Resource-constrained devices in security-sensitive applications require security techniques to ensure cyber-resilience, data privacy and integrity, and trust in open networks. To effectively address the above requirement, a physical layer security technique based on chaotic digital filters (CDFs), which have chaotic amplitude and phase frequency responses, is proposed and experimentally demonstrated, for the first time, in an intensity modulation and direct detection intensity (IMDD) 12Gb/s@25km standard single-mode-fiber (SSMF) optical link. The CDF-based technique provides signal protection by directly distorting signals and inducing interferences between data signals. The low-complexity and cost-efficient CDF-based security technique offers salient features of ‘security-by-design’, ‘openness-by-design’ and ‘dynamic security at the traffic level’, which are of great importance for applications in heterogeneous access networks.