Chaos secure communication offering high-level security and high transmission rates has been rapidly developed in relation to optical fiber links [[1], [2], [3], [4], [5], [6]], where the key distribution rate is from 0.75 Gb/s [3] to 2.55 Gb/s [4], the encryption date rate reaches 60 Gb/s [5] and achieves 100 Gb/s [6]. In free space optical (FSO) communication, chaotic secure communication data rate began at a few Kbit/s and has approached Gb/s using the generation of broadband chaotic carrier for conventional communication wavelengths [7,8]. Considering atmospheric transmission characteristics and private communication in free space, the focus is on mid-infrared chaotic lasers, because in the 3–5 μm band of the atmospheric transmission window has good atmospheric transmission characteristics, lower transmission losses than other bands, and is less susceptible to weather factors such as the presence of rain, dust, fog, and haze [9,10].

Interband cascade lasers (ICL) and quantum cascade lasers (QCL) are typical mid-infrared laser devices, which has been used to realize of order hundred Mb/s and Gb/s data rate in FSO communication [[11], [12], [13], [14]]. Based on the atmospheric transmission characteristics of mid-infrared laser, rapid development of mid-infrared laser devices, and demand of high-speed communication, research into mid-infrared chaotic secure communication has occurred. QCL or ICL with external optical feedback, optical injection, and bias modulation have been investigated in parallel both experimentally and theoretically in order to obtain mid-infrared chaotic lasers [[15], [16], [17], [18]]. To date, a 0.5 Mbit/s transmission data rate has been achieved by unidirectionally coupled QCLs with external optical feedback operating in the chaotic regime, where the message is added to the bias current of the transmission QCL [19]. It is noted that the absence of high-frequency oscillations chaos limits the data rate. Fully-developed hyperchaos has been observed experimentally in ICL subjects to external optical feedback, where the electrical power spectrum exhibits a frequency span as broad as 2.0 GHz [20]. Our previous theoretical research confirmed that ICL with external optical feedback, under certain conditions, can generate several GHz broadband chaos [21]. This opens a route towards high speed FSO secure communication.

In this paper, we demonstrate theoretically a Gb/s secure communication system based on chaos synchronization in ICLs. The bandwidth of the high-dimensional chaos is several GHz and synchronization correlation is above 0.9. A transmission rate at 4 Gb/s is achieved with an BER compatible with that of regular telecommunication systems.