# Recent Advances in Optical Injection Locking for Visible Light Communication Applications
## 2 Applications of Optical Injection Locking (OIL) in Different Fields  
In the 1980s, following the development of semiconductor lasers for telecom applications, scientists actively began investigating the applications of OIL in optical communication systems [4]. Over the course of its history, the OIL technique has been utilized in many fields, prior to the development of VLC systems. OIL has been successfully applied in the infrared wavelength field. Research regarding the effects of OIL on the noise properties of mid-infrared quantum cascade lasers has produced results that indicate that locked slave lasers could operate under reduced-intensity noise levels compared with the free-running operation [6], thereby corroborating the characteristic enhancement ability of the injection-locking technique. This practical approach to achieving low-noise operation for quantum cascade lasers is critical for the majority of applications in gas sensing and absorption spectroscopy. In 2020, a 448 Gb/s four-level pulse amplitude modulation (PAM4) free-space optical communication system with a 600 m free-space link was constructed, which also utilized polarization multiplexing optical injection-locked vertical cavity surface-emitting lasers (VCSELs) [7]. The VCSELs were optically injected with light from distributed feedback (DFB) LDs via the combination of a three-port optical circulator and a polarization controller. The findings from the experiment demonstrated that four 1.55 um VCSEL transmitters were sufficiently powerful  
Figure 1: Schematic diagrams of the OIL technique. (**a**) External OIL configuration consists of master lasers and slave lasers. The master light can be injected into the slave laser and the output light can be blocked from the master laser via a circulator. (**b**) Self-injection-locking configuration consists of a reflector through which light can be partially fed back. for 448 Gb/s PAM4 signal transmission when using the OIL technique. Additionally, the utilization of an eight-mode self-injection-locked quantum dash laser diode (LD) and a Reed-Solomon encoding technique in a secure PAM4-based free-space optical communication system was presented [8], which could transmit 88 Gbps of data over 555 m optical free-space links. Moreover, the OIL technique has also been implemented in the generation and transmission of microwave and mmWave signals. Recently, there have been several studies on the generation and transmission of microwave and mmWave signals using the OIL technique. For example, [PERSON] et al. presented a frequency-modulated microwave generation setup including one master laser and two slave lasers [9]. In such a system, slow and fast perturbations were used to study frequency-modulated and externally locked P1 dynamics. The frequency-modulated continuous-wave generation at 6 GHz was achieved using externally locked lasers, with a comb contrast of up to 42 dB. In an all-optical Ka-band microwave long-distance dissemination system, which was based on an optoelectronic oscillator (OEO), a single tone with high spectrum purity and low phase noise was excited by an optical injection-locked OEO, thereby achieving the stable phase transmission of mmWave signals [10]. Additionally, self-injection-locked quantum dash LD comb source technology has been used to produce a tunable 50/75 GHz mmWave transmission system in the difficult 1610 nm area [11]. Examples of mmWave generation and transmission systems are shown in Figure 2.  
Furthermore, using a hybrid integrated self-injection-locking DFB laser, a gain-switched optical frequency comb source was proposed, which had eight pure continuous comb lines within 3 dB of the spectral envelope peak and a narrow linewidth of 615 kHz [12]. The carrier to noise ratio and the phase correlation between the comb lines were significantly improved by the self-injection-locking effect that was induced by the silicon nitride mirroring reflector, which could be promising for future radio over fiber and coherent optical communication.