Acknowledgements

This work was supported by the Fonds zur Förderung der wissenschaftlichen Forschung, project No. P6880. We thank M. Tacke (Fraunhoferinstitut für Angewandte Messtechnik, Freiburg, FRG) for supplying the laser diodes and G. Bauer (Johannes Kepler Universität Linz, Austria) for many helpful discussions.

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