鉴于时延信息严重威胁着混沌通信的保密性，提出了一种利用频谱仪直接确定激光器时延信息的新方法。通过记录和观察信号的频谱，发现其频谱受到了时延信息的调制；进一步对频谱作逆傅里叶变换，即可直接获取光反馈混沌激光器的时延信息。实验中对腔长L=11.02 m的单反馈和腔长近似相等(L1=11.02 m, L2=11.25 m)或腔长成整数比(L1=11.02 m, L2=22.03 m) 的双反馈情形进行了研究。分析结果表明：在两外腔长度近似相等的情况下，2个外腔长度信息不能被隐藏；当反馈外腔长度成整数比时可以从频谱中提取出半导体激光器的反馈延迟时间。

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.