## 2 System Design and Experimental Setup  
The scenario of portable optical terminal (OT) based on micro-projection architecture for short-range bidirectional multi-wavelength visible light communications is illustrated as in Figure 1. Each OT is based on an LCoS-based micro-projection architecture [10], for which it has been demonstrated that data transmission is possible while performing micro-projection simultaneously. The physical layer components of each OT are identical. It includes a RGB-LEDs (red-, green- and blue-based light-emitting diodes) light source manufactured from Philip (Luxeon Z series), a PD (photo-detector) for signal detection, a lens, L1/L3 with a diameter of 50 mm for collimating/focusing, a 50:50 PBS (polarization beam splitter) for polarization beam splitting, a LCoS-based SLM (Liquid Crystal on Silicon based Spatial Light Modulator) and an image lens, L2 with a diameter of 50 mm. All the lens components and PBS were manufactured from Thorlabs. The advantages of using RGB-based LEDs as the light source of an OT are that a higher modulation bandwidth as well as multiple wavelengths can be used compared with the conventional light source, i.e., a blue LED based white light source, in which the available modulation bandwidth is inherently restricted by the phosphor. However, the main challenge for short-range bidirectional multi-wavelength visible light communications based on OTs is that multiple wavelengths received by the OT should be selected dynamically without the need to enforce additional hardware modifications, i.e., employment of more PDs. In this research, the experimental work of using OT as transmitter (OT_Tx) and receiver (OT_Rx) for bidirectional multi-wavelength visible light communications that have investigated is presented and demonstrated. In our experiments, the use of the OT in the transmitter mode (OT_Tx) is similar to the work presented previously [10]. As illustrated in Figure 1, a Philip Luxeon Z series RGB-based LEDs [11], which is a low cost component and available commercially, was used as the light source of the OT_Tx. Three LED chips, which have the central wavelengths of 650 nm, 550 nm and 450 nm respectively, are included within this light source. The 3-dB modulation bandwidths of each LED chip within the OT architecture were measured to be 8 MHz, 16 MHz and 12 MHz for 650 nm, 550 nm and 450 nm LED chips respectively. These LED chips have also mounted on a MCPCB (metal core printed circuit board, 20 mm \(\times\) 20 mm) for data transmission (OT_Tx). The data for OT_Tx transmission were generated randomly and modulated offline by a computer software (Matlab) from a PC (Personal Computer). These signals were transferred to the memory of an AWG (arbitrary waveform generator, Keysight 33621A) which has a sampling rate of 1 GSa/s. The emitted lights of these LED chips were driven individually by signals that have been modulated offline from AWG and biased separately by DC currents through low-frequency Bias Tees. The DC current used was fixed to 350 mA since it has a better performance observed from our research works. The operation principle of the OT in the forward direction link is that multiple wavelengths (i.e., red, green and blue wavelengths) transmitted from light sources are collimated by the collimating lens, L1 first and are then selected by a PBS before incident on the LCoS-based SLM. Since the LCoS-based SLM is sensitive to the polarization property of the incident light, only one polarization state of the incident light which was selected by a PBS in advance can be modulated. Although this polarization sensitivity problem will result a higher system light loss, a typical polarization conversion system (PCS) [12; 13] can be used to compensate this loss. The LCoS-based SLM device used was a phase only microdisplay (JD 9554, JDC, Hsinchu, Taiwan) and all pixels on the device can be controlled through the upload of a hologram pattern that has designed. These patterns can be binary, multilevel or continuous.