Four modes of optical parametric operation for squeezed state generationU. L. Andersen, B. C. Buchler, P. K. Lam, J. W. Wu, J. R. Gao and H.-A. BachorEur. Phys. J. D, 27 2 (2003) 181-191DOI: https://doi.org/10.1140/epjd/e2003-00263-9