LM-0120 Laser interferometer technical extension
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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
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.
Datasheet
https://luhs.de/assets/lm-0120.pdf
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.
Datasheet
https://luhs.de/assets/lm-0120.pdf