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HNLS008R(L) Red HeNe Laser System Chapter 2: Description
Rev C, March 23, 2015 Page 8
2.1.3. HeNe Laser Overview
A helium-neon laser, typically called a HeNe laser, is a small
gas laser with many industrial and scientific uses. The
primary wavelength they are used at is 632.8 nm, in the red
portion of the visible spectrum.
The gain medium of the laser is a mixture of helium and neon
gases in a 5:1 to 20:1 ratio that is contained at low pressure
in a sealed glass tube. The excitation source for these lasers
is a high-voltage electrical discharge through an anode and
cathode at each end of the glass tube. The optical cavity of
the laser consists of a flat, high-reflecting mirror at one end of
the laser tube and an output coupler mirror with
approximately 1% transmission at the other end.
Optical Resonator Cavity
Figure 2
Typical HeNe Parameters
Beam Diameter (D)
≤ 1 mm (1/e^2)
Cavity Length (L)
0.15 m (0.5 mW) to 1 m (50 mW)
Reflectivity at High Reflector (HR)
>99.99%
Transmission at Output Coupler (OC)
~1%
HeNe lasers tend to be small, with cavity lengths of around 15 cm up to 0.5 m and optical output powers ranging
from 1 mW to 100 mW. Thorlabs offers output powers up to 22.5 mW.
2.1.4. HeNe Linewidth
A red HeNe laser wavelength is 632.816 nm in air, though it is often reported as either 632 nm or 633 nm. The
wavelength gain curve is actually made of several longitudinal modes that fluctuate within the range due to
thermal expansion of the cavity and other external factors.
The linewidth of a HeNe laser is specific to the application. The axial mode structure of the HeNe laser is
characterized by the number of modes, the free spectral range (FSR), and the Doppler width (see Figure 3 on
page 9). The linewidth of individual axial modes is usually small (~kHz) and is primarily determined by external
factors and measurement timescales rather than fundamental laser parameters. In most interferometric
applications, the most relevant parameter is the coherence length, which is determined by the axial modes that
are furthest apart. For a red HeNe laser, the coherence length is approximately 30 cm.
D
L
Anode
High
Reflector
Output
Coupler
Helium-Neon Gas Mixture
Glass Tube Laser Bore Tube
Cathode
HeNe Applications

Metrology

Cleanroom Monitoring Equipment

Food Sorting

Flow Cytometry

Confocal Microscopy

Imaging and Medical Equipment

Opacity Monitoring

Alignment

Maritime Visual Guidance Systems