Capillary-based fluorescent antenna for visible light communications

The use of fluorescent optical antennas in visible light communications (VLC) systems can enhance their performance by selectively absorbing light from the transmitter and concentrating the resulting fluorescence, whilst preserving a wide field of view. In this paper, we introduce a new and flexible way of creating fluorescent optical antennas. This new antenna structure is a glass capillary which is filled with a mixture of epoxy and a fluorophore before the epoxy is cured. Using this structure, an antenna can be easily and efficiently coupled to a typical photodiode. Consequently, the leakage of photons from the antenna can be significantly reduced when compared to previous antennas created using microscope slides. Moreover, the process of creating the antenna is simple enough for the performance of antennas containing different fluorophores to be compared. In particular, this flexibility has been used to compare VLC systems that incorporate optical antennas containing three different organic fluorescent materials, Coumarin 504 (Cm504), Coumarin 6 (Cm6), and 4-(Dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran (DCM), when a white light-emitting diode (LED) is used as the transmitter. Results show that, since it only absorbs light emitted from the gallium nitride (GaN) LED, a fluorophore that hasn't previously been used in a VLC system, Cm504, can result in a significantly higher modulation bandwidth. In addition, the bit error rate (BER) performance at different orthogonal frequency-division multiplexing (OFDM) data rates of antennas containing different fluorophores is reported. These experiments show for the first time that the best choice of fluorophore depends on the illuminance at the receiver. In particular, when the illuminance is low, the overall performance of the system is dominated by the signal-to-noise ratio (SNR). Under these conditions, the fluorophore with the highest signal gain is the best choice. In contrast, when the illuminance is high, the achievable data rate is determined by the bandwidth of the system and therefore the fluorophore that results in the highest bandwidth is the best choice.