| Visible Light Communication(VLC), which uses LED as the light source, takes great advantages of LED's rapid modulation performance. By embedding the communication data information into LED, the communication process can be realized through the alternatively switching between the LED's bright state and dark state. Since 2000 when the Japanese researchers proposed the concept of visible light communication, this technology is developed rapidly and has become a research hotspot of light communication filed. Currently, VLC is still at its early stage, and there are only limited number of prototypes demonstrated, due to the influence of transmission distance, speed and system stability,.etc. For instance, in most traditional VLC systems, LED's intensity is adopted to modulate the digital information, i.e. the bright state and the dark state represents “1” and “0”, respectively. However, the intensitymodulation-based VLC systems greatly suffer from the interference of the background light source(e.g. natural sunlight), especially for the field applications. In order to address the above issue, we are motivated to seek some special light states that are extremely rare in the background light source. By modulating the input LED light to the aforesaid special states, even the signal light is completely merged with the background light, it can still be well separated then demodulated. Specifically, we propose to utilize the light's circularly polarized states for modulating the input light. The main content of this paper is summarized as follows:1. Firstly, this thesis reviewed the LED-based VLC implementations in detail, including the basic principle of transmitter, the free space transmission channel and the receiver parts. Secondly, the hardware platform of the LED visible light communication system based on the circularly polarized light modulation was designed, including the transmitter circuitry, the transmission channel's optical part and the receiver circuitry. In this thesis, the aforesaid VLC system's circuits were simulated by Multisim software, implemented in hardware(PCB-level) and tested using real-time oscilloscope. As indicated by the simulation and experiment results, the transmitter LED circuit's upper-limit transmission speed is measured to be 5Mbit/s; limited by the available photodetector and operational amplifier's performance, the receiver's speed is measured to be 1Mbit/s. Moreover, in order to further elevate the system's overall transmission speed, we used the high speed operation amplifier THS3202 D from TI Company for our simulation, and the overall speed can be up to 20Mbit/s according to the simulation results.2. In this part, the basic principle of the proposed VLC system's optical section was elaborated. According to the basic principle of the polarized light, we modelled the modulation/demodulation of the circularly polarized light based on the Stokes vector and the Mueller matrix. In real-life applications, receiver is usually preferred to be embedded into the mobile terminals, so its volume size is usually limited even integratable to the mainstream silicon chip. As a result, we proposed a dual-layer achromatic micro-quarter-wave-retarder(MQWR) with the detailed design presented in this thesis. Finally, the proposed dual-layer micro-quarter-wave-retarder is compared with the traditional single-layer zero order retarder. The effectiveness of our proposed MQWR is well-proven by the reported achromatic performance. The wavelength range of visible light is in 400nm-700 nm, in this area, our proposed MQWR's retardation is between 0.24 and 0.26, and the traditional single-layer zero order retarder's retardation is between 0.2 and 0.35. In result, the performance is prompted obviously. |
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