In principle the frequency of a laser is defined by its own intrinsic parameters. However in reality the emission frequency f is not stable within a couple of hours. For high precision interferometric 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 setup the Zeeman stabilisation - the most commonly used technique - of a HeNe-Laser is applied and demonstrated. The length of the HeNe laser tube is chosen in such a way that only a single mode can oscillate.
A longitudinal magnetic field is applied to the HeNe tube and the normal linearly polarised splits into two oppositely circular polarised modes due to the Zeeman effect. One can observe the difference or beat frequency with a photodetector behind a polarizer. The beat frequency becomes minimum, when the HeNe laser tube (cavity) is aligned to the centre of the gain profile. The control loop consists of the beat frequency detection and an embedded micro processor based PID - controller. The active actuator is formed by a bifilar heater coil surrounding the laser tube. The task of the students is to understand the stabilisation concept and the underlying control technique of a PID controller. The PID parameter can be set independently from each other and the student will recognize the influence of this parameter on the control loop. The provided software records and displays the controller as well as laser response allowing to record a Bode diagram or the beat frequency drift of the free running laser.
LM-0200 Laser Frequency Stabilisation
The length of the laser tube is designed in such a way, that only one longitudinal mode oscillates. Applying a longitudinal magnetic filed causes at first the splitting of all the atomic energy level, in particular also the level of the Ne - laser transition. This causes the emission of two orthogonally circular polarized modes with a certain beat frequency. By means of the photodetector (PD1) behind the under 45° oriented polarizer (P1) this beat frequency is detected and displayed on an oscilloscope for instance. The microprocessor records the drift of the beat frequency and determines the minimum value which is related to the target of the control circuit. In some further steps the microprocessor learns if it needs to heat or cool to achieve the right control direction. Once these parameters are settled and the initial thermal drift of the tube slowed down, the controller starts the active control.