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Noise Level Modulation (NLM) is a robust physical-layer security technique which uses the injection of random phase noise into the transmitted signal to provide confidentiality in optical networks. The legitimate receiver can seamlessly recover the information by utilizing a feedback loop, while eavesdropping attempts fail, irrespective of the attacker’s location or computational capabilities. This paper outlines the theoretical and implementation principles of NLM when applied to full fiber-optic networks. Our findings reveal that NLM can attain a complete secrecy rate while ensuring compatibility with existing optical devices and protocols. We also propose a practical fiber optic-based implementation scheme, providing a thorough analysis of its variations from the theoretical framework. Furthermore, we delve into the challenges associated with NLM in the context of secure full optical communication systems and explore potential future directions.