All-Digital Device-Compatible Real-Time Physical Layer Secure Transmission Based on Delta-Sigma Modulation

A novel physical layer digital encryption optical transmission method based on delta-sigma modulation is proposed and implemented using field programmable gate arrays (FPGA) for real-time transceivers. This approach converts ultra-high-order ciphertext, encrypted by a first-layer pseudorandom matrix, into 2-level signals via delta-sigma modulator (DSM), enabling all-digital transceiver operation with commercial optical transmission network. By employing a dynamically perturbed parameter matrix, the quantization noise introduced by the DSM is randomized, achieving a secondary masking of the ultra-high-order ciphertext. This method shares conceptual security principles with quantum noise stream cipher (QNSC) but focuses on enhancing device compatibility for physical layer secure optical transmission systems and removes the necessity for additional clock links often required by QNSC and most digital domain secure transmission technologies. In the experiment, the proposed system demonstrated bidirectional quantization noise masking physical layer secure transmission on a coarse wavelength division multiplexing (CWDM) grid, achieving a plaintext capacity of 8 × 10 Gb/s. Subsequently, real-time long-haul transmission was achieved using commercial coherent optical terminal unit (OTU4E) modules. In a 1000 km link, a 96-grid system covering the entire C-band was employed, with 9 grids measured at equal intervals across the band. Each measured grid occupied 50 GHz frequency range and achieved a plaintext transmission rate of 24.88 Gbps. This demonstrates that a potential capacity-distance product of 2.4 Pb/s·km can be achieved.