stabilized Nd:YAG based two-wavelength-two-frequency-lasers for refraction corrected length measurement

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For many interferometric measuring tasks, a compensation of the refractive index of air is either not sufficiently possible at present or would require a disproportionately high amount of technical equipment. In recent years, the two wavelength method has been in the centre of intensive investigations. However, it has not been possible so far to make it applicable because of the unavailability of a suitable laser source.This application aims to eliminate this deficit by combining various physical approaches for generating two wavelengths of two frequencies each, using a Nd:YAG laser thus allowing good stabilization in an easy way. So far, the photoelastic effect occurring in Nd:YAG crystals has been examined intensively for the application as force sensor with a linearity over 12 decades.Conversely, another main focus of this application is to provide a constant beat frequency of the orthogonally polarized laser modes resulting from the application of a piezoelectric force. Thus, in the second step, these two modes can be used for two mode stabilization in this case on an unsaturated absorption peak of a gas absorption cell as it has been the case with HeNe lasers for a long time already. This kind of frequency stabilization promises clearly better relative and absolute stabilities (goal: better 10-9) than the side of fringe technology applied so far. Also, the approach proposed is considerably simpler than stabilization methods on saturated absorption lines.Here, the necessary decoupling of the mechanically and optically coupled closed loop control system from the beat and absolute frequency regulation is a task representing a particularly high scientific challenge. The force controlled beat frequency, which is the prerequisite for the 2 mode stabilization, can now be used in the interferometer directly for the heterodyne technique. The frequency doubling by means of KTP crystal, as it has been used with the Nd:YAG lasers for a long time already, finally results in the stabilized wavelength at 532 nm (which is necessary for the two wavelength method), synchronously to the 1064 nm, with the beat frequency being regulated automatically, too. The goal of this project is to create the scientific technological bases for realizing low cost stabilized two wavelength two frequency lasers, which can well be used for industrial measuring tasks, presenting an absolute frequency uncertainty of 10-9. These lasers should potentially be suitable for a variety of applications of refractive index-compensated interferometric length measurement, in particular for suppressing the effect of air turbulences occurring in long baseline interferometry.