THORLABS HNL100R User Manual page 11

Red HeNe Laser Systems
FSR ~1000 MHz
Short (0.2 m) Laser
The laser process in a HeNe laser starts with the collision of electrons from the electrical discharge with the
helium atoms in the gas. This excites helium from the ground state to a long-lived, metastable excited state.
Collision of excited helium atoms with ground-state neon atoms results in excited neon electrons.
The number of neon atoms entering the excited states builds up until population inversion is achieved.
Spontaneous and stimulated emission between the states results in emission of 632.82 nm light along with other
emission wavelengths. From these states, the electrons quickly decay to the ground state. The HeNe laser's
power output is limited because the neon upper level saturates with higher current, while the lower level varies
linearly with current.
The laser cavity can be designed with the correct mirrors and length to promote other wavelengths of laser
emission. There are infrared transitions at 3.39 μm and 1.15 μm wavelengths, and a variety of visible transitions,
Page 8
FWHM ~1.5 GHz
Figure 2 HeNe Gain Curve Showing Cavity Modes
Helium
21
Metastable Collision
1S
(20.61 eV)
Metastable Collision
20
3S
(19.78 eV)
19
18
17
16
Ground States
Figure 3
Neon Gain
Curve
Lasing
Cavity Modes
Frequency
3S
2
(20.66 eV)
2S
2
(19.78 eV)
(1152.3 nm Laser)
2P
e Impact
1S
Diffusion
HeNe Energy Levels
Chapter 3: Description
Lasing
Threshold
FSR ~200 MHz
Long (1 m) Laser
Neon
(3391.2 nm Laser)
3P
4
(20.30 eV)
(632.8 nm Laser)
1
4
(18.70 eV)
10
2
(16.70 eV)
5
22172-D02