## 6 Underwater Optical Wireless Communication
As mentioned in the above section, underwater optical communication suffers from water absorption, scattering from suspended particles, etc., however, water attenuation is relatively small in the blue-green spectrum region, thus making blue or green emitting LEDs suitable for UOWC. In the work carried out by [PERSON] et al., GaN-based micro-LED array emitting blue light has been manufactured on sapphire substrate, and the LED structure consists of InGaN/GaN MQWs, which is then bonded to a printed circuit board (PCB). The packaged micro-LED array is then tested for underwater communication by analyzing the characteristics of the transmitted light after passing through a water tank. The micro-LED has been driven by a DC bias to measure the -3 dB electrical-to-optical bandwidth using a network analyzer. The frequency response of the micro-LED is shown in Figure 12a for different injection currents ranging from 1 mA to 88 mA, and the extracted -3 dB modulation bandwidth is shown in Figure 12b as a function of the injection current. It is clear from that the bandwidth initially increases up to 160 MHz and then remains constant from 61 mA. The BER has been analyzed at different injection currents from 16 mA to 88 mA, as shown in Figure 12c. It is found that high transmission speed can be achieved at a higher injection current, which is attributed to the increase in modulation bandwidth and light output power. A BER of \(1.3\times 10^{-3}\) is obtained at 88 mA with a data rate of 800 Mbps, and the data rate is limited from the saturation of the modulation bandwidth at 61 mA. However, the data rate can be further improved by using an OFDM modulation scheme, which allows different modulation schemes for UOWC. Figure 11: Overview of underwater optical wireless communication. The obtained light output power at a fixed driving current for UOWC application is the key limiting factor due to the heavy attenuation of power. The BER is again analyzed with respect to the received light output power by maintaining at an injection current of 61 mA. A higher current, i.e., 88 mA, can be used to drive the micro-LED to evaluate the BER and obtain a more efficient value, however, a high injection current may hinder the reliability of the micro-LED. It is observed from Figure 12d and the inset figure that a data rate of 100 Mbps is achieved at received light powers of 80 \(\upmu\)W and 0.1 \(\upmu\)W using p-i-n (PIN) and Avalanche Photodiode (APD) receivers, respectively. The system offers the ability to achieve a data transmission speed of 100 Mbps at distances of about 11 m with a PIN receiver and about 40 m with an APD receiver. The eye diagrams for the micro-LED at 400 Mb/s and 700 Mb/s using a PIN photodetector receiver are shown in Figure 13a,b, and one open diagram can be seen in Figure 13a, implying free error data transmission. The closed eye diagram in Figure 13b corresponds to a BER of \(4.8\times 10^{-7}\). Using the APD receiver, the eye diagram is open for received output power of 80 \(\upmu\)W, as shown in Figure 13c, indicating free error data transmission, and the eye diagram is almost closed in Figure 13d, corresponding to output power of 0.1 \(\upmu\)W.  
Figure 12: (**a**) Frequency response of the micro-LED at different injected currents; (**b**) -3 dB modulation bandwidth as a function of current; (**c**) BER vs. data rate; (d) BER vs. received light output power. [89] Figures reproduced with permission from Optical Society of America.