### Related Works  
Hybrid FSO/RF parallel systems can be divided into two categories: mode switching systems and parallel transmission systems. In a hybrid FSO/RF system with mode switching, the RF link acts as a backup connection. The system switches to the RF link only when the instantaneous SNR of the primary FSO link is lower than a predefined threshold [22; 23; 24]. However, this mode switching system largely depends on the availability of feedback information or channel state information (CSI) on the system transceiver, which increases the system hardware complexity. [PERSON] et al. [25] derived the closed-form expressions of the OP, symbol error rate (SER), and ECC of the hybrid FSO/RF system with hard switching mode.  
On the other hand, in a hybrid FSO/RF system with parallel transmission, the same data will be transmitted on two links at the same time. In addition, the received signals will be processed by diversity combining at the system receiving end before signal demodulation, such as maximum ratio combining (MRC), equivalent gain combining (EGC), selective combining (SC), and adaptive combining (AC) [26; 27; 28]. Therefore, this scheme does not require feedback information or CSI to realize the switching operation between two links. Compared with the mode switching system, the parallel transmission system has the advantages of simplicity and economy. [PERSON] et al. [29] derived the closed-form expressions of the OP and BER of the hybrid FSO/RF system with SC and pointing errors (PE). [PERSON] et al. [30] approximately analyzed the BER performance of hybrid FSO/RF systems with MRC and SC schemes.  
In the recent years, as an effective scheme against eavesdropping in the wireless communication system, physical layer security has been introduced into the RF or FSOcommunication links. This strategy makes use of the inherent random characteristics of the wireless channel to improve system security [31]. [PERSON] et al. [32] studied the security performance of the FSO links subject to Gamma-Gamma distribution and deduced the closed expressions of the lower bounds of strict positive security capacity (SPSC) and security outage probability (SOP).  
At present, research on physical layer security mainly focuses on hybrid FSO/RF systems in the open literature, while research on physical layer security of hybrid FSO/RF communication systems is rare. [PERSON] et al. [33] deduced the new closed-form expressions of average security capacity (ASC), SOP, and SPSC of mixed dual-hop RF/FSO system when the relay scheme is based on an AF relay and RF link eavesdropping. [PERSON] et al. [34] studied the average security rate (ASR) and SOP performance of the mixed FSO/RF dual-hop communication system during RF link eavesdropping under fixed gain and variable gain relay schemes. [PERSON] et al. [35] derived and analyzed the expressions of the OP, outage capacity (OC) and SOP of the hybrid FSO/RF system with modified switching. [PERSON] et al. [36] derived and analyzed the expressions of the SOP, SPSC, and ASC of the hybrid FSO/RF system with selective combining under RF colluding and non-colluding eavesdropping scenarios.  
However, current research on the physical layer security of the hybrid FSO/RF system mainly considers RF eavesdropping, while research on the physical layer security of the hybrid system with FSO link eavesdropping is rare. [PERSON] et al. [37] derived and analyzed the expressions of the SPSC of the hybrid FSO/RF system with selective combining under three different types of eavesdroppers. However, the impact of the FSO link pointing error on security performance is not taken into account.