatism, a long coherence length in single-frequency versions, and a wavelength fixed by an atomic transition, with no dependence on drive current or chip temperature. Their drawbacks are wall-plug efficiency below 0.1 % (a tube drawing 5 mA at 1.5 kV, 7.5 W, for 2 mW of output is at 0.027 %), size, a high-voltage supply and a warm-up drift of tens of minutes. Visible laser pointers and most alignment tools now use diode lasers.

Pitfalls

Power drifts by several percent as the tube warms and as modes sweep through the gain curve, producing mode-sweep noise during warm-up; this matters in power-sensitive measurements. Back-reflections into the cavity destabilize the output. The supply runs at kilovolts and its capacitors can hold a dangerous charge after switch-off. Tubes slowly lose output over tens of thousands of hours as helium diffuses through the glass.

Common questions

Why is the HeNe wavelength given as 632.8 nm and sometimes 633 nm?

632.8 nm is the air wavelength rounded to 0.1 nm; 633 nm is the same line rounded further. The vacuum value, 632.99 nm, is used in metrology.

Can a HeNe laser be modulated quickly?

Not by its drive current: modulating the discharge changes the output only slowly and with noise. Fast modulation uses an external acousto-optic or electro-optic modulator.

References: A. Javan, W. R. Bennett Jr. and D. R. Herriott, Population inversion and continuous optical maser oscillation in a gas discharge containing a He-Ne mixture, Phys. Rev. Lett. 6, 106 (1961); A. E. Siegman, Lasers (University Science Books, 1986); T. J. Quinn, Practical realization of the definition of the metre, including recommended radiations of other optical frequency standards (2001), Metrologia 40, 103 (2003).