# Visible Light Communication System Technology Review: Devices, Architectures, and Applications
## 2 VLC Transmitter Light Source
### Theoretical Background of Laser for VLC Application  
Due to the low data transmission rate of the LED modulation, Professor [PERSON] was the first to use laser diodes to replace LEDs for Li-Fi. By utilizing lasers' high energy and luminous efficiency, the data transmission rate can be ten times higher than LEDs. Laser lighting can produce white light through color mixing technology, similar to RGB LEDs. Although an LED-based VLC can achieve a transmission rate of 10 Gb/s and increase the upper limit of the data transmission rate of 7 Gb/s in Wi-Fi, the laser data transmission rate can easily exceed 100 Gb/s. The R&D team of Arizona State University developed a nano-scale white light laser which can be a potential Li-Fi light source. Laser diodes have several advantages in communication applications: faster response speed, direct modulation, and high coupling efficiency. For general semiconductor lasers, the LD can modulate light controlled by the input current when the injected current exceeds the threshold current (\(I_{th}\)), as the modulation characteristics show in Figure 4. The linear region above the threshold current to the saturation area is the LD working area, the modulation range. Hence, reducing the device threshold current and obtaining a more extensive modulation working area is crucial.  
The threshold current density is shown in Equation (2):  
\[J_{th}=\frac{ed}{I_{inj}}\bigg{(}BN^{\prime}2e^{2\frac{\upmu\upmu+\upmu_{2}}{ \upmu_{0}}}+CN^{\prime 3}e^{3\frac{\upmu\upmu+\upmu_{2}}{\upmu_{0}}}\bigg{)} \tag{2}\]  
where \(J_{th}\) is threshold current density; \(e\) is the electron element charge; \(d\) is the active layer thickness; \(I_{inj}\) is the injection current; \(N^{\prime}\) is the transparent carrier concentration; \(\upalpha_{m}\) and \(\upalpha_{i}\) are the mirror loss and optical absorption loss, respectively; \(\Upzeta_{g0}\) is Maximum mode gain; and \(B\) and \(C\) are the radiation recombination coefficient and Auger recombination coefficient, respectively.  
Figure 3: Benchmark for \(\upmu\)-LEDs used in VLC system.  
A laser-based VLC system reaches tens of gigabit-per-second of data rates, benefiting from its extensive modulation bandwidth. The modulation speed of the laser diode is controlled by the photon lifetime (\(\sim\)ps) rather than the carrier lifetime (\(\sim\)ns) in the LED. In addition, a laser-based VLC system enables narrow beam transmission for long-distance applications. A 100 m OOK-modulated VLC link was demonstrated with a data rate of 2.3 Gbps in 2017 [65]. While the OOK modulation technique is commonly used due to its simplicity, many advanced modulation schemes have been used in laser-based VLC to realize high data rates, with the feasibility to exceed 100 Gbps.  
In a high-speed VLC system, the transmitter is critical to achieving a high data rate. A promising light source in a VLC system requires high optical output power, large modulation bandwidth, high energy conversion efficiency, low operation voltage, small form-factor, and long lifetime. Since the blue-green lasers were based on gallium nitride materials, the high-speed laser-based VLC system requires a fast GaN-based transmitter. Three types of lasers were commonly used as transmitters: edge emitting laser diodes (EELDs), super luminescent diodes (SLDs), and vertical-cavity surface-emitting lasers (VCSELs).