## 2 System Design and Experimental Setup
In our experimental setup, the LCoS-based SLM was sectioned into a different area in which each area has a sub-hologram uploaded. The majority of the sub-holograms on the LCoS device were used to perform the polarization modulation of the incident light, which reflects the light into free-space through the image lens (L2) except that only one sub-hologram corresponding to a SVGA (super video graphics array) resolution (800 \(\times\) 600 pixel) was used for micro-projection application in our proof of concept research. When multiple wavelengths of lights were transmitted from OT_Tx, another OT (OT_Rx) in the reverse direction link can be used to receive these lights for communications. However, in order to select the wavelength accordingly, the OT_Rx should be operated as a dynamic bandpass optical filter given that only one PD is used. For OT_Rx to be operated as a dynamic bandpass optical filter and select wavelength accordingly, the LCoS-based SLM can be collaborated with or without physical bandpass optical filters, as illustrated in Figure 2. In the configuration of an LCoS-based SLM without physical bandpass optical filters, the LCoS-based SLM is operated as a tunable filter to reflect a desired wavelength into the PD accordingly. In order to reflect or block the wavelengths of 650 nm, 550 nm and 450 nm from red-, green- and blue- LED chips respectively, the LCoS-based SLM as illustrated in Figure 3, was divided into three section areas, which are \"Red\", \"Green\" and \"Blue\". Each section area was responsible for each corresponding wavelength separately. As shown in Figure 3a, when the LCoS-based SLM is in Red filter mode in which the red wavelength is a desired one to pass, the section area of \"Red\" on LCoS device was setup to have the maximum reflection power for red wavelength. The other section areas on LCoS device were setup to have the minimum power reflection for green  
Figure 1: Architecture of short-range bidirectional visible light communication (VLC) based on optical terminals (OTs). and blue wavelengths respectively. The design of the LCoS-based SLM to modulate the reflected intensity of 650 nm, 550 nm and 450 nm wavelengths respectively was based on our calibration results in terms of using RGB-LEDs light source as shown in Figure 4.  
Although in each filter mode, the LCoS-based SLM can be setup to have the best reflection efficiency for each corresponding wavelength, the crosstalk resulting from the light of unwanted wavelengths is still unavoidable. Therefore, in order to improve the system crosstalk, physical bandpass optical filters were in turn used in front of the PD to further investigate the property of the crosstalk. Figure 4: Calibration results of LCoS-based SLM using RGB-LEDs. Figure 3: Design of LCoS-based SLM as tunable filter. Figure 2: Design of OT_Rx for reverse directional link. At the receiver part, a focusing lens L3 was setup in front of a PD in order to improve the collection efficiency of the incident light transmitted from free-space. The PD (PDA 10A, Thorlabs) used was a Si free-space amplified photodetector which has an active area of 0.8 mm\({}^{2}\)[14] and a responsivity of 0.385, 0.25 and 0.175 amperes per watt (A/W) for red, green and blue wavelengths respectively. For the multi-wavelength communication scenario, several PDs, which were devoted to each corresponding wavelength within the receiver, should be used and physical bandpass optical filters are also essential components that are normally used in front of each detector in order to choose a desired wavelength and cut off the other unnecessary wavelengths. Although this kind of configuration offers the possibility for the terminal users to receive multi-wavelength simultaneously, it is not so practical for a terminal device to accommodate these PDs within a limited physical space. Moreover, the effectiveness resulted from the physical bandpass optical filters in the proposed optical terminal architecture has also not evaluated properly.