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Abstract
The growing need for ocean monitoring and exploration has boosted underwater wireless optical communication (UWOC) technology. To solve the challenges of pointing, acquisition, and tracking (PAT) in UWOC technology, herein, we propose a 450-nm-laser/scintillating-fiber-based full-duplex (FD)-UWOC system for omnidirectional signal detection in real scenarios. The FD-UWOC system has a −3 dB bandwidth of 67 MHz with a low self-interference level of −44.59 dB. It can achieve a 250-Mbit/s data rate with on–off keying modulation scheme. The system’s robustness was validated by operating over 1.5-m underwater channel with air-bubble-, temperature-, salinity-, turbidity-, and mobility-induced turbulence with a low outage probability. Under air-bubble-induced turbulence, the highest outage probability was 28%. With temperature-, salinity-, and turbidity-induced turbulence, the system performed adequately, showing a highest outage probability of 0%, 3%, and 4%, respectively. In mobile cases, the highest outage probability of the FD-UWOC system was 14%, compared to an outage probability of 100% without utilizing the fluorescent optical antenna. To further validate its robustness, a deployment test was conducted in an outdoor diving pool. The system achieved a 250-Mbit/s data rate over a 7.5-m working distance in the stationary case and a 1-m working range in the mobile case with a 0% outage probability. The scintillating-fiber-based detector can be employed in UWOC systems and would help relieve PAT issues.
© 2021 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
The marine economy has become a vital part of the world and an essential carrier for economic growth owing to the shortage of terrestrial resources. The demands on remote underwater observation, ocean sensing technologies, and underwater wireless communication (UWC) are growing as humanity enters the marine era [1]. Establishing real-time, large-scale, and low-cost underwater sensor networks can boost remote underwater observation technologies. The Internet of underwater things has been proposed to interconnect underwater sensors, ships, underwater vehicles, and relays [2]. It is crucial for data exchange between various underwater vehicles and sensors for oceanic seismic monitoring, seafloor survey, pollution control, etc. In the last decade, UWC has moved from sonar technology to more advanced underwater wireless optical communication (UWOC) technology, which employs light as a signal carrier in the UWC link. UWOC offers wide bandwidth, high transmission speed, and low latency characteristics; thus, it is superior to underwater acoustic and radiofrequency (RF) communication [3,4].
However, complex underwater channel conditions hinder the development of UWOC [5]. The working distance of a UWOC system is limited by the attenuation, which is the sum effects of absorption, scattering, and various underwater turbulence [6]. Absorption and scattering effects depend on the frequency of light and water properties, such as salinity and particle suspension [7]. Underwater turbulence affects the optical beam propagation path by changing the refractive index (RI) of water due to the random variations in the presence of air bubbles, temperature, salinity, etc. [8–12]. The turbulence results in beam distortion and then degrades the performance of the UWOC system. Hence, a UWOC system has strict requirements for positioning, aligning, and tracking (PAT) to maintain steady performance and increase its robustness [13].
An increasing number of studies have been conducted on UWOC systems owing to the rapidly growing demand for ocean monitoring and exploration. Extensive research has been carried out in this area, but most designs are half-duplex or even one-way communication systems [14–17]. Hence, researchers step forwards to FD-UWOC systems and improve the throughputs gain, power efficiency, and data rates. Works listed in Table 1 present the typical work of FD-UWOC systems. Since 2010, Doniec et al. built a light-emitting diode (LED)-based full-duplex (FD)-UWOC system named AquaOptical II, which can achieve the data rate of 2.28 Mbit/s over 50-m working range by using discrete pulse interval modulation (DPIM) [18]. In 2018, Wu et al. from Dalian University of Technology, established a laser diode (LD)-based FD-UWOC system that can work over 16-m with the data rate of 100 Mbit/s [19]. While Fasham and Dunn from BlueComm commercialize the FD-UWOC module, which can transmit the data at 10 Mbit/s over 150-m channel in 2019 [20], Xu et al., from Zhejiang University, proposed a 100-m FD-UWOC system with the data rate of 200 Mbit/s in 2020 [21]. Apart from studying and improving modulation, data rate, working distance of the FD-UWOC system, numerous factors, such as absorptions, scattering, and turbulence, greatly affect the optical communication range and performance of FD-UWOC systems [22]. Previously mentioned works overlooked the practical considerations of underwater effects such as turbulence. These issues are still open for research.
