Fig. 1. (Color online) An example of UVC system and network^[4].

Fig. 2. The typical configuration of UVC.

Fig. 3. (Color online) The spectrum of solar radiation on earth^[37].

Fig. 4. Typical channel models of UVC: (a) LOS model, (b) NLOS model.

Fig. 5. (Color online) The schematic of the experimental setup for UV laser-based NLOS UWOC^[33].

Fig. 6. (Color online) (a) Optical spectra of the LED under a bias voltage of 7 V^[64]. (b) The small-signal frequency response of the system. The dashed line indicates the –3 dB bandwidth, which is approximately 29 MHz at distance = 0^[64]. (c) The modulation bandwidth of the system at a distance of 5 m with different injection currents^[60]. (d) The experimental setup and the flow diagram of the signal generation and offline processing^[60].

Fig. 7. (Color online) (a) A 4 × 4 matrix device structure with a single device size of 60 μm, with the corresponding changes in device response frequency and current^[75]. (b) Simplified cross-sectional schematic of a single DUV μLED presented in this work. Dimensions are not to scale^[34]. (c) Plan view optical image of the fabricated DUV μLED array presented in this work^[34]. (d) The 3 dB electrical modulation bandwidth of the DUV μLED as a function of current density^[34].

Fig. 8. (a) Normalized PL decay kinetics for AlGaN MQW structures with different well widths: (1) 5 nm, (2) 4.1 nm, and (3) 2.5 nm. Measurements were performed under excitation energy density of 25 mJ/cm^2[90]. (b) Well-width dependence of carrier lifetimes for AlGaN MQW structures at excitation energy density of 70 μJ/cm^2[90]. (c) Lifetime for different temperatures derived from the TD-TRPL results^[91].

Fig. 9. (Color online) (a) The experimental setup of the receiver side^[66]. (b) The experimental setup^[19]. (c) Experimental setup for solar-blind NLOS UV communication with diversity reception^[30].

Fig. 10. Application of UVC in aircraft squad.

Table 1. Comparison of DUV light sources in UVC system.

Table 2. Recent progress in UVC using LED as the light source.

Table 3. Comparison of DUV detectors in UVC system.