Helium neon (HeNe) lasers are gas lasers which use a mixture of helium and neon to achieve optical gain. All lasers consist of three components: an energy source (or pump), a gain medium, and an optical resonator; these components are shown in the diagram below. Essentially, the pump provides energy which is amplified by the gain medium. This energy is eventually converted into light and is reflected through the optical resonator which then emits the final output beam.

Image credit: Enlighten Your Mind

Considering these commonalities, HeNe lasers must consist of three main component groups:

• A high voltage electrical current source, which functions as the pump.
• A mixture of helium and neon gases at a concentration of 10:1, which acts as the laser's gain medium.
• A series of mirrors functioning as the optical resonator. HeNe lasers typically employ at least one highly reflective mirror to redirect the beam to the output point, as well as an output coupler to focus the output beam.

A cutaway view of a typical HeNe laser. Image credit: Olympus

When voltage is applied from the power supply, the cathode — which is directly connected to the gas reservoir containing the HeNe mixture — supplies electrical current. The introduction of current excites the helium atoms, which in turn excite the neon atoms, causing them to radiate photonic energy at a wavelength of 632.8 nm. This energy is then funneled into the laser's bore, where it is reflected back and forth through the laser by two highly reflective mirrors. Each time the beam is reflected it strengthens in intensity. Since one of the mirrors reflects 100% of the radiation while the other reflects only 99%, the resulting 1% is emitted as a continuous, focused beam.

An excellent explanation of HeNe laser operation. Video credit: University College of London / CC BY-SA 4.0

Some HeNe lasers do not use internal mirrors but instead use a single Brewster window. When mounted at a special angle (known as the Brewster angle) the window linearly polarizes the energy, eliminating the need for the radiation to be continuously reflected within the laser. The use of a Brewster window results in an extremely efficient laser due to the fact that all radiation is essentially emitted as an output beam.

Helium neon lasers are famed for their long operating lives (up to 50,000 hours of continuous use) mostly due to the fact that only two main conditions can cause a decrease in efficiency or device failure:

• Deterioration of the cathode's oxide coating, which can eventually cause sputtering of aluminum.
• Helium leakage due to a broken seal or diffusion through glass; this can cause reduced lasing ability until the laser fails completely.

Applications

Helium neon lasers are low-cost and simple to use when compared with other gas lasers. They have historically been used for many applications within the fields of microscopy, barcode scanning, spectroscopy, optical disc reading, biomedical engineering, metrology, and holography. Since the introduction and improvement of semiconductor lasers, however, many users are now utilizing smaller and more efficient laser diodes in place of HeNe lasers, especially in confocal microscopy.

Specifications

Wavelength

As stated above, the interaction of excited helium atoms with neon atoms causes emission of 632.8 nm radiation. For this reason, a large majority of HeNe lasers emit 633 nm (red) beams, as this wavelength results in the simplest laser and relies on the basic interaction of the electrical pump and the gas medium. However, by using specially-coated optical components, the basic 633 nm beam can be altered to other wavelengths and colors. Other common helium neon laser wavelengths include 543.5 nm (green), 594.1 (yellow), 611.9 (orange), and infrared at 1523 or 3921 nm.

Power

A laser's power is measured in watts (W) or milliwatts (mW) and is used to describe the strength of the laser's beam. HeNe lasers are relatively low-power devices, especially when compared with carbon dioxide gas lasers, and typically have power ratings between 0.5 mW and 50 mW.

Safety

Despite the low power output capabilities, laser safety is an important topic to consider when discussing HeNe lasers despite their typically low power output. Even a split-second direct exposure to a 200 mW laser emitting 100 yards away can cause permanent eye damage.

A laser warning sign, including specs and class. Image credit: Keller Studio

To address the concerns above, the Center for Devices and Radiological Health (CDRH) — a division of the US Food and Drug Administration (FDA) — provides a laser safety classification scheme based on six product classes. Lasers are also specified by different classes described in the international IEC 60825 standard. The table below describes both US and international classes for laser safety.

Laser safety classes. Image credit: Erchonia

References

Olympus - Helium neon lasers