## 1 Introduction  
With the rapid development of digitization and computing power in information society, communication security is facing imminent threat. The majority of previously announced security enhancement strategies use algorithmic encryption, most commonly the advanced encryption standard (AES) [1, 2, 3, 4], while algorithm-level encryption may be intercepted and cracked as a result of the advent of quantum computing [5]. The chaotic system has the characteristics of broad bandwidth, good randomness and high sensitivity [6, 7, 8, 9, 10]. At present, chaotic secure optical communication at the physical layer has become an effective strategy to protect the security of large-scale data exchange in modern networks [11].  
Chaotic signals are generated by analog and digital methods. The former can generate highly complex nonlinear dynamic broadband signals [12]. In the field of optical communication, most chaotic signals in the analog domain are generated by chaotic lasers,which are large in volume, high in cost and complex in structure. At the same time, these schemes pose higher requirements on chaotic synchronization, which is very important for a secure communication system based on a chaotic system [13]. The latter will pay for computational time when obtaining a more complex nonlinear dynamic behavior [14; 15; 16]. When the chaotic signal is used as the carrier for chaotic secure communication, its spectral efficiency is low, and when the transmission rate is low, the information is easy to be intercepted [17]. On the other hand, some scholars also adopt chaotic signal to drive phase modulator (PM) and scramble private chaotic phase [18]. The process should convert the complicated nonlinear electro-optical signal and be injected into PM, which makes the optical layer encryption possibly. It can also encrypt the whole network data of low delay and high speed [19]. However, the chaotic laser produces complexly and carries out nonlinear electro-optical signal converting. The variety of optical damage will inevitably cause signal distortion [20]. It also significantly increases the cost, and the application of the scheme is impeded by these problems.  
In this paper, we propose an analog-digital combined high-secure optical communication system. The scheme adopts a 3D chaotic system, and \(\mathbf{x}\) is the injected signal of the PM to carry out phase disturbance in the analog domain. With a chaotic circuit instead of chaotic laser as a driving signal to be injected into the PM, the complicated nonlinear electro-optical signal conversion problem of existing schemes is solved. This scheme reduces the cost and optimizes the size. The \(\mathbf{y}\) and \(\mathbf{z}\) are used for the time-frequency domain encryption in the digital domain. Moreover, we demonstrate a 13.3 Gb/s encrypted signal transmission over 25 km standard single-mode fiber (SSMF). If the chaotic driving circuit produces a delay of 3 s, the bit error rate (BER) reaches more than 0.3 at the receiver. The results show that the analog-digital combined encrypted scheme based on the chaotic circuit as driving signal has a good performance in optical communication system and is compatible with the existing optical networks.