Time-delay signature identification in optical feedback chaotic lasers from power spectrum analysis

Considering that the time delay signature has great threat to the security of chaotic communication, we proposed a novel method to directly identify the time delay signature of chaotic laser using a radio-frequency (RF) spectrum analyzer. Through recording and observing the RF power spectrum, it could be found that the power spectrum was actually modulated by the time delay signature; further performing an inverse Fourier transform to the RF power spectrum of chaotic laser, we could directly unveil the time delay signature. We experimentally studied two kinds of optical feedbacks: one was the single feedback cavity with a cavity length L=11-02 m and the other has two feedback cavities that the two cavity lengths were roughly equal to each other (L1=11-02 m and L2=11-25 m) or one was half of another (L1=11-02 m, L2=22-03 m). The results show that the time delay signatures can not be concealed when the two cavity lengths are roughly equal to each other; when one cavity is an integral multiple of another, the time delay signatures can still be extracted from the RF power spectrum of chaotic laser.