Figure 10

Illustration of a typical QPSK data modulation encrypted with \(\hat{d}(\alpha ) = \hat{d}(\alpha _{1})\hat{d}(\alpha _{2})\) with \(\hat{d}(\alpha _{1})\) applying a 16-QAM encryption and \(\hat{d}(\alpha _{2})\) applying a random phase shift operator for back-to-back without fibre for the four graphs on the left hand side and for 80 km fiber for the four graphs on the right hand side. (a) is the constellation diagram with \(\hat{d}(\alpha )\) encryption and then direction coherent detection without decryption using \(\hat{d}(-\alpha )\), and (b) denotes the constellation diagram after \(\hat{d}(-\alpha )\) decryption from (a). (c) is a constellation diagram applying DSP directly to (a). (d) is the constellation diagram applying DSP to (b). (A), (B), (C), (D) are the corresponding constellation diagrams at 80 km, respectively