This paper presents a novel method of hidden data transmission in the physical layer of the optical networks. It is based on direct spread spectrum technique, which is a simplistic and robust approach. There has been a description provided of a concept in which public signal sequences and noise in optical networks were used to hide information. A number of experiments have been conducted according to this work, namely verification of the impact of spreading code lengths on system performance (stealth signal throughput possible to achieve, influence on the public channel);moreover, optical power level dependencies (the stealth versus public channel power levels) were investigated. Finally, there was also a concept presented of multi-code transmission for data rete improvement. Performed experiments proved the feasibility and proper operation of the proposed system and show its potential performance. The maximum achieved hidden data transfer rate was about 20 kbit/s at 25 km fiber length. Copyright © 2015 John Wiley & Sons, Ltd.