To improve the physical-layer data confidentiality in short-reach optical networks, we have proposed an M-ary blockciphered system using an optical code multiplexing approach. In a previous system, each optical code (OC), that corresponds to a frequency subcarrier, was mapped onto one of the log2M bits, and the computational security is related to the correspondence between the bit-block and the OC, and the number of possible combinations equates M!. However, this system presents two critical issues (1) Computational Security: the number M of OCs that can be generated by an optical arrayed waveguide grating (AWG) is limited, due to the port number and the subcarrier crosstalk. (2) Physical security: a careful differential analysis of the corresponding time waveform, optical power, and/or optical spectrum can be used to identify the symbol pattern. To mitigate the effects of interchannel interference, we propose a new differential phase-shift keying (DPSK)-based multi-dimensional M-ary block ciphering system, that assigns binary phase difference patterns to adjacent symbols and demonstrate a 16-dimensional 216-ary ciphered system. In addition, to increase the M-ary number, without increasing the number of OCs, we consider also a differential quadrature phase-shift keying (DQPSK)-based multi-dimensional M-ary block ciphered system and demonstrate a 16-dimensional 232-ary ciphered system.
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