Can Fixed Time Delay Signature be Concealed in Chaotic Semiconductor Laser With Optical Feedback?

External-cavity lasers are usually used for chaos encryption in optical chaos-based communication systems. The external-cavity round-trip time (the time delay in the laser dynamics) is often regarded as an additional key to encode messages, which is a critical security parameter. The feasibility of identifying the time delay has been a crucial issue in chaotic optical communication. Some researchers propose that the time delay can be hidden by modulating the value of feedback strength or increasing the number of feedback cavities. In this paper, we experimentally and numerically demonstrate that the time delay signatures cannot be concealed in optical feedback semiconductor lasers. Whether single or double optical feedback, the time delay signatures can all be identified by the power spectrum analysis method. Furthermore, adjusting the feedback strength, the pumping current and the time-delay value, we find that the extraction of the time delay signatures still cannot be influenced.