Available Photonics Experiments:

P5856 Laser frequency stabilisation
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  • Laser cavity
  • Doppler broadened gain profile
  • Two mode HeNe-laser
  • Frequency pulling and pushing
  • Mode separation
  • Digital - controller
  • Zeeman effect
  • Examples of investigations and measurements
Principle of operation
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.
OX General set-up
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.

Aperiodic limit

Almost perfect parameters

  • P5856 Laser frequency stabilisation consisting of:
18X-XP111Manual Laser frequency stabilisation
2ED-00341HeNe Laser high voltage supply 6.5 mA
3ED-01201Laser frequency stabilizer LSF-01
4ED-02001Photodetector Si PIN
5MC-02601Optical rail MG 65, 500 mm
6PM-03701Mode separator and photodiode
7PM-03801Two mode HeNe-laser
8TP-02101PC Pad
9XM-01201Rotary polarisation analyzer on carrier 20 mm
10XM-10901Mounting plate C25 with carrier 20 mm
 TP-01001Oscilloscope 100 MHz digital, two channel