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European Conference on Optical Communication

Secure FSO Transmission System Based on Y-00 Protocol Using Optical Decryption Incorporated into Coherent Receiver

Ken Tanizawa, Fumio Futami

Published Sep 28, 2025• 0 citations
Publisher

Abstract

We demonstrate an intradyne coherent receiver with LO phase modulation in a free-space-optics PSK Y-OO cipher system with key-less deliberate signal randomization. The optical decryption achieves a small Q penalty while maintaining high security against signal interception, facilitating the migration to secure coherent FSO communications. ©2025 The Author(s)

DOI
10.1109/ecoc66593.2025.11263034

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IET conference proceedings.•Nov 13, 2023•2 citations
We demonstrate an intradyne coherent receiver with LO phase modulation in a free-space-optics PSK Y-OO cipher system with key-less deliberate signal randomization. The optical decryption achieves a small Q penalty while maintaining high security against signal interception, facilitating the migration to secure coherent FSO communications. ©2025 The Author(s)
The Y-00 quantum stream cipher utilizes the effect of quantum noise on optical signals and realizes signal-level symmetric-key encryption to protect interception in optical communication. A transceiver architecture that disaggregates encryption/decryption and modulation/demodulation in the optical domain facilitates the migration of Y-00 cipher to current digital-coherent communication systems. This paper presents a polarization-diversity intradyne Y-00 coherent receiver where local oscillator light is modulated for decryption. Polarization-diversity optical decryption and detection are simultaneously achieved using a typical optical 90-degree hybrid circuit and a conventional polarization-sensitive electro-optic modulator. Penalty-less detection with the decryption is demonstrated in 5-Gbit/s PSK Y-00 cipher transmission over fiber and free-space links, facilitating the disaggregation of decryption and demodulation.
We present a symmetric-key direct data encryption technique utilizing signal masking by quantum noise, called Y-00 quantum stream cipher. Physical encryption of phase-shift-keying (PSK) Y-00 quantum stream cipher is achieved by our proposed extremely high-order phase modulation. Two cascaded phase modulators are driven with two synchronized digital-to-analog converters for coarse-to-fine modulations, resulting in a record 217 phase levels for the quantum noise masking. Decryption of the cipher is implemented in digital signal processing after intra-dyne coherent detection with a free-running local oscillator. We experimentally demonstrate transmission of 10-Gbaud digital-coherent PSK Y-00 cipher with 217 levels over a 240-km field-installed single-mode fiber.