The use of autonomous underwater vehicles (AUVs), to potentially carry out underwater exploration missions, is limited due to insufficient onboard battery and data storage capacity. To overcome this problem, underwater docking stations are used to provide the facility of underwater charging and data transfer for AUVs. These docking stations are designed to be installed in the dynamic ocean environment, where the turbidity and low-light conditions are key challenges to hinder the successful docking operation. The vision guidance algorithms based on active or passive markers are typically used to precisely guide the AUV towards the docking station. In this paper, we propose a vision-based guidance method, using lock-in detection, to mitigate the effect of turbidity, and to reject the unwanted light sources or noisy luminaries, simultaneously. The lock-in detection method locks on the blinking frequency of light beacons located at the docking station and successfully vanishes the effect of unwanted light at other frequencies. The proposed method uses two light beacons, emitting at a fixed frequency, installed at the simulated docking station and a single CMOS camera. Proof-of-the-concept experiments are performed to show the validity of the proposed approach. The obtained results show that our method is capable of recognizing the light beacons at different turbidity levels, and it can efficiently reject the unwanted light without using separate image processing for this step of the vision-based guidance algorithm. The effectiveness of the proposed method is validated by calculating the true positive rate of the detection method at each turbidity level.
A simple and low cost, integrated, fully passive optical network of free-space optics (FSO) and indoor visible light communication (VLC) technique, which can be used for both solid-state lighting and last-mile access network, is proposed in this paper. As a proof of a concept, a non-return-to-zero on-off keying (NRZ-OOK) modulation scheme for transmission over an integrated fully passive optical link of 7-m FSO, 2-m plastic optical fiber and 30-cm VLC was demonstrated by achieving 1.4-Gbps data rate with a bit error rate of 2.6 ×10−3. The phosphor film diverges the blue laser beam to a white light spot covering a radian angle up to approximately 120° with Commission Internationale de l'Eclairage CIE of (0.3439, 0.3541), which is very close to the perfect white area of CIE 1931 chromaticity coordinates (0.3333, 0.3333). Additionally, the generated white-light exhibits low correlated color temperature (CCT) of 5056 K and a high color rendering index (CRI) of 91. The proposed technique will find a wide range of applications in integrated, fully passive optical networks of free-space optics and indoor visible light communication.
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