In principle the frequency of a laser is defined by its own parameters. However in reality the emission frequency f is not stable within a couple of hours. For high precision interferometrical length measurements at least a long term stability of df/f ‰¤ 10-8 must be provided within 8 hours. To obtain such a performance a stabilisation loop must be added to the laser.
Within this set-up the two mode stabilisation -the most commonly used technique- of a HeNe-Laser is applied and demonstrated. The control loop consists of the mode separator with provides the intensity of each mode and a micro processor based PI-controller. The active actuator is formed by a bifilar heater coil surrounding the laser tube. By applying a longitudinal magnetic field along the laser tube the occurrence of a beat frequency is demonstrated caused by the level splitting due to the Zeeman effect. By means of the provided fast photo detector the beat frequency (app. 800 MHz) of the two modes can be displayed by means of an electronic frequency analyser or counter.

Principle of Operation

The radiation of the backside of the laser tube is used to generate the control signal. At the polarizing beam splitter cube PBSC the two orthogonally modes are separated and the intensity measured by PD1 and PD 2. The first amplifier stage generates the difference of both intensity.
The next stage consists of the controller part formed by a proportional (P) and integrating (I) amplifier. Finally the resulting control signal is fed via an power amplifier (PA) to the heater coil which is wrapped around the tube. By changing the temperature the glass tube changes its length according to its thermal expansion coefficient resulting in a change of the length of the optical resonator.
The parameter of all stages can be set by the digital controller either locally via the push buttons or by a remote computer. Therefore this experiment is also addressed to the understanding of PI controller which are commonly used in industry and in Photonic laboratories.