# Recent Advances in Optical Injection Locking for Visible Light Communication Applications
## 4 Applications of OIL in Visible Light Communication Systems
### The Applications of OIL in Underwater Wireless Optical Communication (UWOC)  
Underwater wireless communication plays a crucial role in marine activities, such as environmental monitoring, underwater exploration and scientific data collection. In recent years, UWOC systems have emerged as promising wireless carrier candidates for signal transmission systems in acrimonious, uncharted and turbulent water environments, such as seas. Due to their characteristics of high output power and considerable stability, optical injection-locked sources have demonstrated the tendency to cope well with optical signal propagation issues that occur in UWOC systems, such as strong water turbulence and significant signal attenuation. Figure 4 displays several recent applications of OIL in UWOC systems.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Characteristic Enhancement Compared to Free-Running System**} & \multicolumn{4}{c}{**Working Power (mW)**} \\ \cline{2-5}  & \multicolumn{2}{c}{**Self-Injection-Locked System**} & \multicolumn{2}{c}{**External Optical Injection System**} \\ \cline{2-5}  & \multicolumn{2}{c}{**System**} & \multicolumn{2}{c}{**System**} \\ \cline{2-5}  & **Blue** & **Red** & **Blue** & **Red** \\  & **(-450 nm)** & **(-650 nm)** & **(-450 nm)** & **(-650 nm)** \\ \hline Peak power improvement & & & & \\ (times) & \(\sim\)2.8 & \(\sim\)1.7 & \(\sim\)1.7 & \(\sim\)1.4 \\ Linewidth reduction & & & & \\ (times) & \(\sim\)6.4 & \(\sim\)1.65 & \(\sim\)2.8 & \(\sim\)1.6 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Comparison of the characteristic enhancements of blue and red LDs in SIL and external optical injection cases [29].  
In 2020, Ming Chi University of Technology established a 500 Gb/s PAM4 free-space optical UWOC convergent system for 100 m free-space transmission using either a 10 m piped underwater link or a 5 m turbid underwater link, which integrated PAM4 modulation with a five-wavelength polarization multiplexing scheme [33]. The critical part of that scheme was the utilization of two-stage OIL and optoelectronic feedback on the LDs. With the two-stage OIL and optoelectronic feedback technique, the \(-3\) dB bandwidths were enhanced by \(\sim\)10 times compared to a free-running counterpart.  
Similarly, another wavelength-division multiplexing PAM4 free-space optical integrated FSO UWOC system was proposed, which had a channel capacity of 100 Gb/s [34]. The system applied 405 nm blue-violet light LDs and 1.7 GHz 450 nm blue light LDs via the two-stage OIL and optoelectronic feedback technique, which were adequately adopted for 100 Gb/s PAM4 signal transmission using a 500 m free-space transmission with a 5 m clear ocean underwater link. Apart from the critical deployment of the two-stage OIL and optoelectronic feedback technique, doublet lenses in the FSO, laser beam reducers and transmissive spatial light modulators were also crucial elements that helped the system to achieve a low bit error rate.  
To sum up, with the development of signal modulation and multistage injection locking techniques, practical and high-speed underwater optical wireless links, as shown in Figure 5, which could enable high data rate transmission, are just around the corner.  
Figure 4: Recent applications of OIL in hybrid free space—UWOC systems. The total length of transmission link (x-axis) contains both underwater portion (blue) and free-space counterpart [33; 34; 35; 36].  
Figure 5: OIL in different VLC applications.