What is a He–Ne laser?

A He–Ne (Helium–Neon) laser is a gas laser that uses a mixture of helium and neon gases as the active medium. It commonly emits red light at 632.8 nm.

Which gases are used in a He–Ne laser?

A He–Ne laser uses a mixture of helium and neon gases, typically in a ratio of about 10:1, contained in a discharge tube.

How does a He–Ne laser work?

In a He–Ne laser, an electrical discharge excites helium atoms, which transfer energy to neon atoms through collisions. The neon atoms then undergo stimulated emission, producing coherent laser light.

What is the wavelength of a He–Ne laser?

The most common emission wavelength of a He–Ne laser is 632.8 nm (red light), although other wavelengths such as 1152 nm and 3391 nm are also possible.

What are the applications of He–Ne lasers?

He–Ne lasers are used in barcode scanners, holography, interferometry, optical experiments, alignment, and laboratory demonstrations due to their high coherence and stability.

He-Ne Laser: Construction, Working, Diagram and Applications

Quick Exam Notes

Type: Gas laser
Active Medium: Helium–Neon gas mixture (ratio 10:1)
Wavelength: 632.8 nm (red region)
Pumping Method: Electrical discharge in gas tube
Applications: Barcode scanners, holography, optical communication, lab demonstrations

Construction of He-Ne Laser

The first gas laser to be operated successfully was the He–Ne laser. The He– Ne laser consists of a long and narrow discharge tube (diameter 2 - 8 mm and length 10–100 cm) which is filled with helium and neon with typical pressures of 1 torr and 0.1 torr (1 torr = 1mm Hg). Figure 1 shows the schematic diagram of a typical He-Ne laser. Electrical discharge mechanism is used to pump atoms from ground state.

He-Ne Laser Construction Diagram with Gas Discharge Tube

Figure 1. Construction of a He-Ne laser

Working principle of He-Ne laser

When an electrical discharge is applied to the gas tube, fast-moving electrons collide with helium and neon atoms.
The helium atoms tend to accumulate at levels F₂ and F₃ due to their long lifetimes of 10⁻⁴ s and 5x10^-6 s, respectively.
Since the levels E₄ and E₆ of neon atoms have almost the same energy as F₂ and F₃ of He atom, excited helium atoms colliding with neon atoms in the ground state can excite the neon atoms to E₄ and E₆.
The pressure of helium is ten times that of neon, the levels E₄ and E₆ of neon are selectively populated as compared to other levels of neon.
Transition between E₆ and E₃ releases coherent photon at 632.8 nm. These photons bounce between mirrors, undergo stimulated emission, and get amplified.The partially reflecting mirror allows a stable red laser beam to emerge as output
Neon atoms de-excite through spontaneous emission from E₃ to E₂.
The atoms from E2 level relax back to the ground level mainly through collisions with the walls of the tube.

He-Ne Laser Energy Level Diagram showing Population Inversion

Figure 2. Energy diagram of He-Ne laser

Applications of He-Ne laser

Barcode scanners
Non-contact measuring and monitoring
Blood analysis
Particle counting and food sorting
Tool alignment

Advantages of He–Ne Laser

Produces a stable, coherent, and monochromatic beam.
Long operational life with continuous output.
Cost-effective and widely available.

Limitations of He–Ne Laser

Low power output compared to solid-state lasers.
Large and bulky setup due to discharge tube.
Operates at limited wavelengths (mainly 632.8 nm).

Summary

A He–Ne laser is a gas laser that uses a helium–neon mixture inside a discharge tube with mirrors at both ends. Excited helium atoms transfer energy to neon atoms, producing population inversion. Stimulated emission in neon releases a stable red beam at 632.8 nm, widely used in education, holography, and communication.

MCQs on He–Ne Laser

The active medium of a He–Ne laser consists of:
- a) Only Neon gas
- b) Only Helium gas
- c) A mixture of Helium and Neon gases
- d) Argon gas
Answer
c) A mixture of Helium and Neon gases
The most common emission wavelength of a He–Ne laser is:
- a) 1064 nm
- b) 694.3 nm
- c) 632.8 nm
- d) 1550 nm
Answer
c) 632.8 nm
In a He–Ne laser, population inversion is achieved by:
- a) Direct excitation of Neon atoms
- b) Optical pumping with flash lamps
- c) Energy transfer from excited Helium atoms to Neon atoms
- d) Spontaneous emission
Answer
c) Energy transfer from excited Helium atoms to Neon atoms
A He–Ne laser is classified as a:
- a) Solid-state laser
- b) Semiconductor laser
- c) Gas laser
- d) Dye laser
Answer
c) Gas laser
A common application of the He–Ne laser is:
- a) Tattoo removal
- b) Barcode scanning and alignment
- c) Optical fiber amplification
- d) Metal cutting
Answer
b) Barcode scanning and alignment

Laser Physics