Table 1. Review of FD-UWOC systems.
Herein, we propose a scintillating-fiber-based FD-UWOC system to solve the PAT issue and achieve simultaneous data transmission and reception between sensor nodes and underwater vehicles. Each communication transceiver is packed together with the 450-nm laser transmitter and an array of scintillating fibers coupled to an avalanche photodetector (APD) in the underwater capsule. The large-area and large-field of view (FOV) scintillating fiber serves as an omnidirectional detector by absorbing an incident optical radiation and re-emitting it at a longer wavelength. The re-emitted photons from the fluorescent dyes are then guided to the end of the fibers by total internal reflection (TIR) [23]. The coupled APD collects the guided optical waves to achieve high-data-rate communication and simultaneously relieve the requirement of PAT. The large-area and wide FOV fluorescent receivers increase the probability of receiving signals, thus, increase the system’s robustness over combating the temporal random variations of beam positions due to the turbulence effects [24]. Kang et al. used a fiber-based detector to achieve a 1.15-m ultraviolet-UWOC system at a data rate of 250 Mbps with OOK modulation [23], and Sait et al. demonstrated a practical wavelength-division multiplexing method to achieve an aggregated data rate of 1 Gbit/s using two scintillating fibers, emitting at different wavelengths with OOK modulation scheme [25]. Our system was tested under different types of underwater turbulence, including air-bubble-, temperature-, salinity-, and mobility-induced turbulence in the laboratory tests. Compared with other FD-UWOC systems, the proposed system shows superiority in robustness under turbulence and relieves the strict requirement on the PAT system. The deployment test of the system proved that this large-area high-bandwidth FD-UWOC system could reliably work in practical scenarios when used in conjunction with a 450-nm laser transmitter.
2. Full-duplex communication system and characterization
2.1 Design of full-duplex underwater wireless optical communication systems
The proposed FD-UWOC system mainly relies on the large-active-area scintillating fiber as a photodetector to relieve the strict requirement on the PAT system. The system contains high-power blue lasers, large-area optical fiber-based detectors, avalanche photodetectors, arbitrary waveform generator (AWG), oscilloscope, and a system controller. Figure 1 shows a schematic of the system.
Using the MATLAB platform, we generate the “call” signals and upload them to an AWG (Tektronix AWG70002A) using SourceXpress software from Tektronix (connecting laptop through LAN cable). After being transmitted through a 25-dB amplifier (AMP, Mini-Circuits ZHL-6A+), the modulated signal is combined with the bias current through a bias-Tee to the blue laser (SaNoor Technologies, 450 nm, 1.6 W) of transceiver 1. The bias current of the blue laser was set to 500 mA, giving an optical power of 291.7 mW in the experiments. The directly modulated optical signals strike the color-converting scintillating fibers (Saint-Gobain, BCF-92) after propagating through different channels. The fibers convert the received 450-nm light to 492-nm light and guide it due to TIR to the Si variable-gain APD (Thorlabs, APD430A2/M) of transceiver 2. Finally, after being amplified by the same 25-dB amplifier, the output signals are captured by a mixed domain oscilloscope (MDO, Tektronix MDO3104). The captured signals are uploaded to a laptop using the Test & Measurement Tool in MATLAB and demodulated using the MATLAB program. Then, the program codes the “reply” signal to AWG and sends the modulated signal to the blue laser of transceiver 2. The “reply” signal propagates through the same channel to be detected by transceiver 1, captured by the oscilloscope, and demodulated by MATLAB. Signal transmission and reception between two transceivers can be achieved simultaneously.
2.2 UWOC system characterization
The light–current–voltage (L–I–V) characteristic of the SaNoor blue laser was measured, and the results are shown in Fig. 2(a). The threshold current and voltage were approximately 290 mA and 3.91 V, respectively. The frequency response of the system was measured using a vector network analyzer (VNA). The –3 dB bandwidth of two transceivers were up to 67.61 and 67.58 MHz, shown in Fig, 2(b) and 2(c), respectively. Based on a previous study [3], on–off keying (OOK) and pulse position modulation (PPM) schemes are suitable for dynamic underwater channels and mobile underwater communication cases in comparison with OFDM. Thus, OOK and 4-PPM signals were tested using the scintillating-fiber-based FD-UWOC system.
