## 1 Introduction  
Free space optical (FSO) communications were developed and researched extensively as a potential technology for sixth generation (6G) wireless communication networks [1]. Different from radio frequency (RF) communications, the main feature of FSO communications is to use the laser beam to carry information, which brings various benefits, including high transmission rate, free of eavesdroppers by transmission directivity, and free of frequency spectrum crowding [2]. However, there are two problems that affect the performance of FSO communications. The first one is that FSO communication requires line-of-sight (LOS) links between the light source (LS) and the photodetector (PD); however, in practical circumstances, the connection between LS and PD could be blocked by obstacles [3]. The second problem is the susceptibility to the varying propagation environment caused by rain, fog, and other low-visibility atmospheric phenomena [4]. With the intention of dealing with the non-LOS requirement and maximizing the throughput, the authors in [5] utilized a fixed position relay. Usually, this type of relay is deployed on a building to assist non-LOS transmission. Following that, inter-relay cooperation was proposed in FSO communication to reduce the outage probability in [6]. However, the relay is fixed in a position, so that the communication between the transmitter and receiver lacks flexible connectivity capability. To tackle this issue, a UAV was introduced as an operating platform to assist communication and improve system performance flexibility and efficiency. For example, in [7], the authors considered a UAV decode-and-forward relay in a downlink maritime communication scenario, solving the average achievable rate maximization problem. Furthermore, the authors in [8] proposed a multi-UAV assisted solution to achieve the capacity maximization criterion assuming a cognitive network. Motivated by using a UAV in RF communications, there are existing works that consider a UAV relay for improving FSO communication. For example, in [9], the authors introduced a dual-hop space-air-ground integrated RF/FSO network; with the assistance of a UAV, the satellite could link the terrestrial users with low outage probability. In [10], a similar scenario was proposed for the multi-users on the ground using quadrature amplitude modulation. Furthermore the UAV facilitating the link between space and air, in [11], was utilized to assist the FSO communication between ground non-LOS transmitter and receiver deployed in an urban area. Therefore, the deployment of the UAVs for FSO communication is a valid solution for high data rate purposes, especially under complex circumstances. However, the power consumption and additional signal processing are significant problems for a UAV equipped-relay mobile platform [12], which is a limitation for performance and UAV operating endurance. For the purpose of reducing power consumption and processing time delay, reflecting intelligent surface (RIS) technology was widely utilized in wireless networks. Thanks to the feature of RIS that can control wireless signal transmission through passive phase adjustment with low cost [13]. In [14], the feasibility was described for RIS application on new generation RF communication, where the channel has been turned into a controllable system that can apply the optimization. Furthermore, in paper [15], the phase shifts of RIS can be designed and improved by the joint transmit beamforming method. Not only in RF communication but the work [16] also indicated that RIS and its feature of freedom rotations could be utilized in FSO communication. What is more, the work in [17] showed that RIS could be regarded as a mirror with a phase shift function in FSO communication, which means eliminating the time delay and power consumption caused by signal processing. The works in [16; 17] showed the fixed position mounted RIS could solve the LOS requirement of FSO communication. However, the flexibility and capability coverage for fixed mounted RIS is limited.