# Spatial Multiplexing Holography for Multi-User Visible Light Communication
## 4 Experimental Setup  
### Experimental Platform  
Figure 7a presents the experimental platform utilized in this study. The signal generation process begins with a bit-power-loading discrete multi-tone (BPL-DMT) signal, which is fed into an arbitrary waveform generator (AWG, Tektronix AWG710B, Tech Technology, Shanghai, China) capable of a maximum sampling rate of 4.2 GSa/s. These signals are subsequently amplified using a Mini-Circuits ZFL-2500 VH+ amplifier (Mini-Circuits, New York, NY, USA) and passed through an adjustable attenuator (SMA, KT3.0-30/1S-2S, C&T RF Antennas Inc., Dongguan, China) to mitigate nonlinearity effects.  
The enhanced signals are directed into a laser diode mount (Thorlabs LDM9T/M, Thorlabs, Newton, NJ, USA), which is integrated with temperature-electric coolers (TECs) and controllers. A 638 nm laser diode (Mitsubishi, ML501P73, Tokyo, Japan) is employed to generate the visible light signals. These signals undergo polarization adjustment via a polarizer (insertion loss 0.03 dB @ 632 nm) to ensure compatibility with the spatial light modulator (SLM, Meadowlark, 1920 \(\times\) 1200 XY Phase Series, insertion loss 1 dB @ 632 nm, Meadowlark Optics, Frederick, MD, USA). After polarization, the light is focused through a lens onto a 50 \(\upmu\)m diameter pinhole, serving to eliminate extraneous spatial modes. Subsequently, the light is expanded by the 4-f system (total insertion loss \(\approx\) 3 dB @ 632 nm) to increase the light spot size.  
Figure 7: Experimental setup. (**a**) Experiment platform; (**b**) Signal processing.  
For modulation, visible light is directed through SLM1, which loads the SMH for spatial region transmission. The light then traverses free-space to SLM2. In the context of OAM secret communication, SLM2 decodes the signal using inverse OAM modes. While, for experiments focusing on power allocation and coverage, SLM2 functions similarly to a mirror, reflecting the signals. The transmission distance is calculated from SLM1 for power allocation and coverage experiments, while SLM2 serves as a reflective mirror that does not affect the transmission. In OAM secure communication, SLM2 is positioned 0.1m before the APD to ensure the complete decoding of the OAM modes.  
Finally, the signals are received by a photodetector (Thorlabs, APD430x), converted into electrical signals, and fed into an oscilloscope (Agilent Technologies, MSO9254A, Agilent Technologies, Shanghai, China). When it comes to observing the images, the CCDs replace the APD at the same location. The oscilloscope outputs the data to a computer for subsequent signal processing.  
Three users are positioned in a forward free space following SLM1, located at distances of 0.6 m, 0.7 m, and 0.8 m, respectively. For the power allocation experiment, the target power ratios for the three users were set to 0.15, 0.35, and 1, corresponding to the required data rates for different types of video transmission. In the experiment focusing on power coverage, the targeted diameters for the power coverage areas were established as 0.2 mm, 0.4 mm, and 0.6 mm, respectively. This variation in size aimed to demonstrate the system's capability to adapt to different spatial power distribution requirements. For the OAM secure communication experiment, the users were encoded with distinct OAM modes: 10, 20, and 30, respectively.