Fig. 2. (a) Light–current–voltage (L–I–V) of the blue laser (b) Frequency response of transceiver 1. (c) Frequency response of transceiver 2.
2.3 Fiber structure optimization
First, we studied the variation of the received optical power density with the number of fiber pieces. The 450-nm SaNoor Laser was driven by 500 mA, and the output optical power reached 291.7 mW. The impinging beam’s power on the fiber with a diameter of 0.3 mm hitting was attenuated to 51.3 mW after propagating through a 1.5-m water tank with 30 L of pure water type I, obtained from a water deionizer (MilliQ Academic). As the optical beam hitting on 1, 3, 6, 12, and 24 pieces of 1-m long fiber, the received optical power densities were 70.5, 150.4, 170.3, 375.2, and 361.7 µW/cm2 (shown in Fig. 3). There was a nonlinear relationship between the fiber pieces and the power output from the fiber detector due to the power saturation of the narrow beam and the small illumination area on the large numerical aperture of the fiber detector.
Twelve pieces of fiber were tested in an indoor experiment as they resulted in the highest coupling efficiency. Furthermore, the power loss with fiber length was studied. Twelve fibers were bundled with different lengths (1 m and 2 m) and tested following the same procedure. The results are shown in Table 2. There is insignificant power degradation as fiber length increased by 1 m which could be caused by the internal loss of light due to reabsorption by the fluorophore molecules and the reflection efficiency core-cladding interface [26].
Table 2. Guided optical power output with different lengths of fibers.
Besides power loss, various fiber detector structures were explored to examine their effect on communication performance. We tested 1, 3, 6, 12, and 24 pieces of fiber as single straight and circular layers, 24 pieces of double layers, and cross layers (12 fibers for each layer) in the designed UWOC system. The fiber structures are shown in Fig. 4. We tested the fiber-based UWOC system with OOK and PPM to find the best-suited modulation scheme for our system. The selected results are shown in Table 3. All structures could achieve maximum data rates of 250 Mbit/s with OOK and 125 Mbit/s with PPM modulation (Eye diagram of OOK and PPM are shown in Fig. 5). Since BER values of all tested structures are 0, we further increase the channel attenuation by adding the neutral-density (ND) filter before the fiber. Table 3 presents the highest optical density of ND filter that each structure can add before the BER reaches the forward error correction limit.
Fig. 4. (a)-(e) Different fiber structures and (f)-(g) illuminating fibers when a 450-nm laser beam hits the circular layer and cross layers.
Fig. 5. (a) Eye diagram of an OOK signal at 250 Mbit/s; (b) Eye diagram of a 4-PPM signal at 125 Mbit/s.
Table 3. Performance of various fiber structures in UWOC system.
Because the bandwidth efficiency of OOK is double the bandwidth of 4-PPM [27], the maximum achievable data rate of OOK in our system is double that of PPM. Because of the higher data rate achieved by OOK than PPM, OOK was adopted as the modulation scheme for communications in subsequent experiments.
2.4 Self-interference characterization
An FD system allows simultaneous communication between two systems, but the key deterrence in implementing the FD transceiver is the large self-interference [28]. Self-interference is the interference caused to the transceiver system by its transmitted signal. RF communication is usually of several orders of magnitude higher than the intended signal because the latter crosses a much longer distance than the former [29]. Different ways of canceling self-interference have been explored [28,30]. In underwater optical communication, self-interference mainly comes from diffused light from the LED, scattering light in the ultraviolet (UV) region when using UV light source, or backscattering during propagation and reflections from the interface. In our setup, self-interference primarily comes from the backscattering of light propagation in water and light reflections from the air–acrylic and acrylic–water interfaces introduced by the capsule on which the UWOC system is mounted. Figure 6(a) shows the self-interference source of a line-of-sight FD-UWOC system, indicating the desired (blue beam) and self-interference (red) paths. Figure 6(b) presents the actual transceiver in the capsule, and the self-interference is measured with shown structure. The measured FOV of the shown structure is 133.6 degrees.
Fig. 6. (a) Self-interference of a full duplex-UWOC system; (b) Transceiver in a capsule with a laser placed 3 cm away from the fibers.
