Home Laser Metrology LM-0140 CNC Calibration Extension

Keywords:

• Definition of Length
• HeNe-Laser
• Two Beam Interference
• Homodyne Interferometer
• Fringe Detection and Counting
• Interpolation of Interference Fringes
• Computer controlled stepper motor
• Calibration of Translation Stage

Basic experiment

Intended institutions and users:

Physics Laboratory

Engineering department

Electronic department

Biophotonics department

Physics education in Medicine

Introduction

How it works ...

LM-0140 CNC Calibration Extension

Measuring a length is the comparison of an unknown length with a known one. Since 1983 the standard of one meter is defined as the length of the path travelled by light in vacuum during a time interval of 1/299792458 of a second. The effect of this definition is to fix the speed of light (c) in vacuum at exactly 299 792 458 m/s. If we consider the relation ν=c/λ where ν denotes the frequency and λ the wavelength of the light radiation it becomes clear that in case the frequency of the radiation is known the wavelength λ is known as well. If the used light source has a known and constant frequency it represents a secondary standard of the meter. Preferentially a laser can fulfil the demand of a defined and stable frequency. In practise HeNe-Laser systems are used which frequency is stabilised using optical transitions of the Iodine 127 isotope. The uncertainty of the frequency stabilisation by using this method is better than 1x10E-12. For technical applications like calibrating CNC machines an uncertainty of 1x10E-7 is sufficient. This value corresponds to an accuracy of 0.1 µm per one meter. A HeNe-Laser without any frequency stabilisation means has a fairly good uncertainty of 1x10E-6 and will be used in this experiment. By using the Michelson interferometer we count how many λ/2 bright/dark transition  (fringe) occur along the distance to be measured. The movement will be done by using a computer controlled motorised translation stage with built-in incremental encoder. The travelled distance is compared with the result of  the Michelson interferometer which represents the standard of the meter.