# Recent Advances in Optical Injection Locking for Visible Light Communication Applications
## 4 Applications of OIL in Visible Light Communication Systems  
### Applications of OIL in Free-Space Visible Light Communication Systems  
The adoption of the OIL approach in free-space VLC systems can improve the system performance in many distinct ways.  
In 2013, a novel bidirectional lightwave transport system was proposed, which employed a phase modulation scheme and an optical injection-locked DFB LD as a duplex transceiver for passive optical networks [25]. The critical part of the OIL technique in that study was that it could be achieved when the frequency of the master laser (DFB LD1) was lower than that of the slave laser (DFB LD2), through which the system obtained a low bit error rate. In 2020, another bidirectional free-space optical communication system was built with a 600 m free-space transmission, which applied a phase modulation scheme and a remotely optical injection-locked DFB LD [26].  
As well as DFB LDs, optical injection-locked VCSELs have also been deployed successfully. In 2015, a 10 m/25 Gbps Li-Fi transmission system was proposed, which was based on a two-stage optical injection-locked 680 nm VCSEL transmitter [27]. Compared to the free-running system (5.2 GHz), the two-stage optical injection-locked system achieved a pronounced increase in the 3 dB bandwidth of up to 26.2 GHz.  
In 2020, a tunable external cavity self-injection-locked violet LD system was reported, which exhibited a continuous wavelength tunability of 5.15 nm (400.28-405.43 nm), with mean SMSR and linewidth values of \(\sim\)23 dB and \(\sim\)190 pm, respectively [28]. The salient setup of that system was a pellicle beam splitter with a 92:8% splitting ratio within the external cavity, which could transmit 92% of the optical power back to the front facet of the laser, thereby realizing high-quality external cavity self-injection locking.  
In addition to the distinctive qualities of the various device categories, there are some differences between self-injection and external injection locking. In 2018, a team of researchers from KAUST launched an investigation into the performance enhancement effects of self-injection and external injection locking for high-bitrate VLC systems [29]. They discovered that \(\sim\)1.4- and \(\sim\)1.1-fold improvements in the modulation bandwidth and \(\sim\)6.5- and \(\sim\)3.2-fold reductions in the spectral linewidth were achieved using self-injection-locked blue and red LDs, respectively. The short external cavity self-injection-locked system also exhibited superior performance by a factor of 1.1-1.3 compared to the long cavity (26 cm) configuration. Conversely, the external injection system exhibited weak locking signatures but improved linewidths by a factor of \(\sim\)1.6-2.8, reaching as low as \(\sim\)70 and \(\sim\)87 pm for the blue and red LDs, respectively, while almost doubling the peak powers. A comparison of the characteristic enhancements of blue and red LDs in SIL and external optical injection cases is exhibited in Table 3.  
Furthermore, two-stage injection has become more common nowadays. In 2022, a wavelength-division multiplexing visible laser light communication and white light ring network was successfully demonstrated this year, which achieved a 150 Gbit/s accumulative transmission rate at the central station, a 50 Gbit/s transmission rate at the optical node and 604 lux white light at the central station [30]. By utilizing red, green and blue LDs with the two-stage OIL and optoelectronic feedback technique, high-speed laser-based VLC links and white light illumination at the reading or writing level could be accomplished.  
Overall, different OIL techniques, including self-injection, external injection and multi-stage injection, have been applied to enhance a diverse range of devices, with the primary goals of greater \(-\)3 dB modulation bandwidth improvements and enhanced data transmission capacities with high bitrates.