Home Laser Metrology LM-0200 Laser Frequency Stabilisation

Keywords:

  • Two Mode HeNe-Laser
  • Doppler Gain Profile
  • Frequency Pulling
  • Mode Separation
  • PID - Controller
  • Longitudinal Zeeman Effect

Basic experiment

Intended institutions and users:

Physics Laboratory

Engineering department

Electronic department

Biophotonics department

Physics education in Medicine

 

Introduction

How it works ...

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LM-0200 Laser Frequency Stabilisation

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 two mode 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 two modes can oscillate. The control loop consists of the mode separator with provides the intensity of each mode and a micro processor based PID-controller. The active actuator is formed by a bifilar heater coil surrounding the laser tube. 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 spectrum analyser or counter.

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 mode drift of the free running laser.

By applying a magnetic field to the Laser tube the occurrence of a beat frequency is demonstrated caused by the level splitting due to the Zeeman effect.

LM-0200 Laser Frequency Stabilisation

 

 

LM-0200 Laser Frequency Stabilisation

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