# High-Speed Visible Light Communications: Enabling Technologies and State of the Art
## 3 Enabling Technologies for a High-Speed VLC System
### LED Technology  
White light is the most widely used form of illumination in the common communication applications, such as color rendering [17]. And the most common and cost effective way to produce white light is to use a blue LED with yellow phosphor coating, as shown in Figure 3a[17]. While due to the slow relaxation time of the phosphor coating, the modulation bandwidth is on the order of about 3 MHz [17]. And the limited bandwidth and slow modulation response of LEDs are the key challenges which limit the achievable data rate in VLC. The White light also can be produced with a combination of ultraviolet emitters and multi-color phosphor. Another more attractive way is using separate red-green-blue (RGB) or red-yellow-green-blue (RYGB) emitters, namely, by using RGB or RYGB LED. RGB LED is the most common multi-chip LED, which contains three jointly packaged LED chips (red, green and blue) to produce white lights (see Figure 3b). By changing the light intensities of the different chips, the color of the output light is modified. Since this kind of LEDs avoid the use of phosphor as well as the slow relaxation time, a much higher modulation bandwidth of more than 100 MHz can be obtained. However, the multi-chip LEDs still suffer from some drawbacks, such as having a lower Color Rendering Index3 (CRI), a more complex structure, and higher costs than the traditional phosphor-based LEDs (ps-LEDs). Besides the above LED technologies, some novel LED architectures and LD transmitters are proposed to improve the modulation bandwidth of the VLC system as well. (1) Resonant Cavity LEDs (RC-LED)  
The light extraction efficiency of the LED refers to the ratio of the output photons to the photons generated by the electron-hole recombination in the active region. In conventional LED devices, the light extraction efficiency is usually less than 10%, due to the factors, such as the substrate absorption, electrode blocking, and total reflection on the out-light surface. Most photons are limited to the inside of the device and are converted into heat, which greatly affects the performance of the device, especially in high power LED devices. To improve light extraction, Bragg reflectors are employed in the LED device to offer high efficiency and high bandwidth, based on the resonant cavity enhancement effect as shown in Figure 4. The emission of RC-LED is typically at \(\sim\)650 nm with a narrow line width and can be modulated in excess of 100 MHz [18; 19]. Ref [20] demonstrates a AlInGaN based RC-LED that providing the lowest-loss transmission window, around 520 nm, and the bandwidth can level up to 330 MHz. With RC-LED, a highly directional output and high data rates of up to 500 Mbps [21] can be obtained, but its fabrication is challenging [22]. Figure 3: Schematic diagrams of ps-LED (**a**) and RGB LED (**b**). Micro LEDs (\(\upmu\)-LED)  
Instead of a single LED, micro LEDs consist of arrays of micro-pixelated LED pixels, based on III-nitride Technology. \(\upmu\)-LEDs offer better contrast, response times, longer lifetime, and energy efficiency. More importantly, by reducing the LED active area to 100 \(\times\) 100 \(\upmu\)m\({}^{2}\) or less, the capacitance is decreased while the current density is increased, and this thereby contributes to a significant increase in the modulation bandwidth. In previous reports, the typical size of each individual pixel ranges from 14 to 84 \(\upmu\)m, and the 3-dB bandwidth can reach 450 MHz, which contributes a data rate up to 1.5 Gb/s [24; 25; 26; 27]. [PERSON] et al. reported a violet micro-LED (shown in Figure 5) with electrical-to-optical bandwidths of up to 655 MHz [28]. With further improvement of these \(\upmu\)-LED sources, researchers from the University of Strathclyde has achieved a high bandwidth that exceeding 800 MHz [29]. (3) Laser diodes (LDs)  
Since it is the photon lifetime, not the carrier lifetime that dominates the dynamics, LDs are capable of handling a data bandwidth enlarged by one or two orders of magnitude greater than LEDs [30].