The background noise of the system was measured using a vector network analyzer (VNA, Agilent, E8361C) pre-calibrated before the measurements using an electronic-calibration module (Agilent, 85093-60010). Transceivers 1 and 2 were solely turned on alternately without sending a modulated signal, and the background noise of the transceivers was measured. Through the S21 measurement from the VNA, the average background noise was at a level of −48.32 dB and lower (Fig. 7(a)). Then, the S21 measurement was repeated with the scan signal from 10 to 100 MHz. When the laser beam propagation path was next to the fibers, self-interference was −19.94 dB, which is larger than noise. We measured the beam path away from the fibers 0–6 cm, and self-interference ranged from −19.94 to −45.25 dB (Fig. 8). In the indoor and outdoor tests, the laser beam path was 3 cm away from the fibers (Fig. 7(b)). The self-interference was −44.59 dB. There was almost no self-interference, and the acceptable self-interference for the FD-UWOC system during the S21 measurement from the real signal was at −14.32 dB, which is more than 36 times larger than the self-interference level. The signal is significant compared to the noise when the laser was 3 cm away from the fibers. Thus, the self-interference is negligible.
Fig. 7. (a) Noise of the FD-UWOC system (laser off); (b) Self-interference when the laser is 3 cm away from the fibers.
3. Experimental setup and results
3.1 Effects of air bubble-induced turbulence on fiber-based UWOC system
An underwater channel is not calm and tranquil in a real scenario. The complex underwater environment introduces turbulence to the underwater optical channel and degrades the system’s performance. To better manifest the robustness improvement of the proposed system, we use the same APD without scintillating fiber as the benchmark system to compare the performances under the same turbulence. Dynamic air bubbles could be generated by the propellers of underwater vehicles, which degrade the performance of optical communication systems. It is vital to study and evaluate fiber-based FD-UWOC system’s performance under air bubble-induced turbulence. Herein, air bubbles were generated by blowing nitrogen gas into a tube and placing it underneath the two laser beam propagation paths to create turbulence by interfering with the beam. The airflow speed was controlled and recorded at two rates: 0.44 mL/s and 0.89 mL/s. The scintillation index (SI) coefficients, $\sigma _I^2$ [Eq. (1)], defined as the normalized variance of the received signal intensity fluctuations at the detector [8].
where I is denoted as the intensity of the received signal from the APD, E[I] is the expected value of the received intensity. According to the study of reciprocal channel turbulence effects [8], the underwater optical channels with opposite propagation directions should have similar SI when they are under the same turbulence. In other words, the FD channels exhibit the same turbulence effect. The averaged SI results of the dual channels under different generated air bubble speeds are shown in Fig. 9(a)–9(c) to characterize the turbulence-caused signal fluctuations of the FD-UWOC system. The beam illuminates the cross-layer fiber, shown in Fig. 9(d), which shows the impact of turbulence on beam attenuation and distortion. The outage probability of the FD-UWOC system during the communication under turbulence was also measured. Outage probability is the probability of a point at which the receiver power value falls below the threshold (where the power value relates to the minimum signal-to-noise ratio (SNR)) [31,32]: where SNRinst is the instantaneous measured SNR, and SNRthr is the threshold SNR when the system’s BER is at 3.8×10−3. In our case, the SNRthr is 1.05. Outage probability reflects the probability of communication loss within a specified period. Herein, the outage probability is calculated by the average value of the outage probability of 100 measurements under the same conditions. Sending every single group of signal takes 400 µs, and sending 100 groups of data takes 40 ms. Each occurred outage to any of the channels of the FD-UWOC system would be counted as an outage of the whole system. The SNR and BER of each measurement were measured to calculate outage probability. The performance of the UWOC system under air bubble turbulence is shown in Table 4, and the eye diagrams are shown in Fig. 10. With different strengths of air bubble turbulence, the system could receive the signals successfully with a low outage probability compared to the benchmark system, indicating the robustness of the scintillating fiber detector in the UWOC system.Fig. 9. (a) Average scintillation index (SI) of two channels without bubble turbulence; (b) average SI under a low bubble generation rate; (c) average SI under a high bubble generation rate; (d) Fiber illumination when a laser beam hits on them.
Table 4. Performance of scintillating-fiber based FD-UWOC system under bubble turbulence.
