Upgrade to Technical
Interferometer II

Measuring 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 a vacuum at exactly 299 792 458 m/s. If we consider the relation: where ν denotes the frequency and l the wavelength of light radiation, it becomes clear that in case the frequency of the radiation is known the wavelength λ is defined. With this knowledge we can rewrite the definition of one meter as: If the used light source has a known and constant frequency, the above equation can be simplified to: where k is a constant telling us how many wavelength fit into one meter. Preferentially a laser can fulfil the demand of a defined and stable frequency. In practice HeNe-laser systems are used with a stabilised frequency using optical transitions of the Iodine 127 isotope. The uncertainty of the frequency stabilisation by using this method is better than 10-12. For technical applications, like calibrating CNC machines, an uncertainty of 10-7 is sufficient. This value corresponds to an accuracy of 0.1 µm per one meter. A HeNe-laser without any frequency stabilisation means that it has an uncertainty of 10-6 and will be used in this experiment. By using the Michelson interferometer we count how much a half wave-length (fringe) is repeated along the distance to be measured. Let N be the number of counted wavelength, then the relation between the unknown distance L or length and the definition of one meter will be: The movement will be done using a computer controlled motorised translation stage with a built-in incremental encoder. The travelled distance is compared with the result of the Michelson interferometer which forms the standard.
 

Principle of operation

In this set-up the modules U (measuring gauge 5 mm/ 1 µm) and F of the Technical Interferometer up-grade are substituted by the module M1. This module consists of a high precision translation stage driven by a DC motor and a position encoder. The triple reflector Tr2 is mounted on top of the moving part of the stage. The optical arrangement is the same as for the Technical Interferometer. The incoming laser beam is separated into two orthogonally polarised beams by means of the polarising beam splitter (PBS). For the measurement of the position accuracy of module M1 the triple reflector Tr1 stays at a constant position. Both beams are combined again in PBS and deflected to the fringe detection unit where the necessary optical sin, -sin, cos and -cos signals are detected by the photo detectors D1 through D4. These signals are used to determine the travel of Tr2 with an accuracy of 10-6 and a resolution of 0.04 µm. The travel of Tr2 is controlled by the DC motor controller board which is built into a PC. A travel command of e.g. 10.000 mm is launched by the controller. The DC motor starts turning the spindle of the stage. The movement is finished when the number of signals detected by the encoder reaches the desired value. During the movement of the stage, the PC also gathers the interferometer signal giving the exact position of it. A plot of the entire travel range using different kinds of step wise positioning of the stage gives the position accuracy of the translation stage.