text: Doppler broadened gain profile
  Frequency pulling and pushing
  Digital - controller
  The two longitudinal modes emitted by a Helium Neon laser are orthogonally polarised to each other. Their individual intensity is measured by using a polarising beam splitter cube (PBSC) as mode separator and a photodiode for each mode. If the intensity of both modes equals, they are symmetrically centred to the centre frequency of the laser transition. By controlling the temperature of the laser tube the intensity of both modes is kept constant resulting in a fixed frequency of the modes. A PI controller with variable coefficients is used for the stabilisation.
  A helium Neon laser tube (6) is covered by a acrylic housing for security reasons. The beam leaves on the right side and passes a polariser (8) before it hits the photodetector(3). On the left side of the tube a small fraction of the main laser intensity leaves the tube which is used for the stabilisation. The tube is supplied with the necessary high voltage of the unit (1). The interface (2) provides the preamplifier for the photo diodes as well as the control amplifier for the bifilar heater winding. The pad computer (7) is connected to the interface (2) with a USB bus. The provided software running on the pad computer allows the configuration of the parameter of the control loop and monitoring of the stabilisation.
  Operate and optimise a PI Controller
  Each engineer or physicist will be faced with control tasks. This experiment therefore addresses also the operation of a proportional and integral controller. The software based controller allows the configuration of all relevant parameters. The reaction of the system is monitored and recorded so that the cases of the aperiodic limit, oscillating and stable state by modifying the control parameter can be observed. The monitor signal is the difference in intensity of both modes and the set point of the controller is zero.