3.2 Effects of temperature-induced turbulence on fiber-based UWOC system
Temperature variations change the RI of water. In real life, inhomogeneous thermohaline distribution in seawater differs with depth and creates an inhomogeneous RI distribution of water [33]. Thus, the variation in RI changes the beam propagation direction and introduces alignment issues in UWOC systems. To emulate the temperature gradient and tides mixing with different temperatures [34], we created temperature differences of 2°C and 4°C by heating pure water type I from one side of the tank and separating the sides using the transparent acrylic board (Fig. 11(a)). After testing the UWOC system with a constant temperature gradient, we removed the acrylic board to let the water at different temperatures to mix, and we measured the average SI of two channels (Fig. 12), BER, and outage probability of the FD-UWOC system. Then, water at different temperatures mixed with each other when the acrylic board is removed, the RI of the water changes. The inhomogeneous RI distribution of water distorted the beam shape, and the propagation path changed. The laser beam moved out of the view of Thorlabs CMOS camera (Fig. 11(b)). However, the fiber-based detector still managed to capture the deflected beam, and the system could work with a 0% outage probability. The results are shown in Table 5. The fiber-based FD-UWOC system worked perfectly and maintained 0% outage probability in the presence of a temperature difference over an underwater channel. The benchmark system can only retain 0% outage probability when there is no temperature mixing process. Otherwise, outage probability degrades to 100%. The results manifest the resilience of scintillating fiber over temperature-induced turbulence.
Fig. 11. (a) Schematic of the temperature gradient in the tank generated with an acrylic board separation. (d) Fiber illumination upon laser beam illumination.
Fig. 12. Averaged scintillation index (SI) of two channels under 2 and 4 degrees Celsius temperature difference with (w) and without (w/o) temperature mixing process.
Table 5. Performance of scintillating-fiber based FD-UWOC system under temperature-induced turbulence.
3.3 Effects of salinity-induced turbulence on fiber-based UWOC system
The salinity of the Red Sea ranges from 35 part-per-thousand (ppt) to 40 ppt [35], and the gradient of the salinity is approximately 0.2 ppt/km [33]. The higher salinity of water results in higher attenuation of light in water. To emulate the underwater channel in the Red Sea, we tested the system under the salinity of 35 and 40 ppt by adding 1050 and 1200 g of salt into 30 L of pure water type I. Salty water was injected using a pump, and the solution was stirred until the salt was completely dissolved. Figure 13(a) shows the laser beam propagating through the salted water. Figure 13(b) shows the fiber illumination when the laser beam illuminates it and reveals the scattering effects on the beam due to salinity. The SI of the two cases is shown in Fig. 14, and the results are listed in Table 6. At 35 and 40 ppt salinity, the FD-UWOC system could work with low outage probabilities of 2% and 3%, respectively. The proposed system performed robustly under salinity-induced turbulence. The benchmark system shows low outage probabilities due to the delicate alignment of the high-power laser to secure the UWOC system's performance.
Fig. 13. (a) Laser beam (450 nm) passing through a 40-ppt salinity underwater channel and (b) hits on fibers.
Fig. 14. The averaged scintillation index (SI) of two channels under (a) 35- and (b) 40-ppt salinity turbulence.
Table 6. Performance of scintillating-fiber based FD-UWOC system under salinity-induced turbulence.
3.4 Effects of turbidity turbulence on fiber-based UWOC system
To verify the robustness of the system over turbidity, we added different volumes of Maalox, a commercial antacid preparation, which contains l(OH)3 and Mg(OH)2, into pure water. Dissolving Maalox in pure water can create particle suspensions and changes the attenuation coefficient, c, of water. To emulate pure seawater, coastal ocean water, and turbid harbor water, 26, 179, and 1316 µL of Maalox were added to 30 L of pure water type I, resulting in the total attenuation coefficients of 0.043, 0.298, and 2.19 m-1, respectively. The volume of Maalox added was calculated based on a previous study [36]. The communication performances of the FD-UWOC system tested in three types of water are shown in Table 7. The laser beam hitting the cross-layer fibers after propagating through 1.5-m emulated Turbid Harbor water could only illuminate the first layer of the fibers owing to the severe attenuation (Fig. 15). The SI of received signals under different turbidities is shown in Fig. 16. Over the Turbid Harbor water channel, the system showed a 4% outage probability, and when working over the other two types of water channels, it showed 0% outage probability. The benchmark performs better (0% outage probability under all types of turbid water) than the proposed system due to no power attenuation from the scintillating fiber structures, thus can receiver more optical power after the high attenuation through the turbid water channel. Using scintillating fiber may come at the cost of increased power requirements. However, with the low outage probability at 4%, the designed FD-UWOC system is resilient under turbidity-induced turbulence.
