Four modes of optical parametric operation for squeezed state generation U. L. Andersen, B. C. Buchler, P. K. Lam, J. W. Wu, J. R. Gao and H.-A. Bachor Eur. Phys. J. D, 27 2 (2003) 181-191 Published online: 26 August 2003 DOI: 10.1140/epjd/e2003-00263-9