## 4 Results and Discussion  
Figure 4 illustrates the optical powers measured by an optical power meter (Thorlab(r), PM100D) when sliding along the 100 cm LDOF. It can be observed that the light intensity is quite uniform in the 20-80 cm range with an average optical power of 25 \(\mu\)W, illustrating that clients Rx locating in the 20-80 cm range can receive similar optical signals. It is also worth pointing out that, as the LDOF is designed to diffuse light 360\({}^{\circ}\) around the fiber circumference, the measured optical powers are nearly the same around the LDOF circumference. As shown in Figure 4, the red DL signal is launched from the left-side facet of the LDOF (i.e., 0 cm position in Figure 4); while the green UL signal is launched from the right-side facet of the LDOF (i.e., 100 cm position in Figure 4). We can observe that both DL and UL signals have the same measured optical power at the 50 cm position. This illustrates the good performance of the LDOF since the scatterings of the DL and UL in the LDOF are the same. Furthermore, this phenomenon also illustrates the uniform light diffusion performance of the LDOF of the two colors at both facets. In addition, we also carried out the power measurement 360\({}^{\circ}\) around the LDOF circumference, and similar powers were received [41]. Figure 3: Experimental setup of the free-space VLC system with bi-direction transmissions supporting multiple moveable Rxs. BERT: bit-error-rate tester; APD: avalanche photodetector; DM: dichroic mirror; OF: optical filter; DSO: digital sampling oscilloscope. Figure 5 illustrates the optical spectra of the DL and UL signals emitted by the 633 nm and the 520 nm LDs measured by a spectrometer (Ocean(r) Insight USB2000). The experimental setup used to obtain the optical spectrum in Figure 5 is the same as that used in Figure 3, in which the APD is replaced by a spectrometer. The optical detector of the spectrometer is located close to the LDOF. It can be observed that both the DL and UL signals have narrow linewidths and high side-mode suppression ratios. Figure 6a-c illustrate the photographs of the LDOF without, with the red light, and with the green light launchings, respectively. We can observe uniform light around the fiber circumference in all cases. The optical signal emitted via the LDOF is safe for human eyes. We purposely make turns in the LDOF to illustrate the flexibility of the LDOF as an optical omni-directional antenna. Figure 7 illustrates the experimental photographs of the CO, in which a directly modulated red LD is used to provide the DL data and an APD is used to receive the UL green data. The DM is used to separate the red DL and green UL signals from the wavelength multiplexed signal, and a lens is used to focus the UL signal into the APD. Figure 8 illustrates the experimental photographs of the client side at different  
Figure 4: Experimentally measured optical powers by a power meter at different positions along the LDOF. Figure 5: Experimental optical spectrum of both DL and UL signals. viewing angles. Two collimators at each side of the LDOF are used to couple optical signals into and out of the LDOF. The client APD mounted on a sliding stage can slide along the whole LDOF to receive the DL signal. As discussed above, we purposely make turns in the LDOF to illustrate the flexibility of the optical antenna. The yellow color emitted via the LDOF can be observed when both red and green lights are launched and combined in the LDOF. Figure 9 shows the DL BER measurements via the LDOF from data rates of 100 Mbit/s to 220 Mbit/s measured at the client side. From a data rate of 100 Mbit/s to 190 Mbit/s, it is error-free. The BER starts to increase at a data rate of 200 Mbit/s, and the BER is 6.50 \(\times\) 10\({}^{-7}\). BER of 3.69 \(\times\) 10\({}^{-4}\) is measured when the DL data rate is 210 Mbit/s, satisfying the 7% pre-FEC threshold (BER = 3.8 \(\times\) 10\({}^{-3}\)). Figure 10 shows the corresponding received DL OOK eye diagrams at different data rates.