Talha's Physics Academy
Construction and Working of Helium-Neon (He-Ne) Laser
Video Lecture: Helium-Neon (He-Ne) Laser
Watch the complete lecture detailing the historical background, components, and resonant energy transfer mechanism of the He-Ne laser:
Helium-Neon (He-Ne) Laser: Introduction
“The He-Ne laser was the first continuous wave (CW) laser ever constructed, built in 1961 by Ali Javan, W. R. Bennett, and D. R. Herriott at Bell Telephone Laboratories.”
Unlike pulsed lasers, the He-Ne laser emits a continuous beam of light. It is commonly utilized in school physics laboratories and older barcode scanners.
Construction of He-Ne Laser
The helium-neon laser consists of three essential components:
- Gain Medium: A gas discharge glass tube containing a low-pressure mixture of helium and neon in a ratio typically between $5:1$ and $20:1$. The partial pressure of helium is maintained at approximately $1\text{ mbar}$, whereas that of neon is $0.1\text{ mbar}$. Helium acts primarily as an energy transfer medium, while neon serves as the active laser medium.
- Pump Source: Electrical discharge powered by a high voltage (several hundred volts up to $10\text{ kV}$) applied across an anode and a cathode situated at opposite ends of the glass tube. A steady operating current of $5\text{ to }100\text{ mA}$ is typical.
- Resonating Cavity: The gas-filled discharge tube is positioned between two parallel mirrors. The left-side mirror is partially silvered and acts as the output coupler (allowing a fraction of light to escape as the laser beam), while the right-side mirror is fully silvered and functions as the high reflector.
Working and Laser Operation
- Electrical Discharge & Electron Collision: When switched on, a high voltage is applied across the gas mixture, accelerating electrons between the electrodes through the discharge tube. These energetic electrons collide with helium atoms, transferring energy and promoting helium electrons into excited metastable states ($F_3$ and $F_5$).
- Resonant Energy Transfer: Because helium metastable states cannot easily return directly to the ground state via spontaneous emission, they collide with neon atoms. Excitation levels in neon atoms closely match the metastable energy levels of helium ($F_3 = E_3$ and $F_5 = E_5$).
- Population Inversion in Neon: Energy transfer from excited helium atoms pumps the neon atoms into higher excited/metastable states ($E_3$ and $E_5$), while the helium atoms drop back to the ground state. This efficient collisional transfer helps achieve the necessary population inversion in neon.
- Laser Emission: As a result of spontaneous and stimulated emissions between these neon energy levels, a continuous monochromatic beam of red light with a wavelength of **$632.82\text{ nm}$** is emitted and amplified inside the optical cavity.
Fig: Energy transfer and laser action mechanism in a Helium-Neon laser system.
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