Time-delay Signature And Its Identification In Optical Feedback Chaotic Semiconductor Lasers

Optical chaos has attracted considerable attention in recent years due to its great applications in various fields, such as chaos-based communication encryption systems, chaotic lidars, chaotic optical time-domain reflectometry, and fast random bit sequences generators (RNGs). Through perturbing a semiconductor laser (SL) with optical injection, optical feedback, or optoelectronic feedback, the laser can be driven into broadband chaotic output. Among the mentioned methods, the semiconductor laser with optical feedback (SL-OFB) may be the most common applied chaotic sources due to its unique virtues, such as simple configuration, small size, feasible controllability, and rich chaotic dynamics.Nevertheless, the output of SL-OFB usually has periodic intensity fluctuations. These fluctuations are nearly identically repeated at the photon round-trip time in the external cavity, and are widely called the time delay (TD) signature. The TD signature seriously affects the performance of chaotic system in some applications. For example, the TD signature provides the clue to break the chaos-based secure communication and can deteriorate the randomness of RNGs based on SL-OFB. Much effort has been focused on to overcome these problems. Typical scenarios are to generate high-dimensional chaos, to modulate the delay time, and to use multiple feedback. However, the mechanisms of time-delay signature suppression in these methods are not clear and there are some disagreements whether they can successfully conceal or suppress time-delay signature.In this paper, we experimentally and theoretically analyzed the concealment of time-delay signature in many kinds of optical feedback semiconductor lasers and obtain some useful results. The main works are summarized as follows:1. Summarize all methods reported for extracting time-delay signature and point out the limitations of these methods; Introduce and describe Wiener-Khintchine theorem which is the basis of the following researches.2. Propose a new method to extract the time-delay signature of chaotic semiconductor laser, named as spectrum analyzing method. This method uses the rf spectrum of chaotic signal as analytic object which are recorded using a spectrum analyzer, then make a Inverse Fourier Transform to the obtained rf spectrum, and finally can acquire the time-delay signature of chaotic semiconductor laser. Further, utilizing this method, we experimentally and theoretically analyzed the time-delay signatures of single optical feedback and double optical feedback semiconductor lasers. Our researches find that:i) in the case of single feedback, the time-delay signature can be successfully obtained with our method, no matter how the feedback strength changes, ii) in the case of double feedback, when the length of one external feedback cavity are a integer of the other, our method is still valid but other methods fails.3. Utilizing the spectrum analyzing method, we further study the effect of the feedback light polarization on the extraction of time-delay signature. Firstly, we establish a theoretical model of optical feedback semiconductor lasers with tunable polarization state. Secondly, we analyze the effect of the parameters such as feedback strength, polarization angle and bias current on time delay signatures of chaotic lasers. Finally, we conclude that the time delay signature can be concealed by adjusting the polarization of feedback light, when the feedback strength and bias current are set properly.