# Chaotic Optical Communication with Wavelength-Hopping Technology Based on Tunable Lasers
## 1 Introduction  
Fiber optic communication has previously been considered a kind of communication technology with high security. The epoch of absolute security in optical fiber communication continued until 2005 when researchers discovered the occurrence of eavesdropping on submarine cables. Therefore, a significant amount of research has recently focused on how to further improve the security of the optical fiber communication system. Chaotic communication has developed as a kind of encryption technology based on physical layers. The message in this type of communication system is added to the chaotic signal in the emitter and recovered by a receiver, which is synchronized with the emitter after transmission [1]. Due to the high bandwidth of laser chaos, researchers mostly pay attention to the field of chaotic laser secure communication [2; 3; 4; 5; 6], and in recent years, more new methods have been introduced to improve the performance of chaotic laser secure communication [7; 8; 9]. There is also a significant amount of research on optoelectronic oscillators [10; 11] and chaos generation [12], which is helpful for designing the chaotic sources and then realizing the chaotic laser communication system.  
Meanwhile, the confidentiality problem of chaotic laser communication systems has also been proposed by some scholars [13; 14]. Attempts or experiments to recover the delay times [15] and messages [16] have been conducted, which have shown that it is necessary to enhance the security of chaotic laser secure communication. Additionally, to prevent the recovery of information, some schemes to further enhance the confidentiality of the chaotic laser communication systems have been proposed, such as subcarrier modulation [17] and double masking [18], and to prevent delay times, concealment technology of time delay has become a popular research topic in recent years [19; 20; 21]. However, the above systems are complex and limited in application.  
In this paper, by introducing the method of wavelength-hopping communication into chaotic laser communication, we propose a structure of a wavelength-hopping chaotic laser communication system based on tunable lasers. Due to the wavelength hopping and the pseudo-randomness of the wavelength-hopping sequence, it is difficult for the eavesdropper to intercept the information and predict to which wavelength the hopping station will hop. This will increase the demodulation mismatch of eavesdroppers and further increase the difficulty of obtaining the transmitting information. It also means that the introduction of wavelength-hopping technology will make the key to a chaotic communication system a dynamic variety process, while the traditional chaotic communication system is a static process. Therefore, it makes the key to a wavelength-hopping chaotic communication system much more complex than in a traditional system. Based on the wavelength-hopping chaotic laser communication system, the numerical results indicate that this structure can increase the bit error rate (BER) of the eavesdropper by five orders of magnitude under the same conditions, which greatly improves the difficulty of message decoding and improves the confidentiality of the system.