He-Ne laser
1. Overview — Most Common Gas Laser (CW)
He-Ne laser = four-level, gas, continuous wave (CW) laser with active medium Neon (Ne) lasing, Helium (He) as sensitizer/energy transfer. Main wavelength 632.8 nm (red), also IR 1.15 µm and 3.39 µm.
Invented 1961 (Javan), low power ~1–50 mW, highly coherent.
Long narrow glass discharge tube (20–50 cm, bore 2–5 mm) with Ne:He ≈1:10 at ~1 torr, Brewster windows at ends (to get polarized output), mirrors M1 (HR 100%) and M2 (T≈1%) outside tube forming cavity, high-voltage DC discharge (~1–2 kV) with ballast.
Brewster windows oriented at θ_B≈56° to transmit p-polarization without loss.
So pumping is electrical discharge + He→Ne collisional transfer, not optical.
4. Operation (CW)
Discharge continuously populates He*, which pumps Ne* upper; lower Ne levels empty quickly via spontaneous + wall collisions ⇒ steady inversion ⇒ gain > loss ⇒ CW oscillation between mirrors; output through M2 is collimated, polarized (Brewster), TEM00 mode.
Need narrow bore to enhance wall collisions for depopulating 1s.
5. Characteristics
- Wavelengths: 632.8 nm dominant (with mirrors coated for that), linewidth ~1 GHz (Doppler), coherence length ~30 cm (multimode) up to km with etalon.
- Low gain (~3% per metre) but easy CW; efficiency ~0.1%.
- Beam: Gaussian, divergence ~1 mrad, polarization linear (Brewster).
6. Applications
Lab alignment, interferometry (Michelson), barcode scanners (old), holography, metrology, education.
7. Exam Points
- Construction diagram with discharge tube and Brewster windows — 7 marks, very frequent.
- Four-level scheme + He→Ne energy transfer — 7-10 marks, almost certain.
- Why CW and low threshold (four-level) vs Ruby — 5 marks, very frequent.
- Wavelengths and wall de-excitation role — 3-5 marks.