Home Laser Metrology LM-0120 Laser interferometer technical extension

Keywords:

  • Definition of Length
  • HeNe-Laser
  • Two Beam Interference
  • Homodyne Interferometer
  • Fringe Detection and Counting
  • Interpolation of Interference Fringes
  • Calibration of Micrometre Gauge

Basic experiment

Intended institutions and users:

Physics Laboratory

Engineering department

Electronic department

Biophotonics department

Physics education in Medicine

 

Introduction

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LM-0120 Laser interferometer technical extension

One essential element of a technical laser interferometer is the secure detection of the bright / dark transitions (fringes) even for variable contrast conditions which may establish due to modifications of the initial adjustment during the displacement of the measuring reflector. To compensate for these influences a fringe signal C phase shifted by 180° with respect to fringe signal A is optically produced. By means of an subsequent electronic comparator disturbing DC-offset parts are eliminated from the resulting signal. To detect the direction of the displacement of the measuring reflector a signal B, phase shifted by 90° with respect to signal A, is created optically. Furthermore such a 90° phase shifted signal D with respect to signal C is created. Also signal B and D are treated with a comparator to remove any offset. As a result two offset free fringe signals are created having a phase shift of 90° to each other allowing the directional discrimination by applying the quadrature encoder principle.

Another important difference to the classical Michelson interferometer lies in the fact that instead of mirrors triple reflectors are used in such a way that no beam travels back into the laser source. Such back reflections lead to frequency and thus intensity fluctuation of the laser source falsifying the counted number of fringes.

This extension provides all necessary optical components to turn the existing Michelson interferometer to a technical one. One triple reflector is mounted onto a translation stage. The piston pin of the provided micrometre gauge firmly touches the back of the moveable triple reflector. The idea of the measurement is to calibrate the display of the gauge against the wavelength of the laser source which forms the secondary standard of a metre. The fringes generated by the movement are counted by the provided forward backward counter in fractions of the wavelength λ like λ/4, λ/8 or even λ/16.

LM-0120 Laser interferometer technical extension

 

 

LM-0120 Laser interferometer technical extension

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