Red HeNe Laser System
12112-S01 Rev CB, August 31, 2010
Page 16
www.thorlabs.com
While a HeNe laser is less sensitive to variations caused by back reflections, they are not immune to it.
Large retro-reflections back into the laser can cause unpredictable power changes. A free-space isolator
can be used to reduce or eliminate these effects. If possible, only a small angular deviation is needed to
eliminate the back reflections.
These HeNe lasers are ill-suited to any application or experiment where single frequency or long
coherence length is required.
Noise, Oscillations, and Fluctuation Sources
There are many sources of noise, oscillations and fluctuations of the output of a HeNe laser. One of the
most common is from longitudinal modes drifting into and out of the gain curve. The frequency of these
fluctuations vary from very slow, less than a hertz, to as much as several kHz depending on the stability
of the environment.
Other, more constant sources of fluctuations include the following (with corresponding frequencies):
•
Line noise from AC high voltage power supplies (50/60 Hz, 100/120 Hz, etc)
•
Switching noise from DC high voltage power supplies (40 kHz)
•
Plasma oscillation (tens of MHz)
•
Transverse mode beat notes (hundreds of MHz)
•
Axial mode beats (at FSR, hundreds of MHz)