## 1 Introduction  
Electromagnetic waves in the MHz band or higher experience significant attenuation underwater due to the conductivity of water, and the attenuation rate generally increases with frequency under identical environmental conditions [1, 2]. As a result, electromagnetic wave-based communication commercially available overland cannot be applied under the water, and acoustic waves with water as a medium or visible light from a light source are instead used for underwater wireless communication [3]. Acoustic wave communication provides a long communication distance but is limited in high-speed communication since the amount of data transmission is small [4]. Underwater wireless optical communication (UWOC) provides a shorter communication distance than acoustic wave communication but allows high-speed communication [5]. Despite signal attenuation and delay caused by variations in salinity, temperature, and bubble presence and by light scattering and absorption, UWOC delivers propagation speeds higher than underwater acoustic communication [6]. Unmanned underwater vehicles (UUVs) are one of the important applications of underwater communication. Underwater communication is essential for sending scan information obtained underwater or the condition of the UUV to other UUVs or to the ground. Also, high-speed communication is required to process underwater images for real-time inspection of marine structures or piers in port facilities [7]. So far, cables have been used for the communication of UUVs such as remotely operated vehicles (ROVs). These cables can break or bend with structures, reefs, rocks, and floating objects, causing damage to the ROVs or restricting their activity [8; 9]. Therefore, high-speed wireless communication without cables is necessary to achieve the smooth operation of ROVs. Underwater acoustic communication has been commonly used so far for underwater communication but is unsuitable as a real-time video communication medium due to the low data rate of 100-5000 bps. To solve the delayed data rate of underwater acoustic communication, numerous studies on UWOC technology have been performed [7]. In the literature, a detailed overview of various research activities on UWOC was presented [10]. Extensive research was carried out to develop complementary technology, which can allow broadband underwater communications such as real-time video transmissions, teleoperation of autonomous underwater vehicles (AUVs), and remote monitoring of underwater stations [11]. Also, in a recent study on practical research, researchers provided a review of practical considerations and solutions in UWOCs. However, most of these studies focus on increasing the underwater optical wireless communication speed and its verification in an indoor water tank [12]. There were some studies on UWOC between AUVs and underwater modems or AUVs and the surface vehicle. In 2023, [PERSON] et al. demonstrated a UWOC system that achieved a communication distance exceeding 35 m at a data rate of 1.9 Mbps [13]. By [PERSON] et al., a cooperative control framework based on potential field theory and a hybrid (optical/acoustic) data transfer policy based on [PERSON]'s analytic hierarchy process (AHP) algorithm was studied [14]. However, in this paper, the validity of the proposed algorithm was presented through simulation instead of experiments. A compact, low power consumption, cost-efficient, and lightweight optical modem designed for fast data transmission between MEDUSA at ranges on the order of 10 m was presented [15]. The experiments have shown the capability of the modem to transmit data at 20 kb/s, over a distance of 10 m in water with 4-5 m visibility. A bi-directional UWOC system, the Woods Hole Oceanographic Institution (WHOI) optical modem, was deployed on a seafloor test node and on the AUV Sentry. A complete optical field map of the signal from the seafloor node was generated with an autonomous data mule mission.