Fig. 16. Averaged scintilation index (SI) under pure sea, coastal ocean, and turbid harbor channels.
Table 7. Performance of scintillating-fiber based FD-UWOC system under turbidity-induced turbulence.
3.5 Effects of mobility in underwater and deployment
The fiber-based FD-UWOC system was also tested in a mobile case to emulate the scene carried by ROV and communicate with an underwater sensor node while ROV moves. The laser was mounted on a mobile platform, and the motor speed of the platform controlled the moving speed of the laser. The motor speed was set to 500, 1000, or 1200 r/min (denoted MS500, MS1000, and MS1200, respectively), and the corresponding moving speed of the laser was 0.86, 1.72, or 2.06 m/s, respectively. Although the laser beam was hitting on different parts of the fibers without a miss when it was moving horizontally, averaged SI of two channels under different speeds was very high compared to that in the stationary case (Fig. 17). In the mobile case, the FD fiber-based UWOC system could still maintain a low outage probability due to the better light coupling from the fiber to the APD. The power fluctuation of the received signal is minimized. The low outage probability of the proposed system with different moving speeds (from 4% to 14%) proves the system's robustness when compared to the benchmark system, which utterly cannot work under mobile cases with 100% outage probability. The results are shown in Table 8.
Fig. 17. Averaged scintilation index (SI) of two channels with the mobility speed at (a) 0.86 m/s (MS500), (b) 1.72 m/s (MS1000), and (c) 2.06 m/s (MS1200).
Table 8. Performance of scintillating-fiber based FD-UWOC system in mobile cases.
3.6 Deployment test of the FD-UWOC system
To further validate the robustness of the scintillating-fiber-based FD-UWOC in real scenarios, we conducted a deployment test in an outdoor diving pool. Figure 18 shows the system operating in the pool. The FD-UWOC system was first tested in a stationary case. The attenuation coefficient of the pool water is 0.37 m−1, higher than the Coastal Ocean’s attenuation coefficient and lower than Turbid Harbor water [37]. It could achieve communications with a 0% outage probability over 7.5 m. Then, a remote-operated vehicle (BlueRobotics ROV) was mounted on transceiver 1 to test the communication performance in a mobile situation. A working distance of 1 m with a 0% outage probability was achieved. The opened-eye diagrams for both cases are shown in Fig. 19.
Fig. 18. Deployment test setup in a pool, showing the transceiver mounted on an ROV working over 7.5 m (see Visualization 1).
Fig. 19. Eye diagrams of OOK scheme for mitigating the effects of mobility in a diving pool: (a) 7.5-m working distance in a stationary case; (b) 1-m working distance in a mobile case.
4. Conclusion
We developed a scintillating-fiber/450-nm-laser-based FD-UWOC system and evaluated its performance under different turbulence and field test conditions. The system was characterized in terms of frequency response at 67.61 and 67.58 MHz of two transceivers, the maximum achievable data rate of 250 and 125 Mbit/s with OOK and PPM schemes, respectively, and self-interference at a level of −40 dB. The effects of turbulence on the UWOC system were also investigated. Outage probability was employed to evaluate the system performance under air-bubble-, temperature-, salinity-, and mobility-induced turbulence. A benchmark system without applying a scintillating fiber detector is tested under the same turbulence for comparison. In comparing the outage performance of the benchmark system and the proposed scintillating-fiber-based FD-UWOC system, laboratory experiments proved the robustness of the scintillating fiber detector for omnidirectional signal detection over a complex underwater channel. Field tests on the FD-UWOC system in an outdoor diving pool further validated the stability of the system when operating in a real scenario. In future studies, the FD-UWOC system would be implemented in the ocean. There is also a need to investigate smart transceivers to overcome the issues of turbulence and PAT in the real sea.
Funding
King Abdullah University of Science and Technology (BAS/1/1614-01-01, GEN/1/6607-01-01, KCR/1/2081-01-01, KCR/1/4114-01-01); King Abdulaziz City for Science and Technology (KACST TIC R2-FP-008).
Acknowledgments
The authors acknowledge Water Sports Center in KAUST for access to the diving pool.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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