# Recent Advances in Optical Injection Locking for Visible Light Communication Applications
## 3 Enhancement Effects of OIL on the Characteristics of Different Devices  
Since GaN-based devices have drawn significant attention for their application in VLC systems, we firstly examined the OIL in violet-blue-green light emitters. GaN-based LDs have demonstrated significantly higher modulation frequencies, greater power and better beam quality in comparison to GaN-based light-emitting diodes (LEDs). These features enable GaN LDs to produce long-distance transmission and open the door for the realization of emerging VLC applications. With the implementation of the OIL technique, the performance of GaN LDs has been further improved. Table 1 summarizes the enhancement effects of OIL on the characteristics of GaN LDs and the comparison with free running LDs. In 2018, researchers from KFUPM and KAUST reported the application of self-injection locking in InGaN/GaN (blue/green) and InGaP/AlGaInP (red) visible light LD systems, in which the free-space optical feedback paths were accomplished using external mirrors [18]. They achieved significant increases of \(\sim\)57% (1.53-2.41 GHz) and \(\sim\)31% (1.72-2.26 GHz) in the modulation bandwidth and \(\sim\)9 (1.0-0.11 nm)- and \(\sim\)9 (0.63-0.07 nm)-fold reductions in the spectral linewidths of the green and blue lasers, respectively. The following year, self-injection-locked green LDs using tunable dual-wavelength systems have been explored [19]. The self-injection-locking scheme was based on an external cavity configuration and utilized either highly or partially reflective mirrors. A tunable longitudinal mode spacing of 0.20-5.96 nm was accomplished, which corresponded to a calculated frequency difference of 0.22-6.51 THz. To further explore the systems, the same group employed the self-injection-locking technique for InGaN/GaN green LDs in an external cavity configuration with a partially reflective mirror using single- and multiwavelength laser systems [20]. The single-stage self-injection-locked laser system was for tunable laser and multiwavelength generation, while the two-stage self-injection-locked laser system was for near-single-wavelength generation. Figure 3 shows the narrow linewidth in single-mode, dual-mode and four-mode self-injection-locked green LDs at \(\sim\)525 nm and the two-stage self-injection-locking setup achieved a narrow locked-mode linewidth of \(\sim\)34 pm at 524.05 nm. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Setup** & **Pros** & **Cons** \\ \hline Free-running blue & Simple structure & Relatively large emission \\ F-P laser diodes & & spectrum linewidth \\ InGaN/GaN DFB LDs & Near-single-mode emission and a high & Fabrication of high-quality DFB grating \\ with gratings & side-mode & is challenging \\  & suppression ratio & System complexity \\ Littrow or Littman external cavity & A narrower linewidth and a high & (i.e., requires dielectric gratings or other \\ LD systems & side-mode suppression ratio & wavelength filtering elements) \\ External cavity semiconductor & A satisfactory tuning range and high & Fine-tuning structure \\ LD systems & output power & is required \\ Self-injection-locked LD systems & Good wavelength tunability and high & Additional system complexity \\  & optical power & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The enhancement effects of OIL on the characteristics of different devices [15; 16; 17]. Later in 2020, a prism-based self-injection-locked seamlessly tunable blue InGaN/GaN LD composite cavity system was presented [21]. The team achieved a clear main peak in the spectrum in contrast to the free-running one. Using an external cavity configuration and the self-injection-locking technique, the robust, simple and compact system achieved a significant enhancement in the wavelength-tuning window of up to 12.11 and 8 nm with \(\sim\)3 and 14.5 mW optical power via high-reflection and low-reflection configurations, respectively, at just above the threshold current.