## 6 Underwater Optical Wireless Communication  
Underwater communication (UC) has received significant interest as part of the VLC applications due to growing ocean exploration demands, and researchers are making substantial efforts into realizing UC systems with a longer distance, a higher transmission rate and lower BER [83,84]. Sonar has been commonly used for this UC application but suffers from a low transmission rate and latency, while using RF for UC is limited with a short range and high cost [85]. UOWC has therefore been introduced and is an innovative technology with numerous advantages, such as high speed, low latency, low cost of implementation and high safety [85]. LEDs and laser diodes have been commonly used as the light source for the UOWC systems, however, the transmission distance is short for the commercial LEDs and it has a limited bandwidth, making it difficult to maintain a high data transmission rate. \(\mu\)-LEDs based on III-nitride semiconductors such as GaN offer efficient solution to address the restricted bandwidth issue by displaying low RC delay [86,87]. However, due to low output power, \(\mu\)-LEDs are not preferable for high attenuation UOWC, but despite having a small bandwidth, they display tremendous potential in UOWC for medium/short distance transmission. An overview of UOWC is shown in Figure 11.  
Recently, [PERSON] et al. demonstrated 2 Gbps/3 m air-underwater optical wireless communication using a single-layer quantum dot (QD) blue \(\mu\)-LED [88]. The QD \(\mu\)-LED structure consists of 75\(\mu\)m single-layer InGaN QDs as the active region and SiO\({}_{2}\) as the passivation layer. The indium tin oxide (ITO) layer is deposited on the top of QD \(\mu\)-LED to form ohmic contact. The passivation layer is used to reduce the current leakage and enhance the output optical power. The experimental set-up for the QD \(\mu\)-LED-based UOWC system uses 1 m free space and 2 m underwater distance. A BER tester (BERT Anritsu MP2100A) is used to evaluate the communication efficiency by generating pseudo-random binary signals (PRBSs) with non-return-to-zero (NRZ)-OOK modulation format. The NRZ-OOK signals are transmitted through an amplifier until the bias-tee superimposes them with the direct current (DC). The superimposed RF and DC signal then drives the micro-LED with the bias tee and activates the modulated light. Using a convex lens, the optical light is focused and then transformed into an electrical digital baseband signal that is further amplified by an amplifier. The transmitted signal first feeds back to the BERT to record the eye-diagram using the digital sampling oscilloscope and measure the system's BER using the BERT error detector at various data transmission rates. The modulation bandwidth has been calculated and found to increase with increasing current density, and it even  
Figure 10: (**a**) Signal-to-noise (SNR) curve plotted against the bandwidth attained for Orthogonal Frequency-Division Multiplexing (OFDM) using the reported \(\mu\)-LED array; (**b**) BER plotted against the data rate attained by adopting OOK, 4-PAM and OFDM modulation schemes. [79] Figures reproduced with permission from IEEE. reaches 1.03 GHz corresponding to a current density of 582.54 A/cm\({}^{2}\), which is a record-breaking bandwidth with such a short-range UOWC system. The eye-diagram of the device set-up becomes unclear when the data rates are 1.6 and 1.8 Gbps corresponding to BERs of \(2.14\times 10^{-4}\) and \(4.20\times 10^{-4}\), respectively. The eye-diagram is nearly closed at 2.0 Gbps corresponding to BER of \(2.03\times 10^{-3}\), but the highest data rate of 2.0 Gbps is achieved under the forward error correction (FEC) criterion of \(3.8\times 10^{-3}\). This is the highest data rate record for the UOWC system based on a single-layer QD \(\upmu\)-LED with a 3 m air-underwater transmission distance. In another research by [PERSON] et al., there is demonstration of an 80 \(\upmu\)m blue-emitting micro-LED for high-speed underwater optical wireless communication [89].