Absolute distance interferometry using variable synthetic wavelength

Since many years there have been attempts to replace conventional laser interferometers by absolutely measuring systems. To remove the ambiguities in interferometrical length measurement in general, the application of several different laser wavelengths is offered. With the development of tunable lasers those methods become important which use a variable wavelength. They me an alternative to the well-known multiwavelength techniques. During the laser tuning the total change of the phase of the interferometric signal is measured, which depends of the reflector distance. However, alterations of the phase are also caused by changes of the refractive index of the optical medium and by vibrations of the retroreflector. These disturbances can be avoided if a variable synthetic wavelength (vsw) is generated by two laser beams instead of using the short wavelength of light. Therefore the vsw-interferometer, developed on this basis, supplies good results also under industrial conditions.