Quantum Noise-Assisted Coherent Radio-over-Fiber Cipher System for Secure Optical Fronthaul and Microwave Wireless Links

This paper reports on a symmetric-key direct-data encryption technique that directly protects the interception of optical and microwave signals from physical layer in a coherent analog radio-over-fiber (RoF) system. Secrecy is realized by signal masking by quantum (shot) noise. The quantum noise masking for the encryption is achieved by converting data into extremely high-order signals at an optical frequency. After signal transmission over an optical fronthaul link, the frequency of the encrypted signal is shifted to a microwave frequency via an optical heterodyne process using a local oscillator for wireless transmission. The effect of the quantum noise masking is naturally and seamlessly kept in the heterodyne process. We experimentally demonstrate 12 Gbit/s coherent RoF cipher systems utilizing the quantum noise masking for a 30 GHz wireless band. Adequate signal quality and high security against interception with sufficient quantum noise masking are simultaneously achieved in the optical fronthaul and microwave wireless links.